Breeding of waders (Charadrii) and Brent Geese Branta bernicla bernicla at Pronchishcheva Lake, northeastern Taimyr, Russia, in a peak and a decreasing lemming year

I B l S 135: 2 7 7 - 2 9 2

Breeding of waders (Charadrii) and Brent Geese Branta bernicla bernicla at Pronchishcheva Lake, northeastern Taimyr, Russia, in a peak

and a decreasing lemming year L. G. UNDERHILL', R. P. PR?S-JONES2.*, E. E. SYROECHKOVSKI, JR.3, N. M. GROEN4.

V. KARPOV3, H. G. LAPPOS, M. W. J . VAN ROOMEN4, A. RYBKIN3, H. SCHEKKERMAN4, H. SPIEKMAN4 & R. W. SUMMERS6

lAvian Demography Unit, Department of Statistical Sciences, University of Cape Town, Rondebosch 7700, South Africa, and Schweizerische Vogelwarte, Sempach CH-6204, Switzerland

ZBritish Trust for Ornithology, The Nunnery, Thetford, Norfolk, lP24 2PU, UK

31nstitute of Evolutionary Morphology and Animal Ecology, Russian Academy of Sciences, Leninsky Prospect 33 , 11 7071 Moscow, Russia

4Working Group International Wader and Waterfowl Research, Vogelbescherming, Driebergseweg 16C, 3708JB Zeist. The Netherlands

Slnstitute of Geography, Russian Academy of Sciences, Moscow, Russia 6Royal Society for the Protection of Birds, North Scotland Regional Office, Etive House,

Beechwood Park, lnverness lV2 3BW. UK

~~

During summer 1991, lemmings occurred at high densities in Arctic tundra at Pronchi- shcheva Lake in the northeastern Taimyr Peninsula, whereas, in 1992, lemming densities were substantially lower and decreased further during the summer. In 1991, avian predators such as Snowy Owls Nyctea scandiaca, gulls and skuas bred well: Arctic foxes Alopex lagopus were rarely observed in the study area but bred in the immediate vicinity. In both years there was a late thaw, but this did not deter breeding by birds. The insect food supply for waders showed similar patterns of abundance in both years. In 1991, 73 nests of nine species of wader were found within a 14-km2 study area, and Dark-bellied Brent Geese Branta bernicla bernicla nested in association with Snowy Owls. The overall density of wader nests was estimated to be 7 per km2. Clutches disappeared at only two wader nests and no Brent Goose nests, and the Mafield estimate of the daily probability of predation for waders was 0.0022. In contrast, the daily probability of predation was 0.20 in 1992, when there was a similar breeding density of waders. Arctic foxes were seen searching for food daily within the study area, and fox droppings were found associated with nests taken by predators. The predicted scenarios for peak and decreasing lemming years (the Roselaar- Summers hypothesis), i.e. low predation and high nest success in the peak year and high predation and low nest success in the decreasing year, therefore occurred.

The relationship between abundances of lemmings Lemmus sibiricus and Dicrostonyx torquatus and Arctic foxes Alopex Zagopus in the Taimyr Peninsula, Russia, and the breeding productivity of migratory waders (Charadrii) and Dark-bellied Brent Geese Brunta berniclu bernicla (the Roselaar-Sum- mers hypothesis [Dhondt 19871) has been the subject of a series of correlational studies which have generated consid- erable interest and debate (Roselaar 1979, Summers 1986. Boyd 1987. Dhondt 1987, Ebbinge 1987,1989, Greenwood 1987. Owen 1987. Summers & Underhill 1987, Underhill 1987,1988, in press, Gromadzka 1988, Martin & Baird 1988, Parkin 1988. Prokosch 1988. Sutherland 1988, de Boer &

* Current address: The Natural History Museum. Tring, Hertford- shire HP23 6AP. UK.

Drent 1989, Underhill et al. 1989, Underhill & Summers 1990). Many of these papers concluded that the controversy could not be resolved by statistical argument and that only direct observations on the breeding grounds in the Taimy~ Peninsula would suffice. An opportunity for such fieldwork was afforded by the International Arctic Expeditions of the Institute for Evolutionary Morphology and Animal Ecology. Russian Academy of Sciences, in the summers of 1991 (F'rj% Jones 1991) and 1992.

A relationship between lemmings, Arctic foxes and breeding birds was postulated by Larson (1960), who pointed oul that, in those regions of the Arctic where lemmings were absent, the number of breeding bird species was small and their anti-predator behaviour (sitting tightly on eggs and elaborate injury-feigning displays) was highly developed. In

3 7 7

278 L . G . U N D E R H I L L E T A L . I B I S 135

LAPTEV

Bolshevik Island

9

S E A

1 ‘ .- KARA S E A

Figure 1. scale line in the inset represents 15 km.

The study area at Pronchishcheva Lake. northeastern Taimyr Peninsula. Russia, showing localities mentioned in the text. The

contrast, in regions where lemmings occurred, the avifauna was richer but many species performed less intense distrac- tion displays. Roselaar (1979) pointed out the similarity in breeding productivity of Curlew Sandpipers Calidris ferruginea and Brent Geese and suggested that this was the result of Arctic foxes switching their diet from lemmings in years of high lemming abundance to eggs and young of waders and geese in years of low lemming abundance. However, Roselaar had no lemming data and merely guessed that there was a relationship. Summers (1986) obtained lemming abundance data from Dorogov (1983) and pointed out the concordance between the lemming peaks and the years of high breeding productivity of Brent Geese. Correlations between the proportions of first-year Brent Geese in western Europe and those of first-year waders at Langebaan Lagoon, South Africa, have been reported for Curlew Sandpipers (Robertson 1981. Underhill 1987), Sanderlings Calidris alba (Summers et al. 1987). Knots C. cnnutus (Underhill et al. 1989) and Turnstones Arenaria interpres (Summers et al. 1989).

In this paper, we report on the breeding density. breeding seasonality and nest success of waders and Brent Geese at a locality in the Taimyr Peninsula during two successive summers. Data on the density of lemmings and potential predators are presented, together with the phenology of snow cover and the insect food supply, factors that could also influence breeding success.

STUDY AREA AND METHODS

The general location of the study area was preselected from a map to be close to a relatively large lake in the foothills of the Byrranga Mountains in the southern part of the Arctic tundra subzone (Chernov 1985). The exact location of the camp, on the west shore of Pronchishcheva Lake (altitude 78 m above sea level, and 30 km from the shore of the Laptev Sea) (75”16”, 112O28‘E) (Fig. 1). was chosen on the arrival of the advance party on 12 June 1991, the criterion being to locate the camp as close to the preselected area as possible but with snow-free patches within walking distance. Three rivers, from south to north, the Bad (Plochoi in Russian transliteration), the “Good” and the “Ugly” (our names), flowed eastwards, draining hilly countryside. The Ugly River was a tributary to the Kuldima River, while the Good and Bad Rivers flowed into the lake. The highest point within the study area was 151 m above sea level.

The watersheds and most of the slopes in the study area consisted of nanopolygonal spotty (medallion) tundra (Cher- nov 1985). The highest parts of prominently drained watersheds were generally gravelly (up to 6OYo bare) with little vegetation other than mosses and lichens surrounding each bare spot of frost-heaved tundra. Flatter watersheds had less bare ground, the vegetation consisting of herbs, dwarf shrubs and mosses, the dominant species being Dryns punctata, Salix volaris, AZovecurus alainus and Hylocomium svlendens var.

1993 B R E E D I N G O F W A D E R S A N D B R E N T GEESE A N D A B U N D A N C E O F LEMMINGS 279

Figure 2. Distribution of Snowy Owl nests (O), Brent Goose nests (Ir) and broods @), Herring and Glaucous Gull colonies (G). and Arctic fox dens (F) in the vicinity of F’ronchishcheva Lake, summer 1991.

alaskanurn. Marshy sites and small pools were found on some of the flatter watersheds. Valley plains and dells contained bog-tundra complexes, with a poorly developed patchwork of polygons in places, the dominant species being Eriophorurn angustijioliurn, Carex stans, Calarnagrostis holrnii, Dupontiafisch- eri and Polytrichurn stricturn. A total of 145 plant species was i.dentified.

The percentage cover of different vegetation types was estimated in each 250-m x 250-m square within the 14- km2 intensive study area (see below). Dry gravelly tundra made up 7% of this area. Wet vegetation types covered 31%, including 7% marshes with water covering the surface. The remaining 61% consisted of various types of well-vegetated and moderately dry tundra, and 1% consisted of gravelly river beds.

The study periods were 15 June to 8 August 1991 and 23 June to 22 July 1992. The percentage snow cover was vi- sually estimated daily until the only remaining snow lay in sheltered gullies. Maximum and minimum air temperatures were recorded daily at the camp. The thermometer was placed 1 cm above the ground and shaded from the sun.

Nests were searched for by flushing incubating birds and watching them come back onto the nest. The nests were marked at a distance of 10 m with small stakes and were checked frequently (median interval 3 days) to determine whether the eggs hatched, were taken by predators or were deserted. An area of 14 km2 to the west of the camp was intensively studied, with the objective of locating all nests and broods, although excursions were made into a larger area (Fie. 2).

Incubation periods (time between laying and hatching of the last egg) were determined where possible and compared with values reported in the literature. In 1991, estimation of the timing of the start of incubation was made at each nest for which egg dimensions and mass were available, using the principle that eggs lose mass at an approximately constant rate during incubation (Drent 1975. L.G. Underhill, unpubl.). Most egg lengths and breadths (dial callipers to 0.1 mm) and masses (50 g Pesola balance, estimated to 0.1 g, shielded in a rucksack on windy days) were measured by one observer (L.G.U.) to limit the problem of inter-observer variability. A large proportion of nests was found within a week of the start of incubation, so the errors made with this method were likely to be small. In 1992, egg mass was measured to 0.5 g, and this method could not be applied. However, in this year, when the rate of predation was so high (see Resufts), this information was used to estimate likely starting dates of incubation at each nest to produce a composite picture of the overall start of incubation at wader nests (see Appendix). Nest survival rates were estimated using the Mayfield method (Mayfield 1975).

Lemming densities were estimated by intensive trapping. Snap traps were set for 2-3 days in seven plots of 0.25 ha. The traps were cleared at least daily, and the data point was accepted only if the numbers of lemmings in the final two trappings were relatively small, indicating that virtually all lemmings in the plot had been trapped and that immigration was negligible (Tupikova & Yemelyanova 1975). In both years lemming studies were conducted by A.R., so that the results obtained are comparable.

280 L . G . UNDERHILL E T A L . I B I S 135

Insects and spiders were trapped in ten pitfall traps set 10 m (1991) or 20 m (1992) apart on a transed through dry. spotty tundra near the camp. The pitfalls (500-ml plastic tubs 11 cm deep and 9 cm diameter) were filled to a depth of 1-2 cm with a watedethanol mixture (to prevent freezing) with a drop of detergent added to disrupt surface tension. The traps were emptied regularly (median interval 2 days in 1991. constantly at 3 days in 1992). and the animals were identified and counted. The capture rate was dependent on the abundance and activity of insects and arthropods and is therefore thought to provide an approximate index of food availability to waders. The data are presented as graphs using 5-day moving averages.

The examination of nest success formed part of a multi- faceted study. The eggs were measured when the nests were found and weighed at intervals. Most of the incubating adults were trapped on the nest, ringed (and individually colour- marked with leg flags), measured and weighed, had blood and feather samples taken and were dyed with picric acid. All these activities created additional disturbance, increasing the likelihood of predation and desertion and tending to decrease the nest success from what it would have been without our presence. Blood- and feather-sampling and dye- marking were done only in 1991. so this additional disturbance was lower in 1992. The only use made of this information in this paper is to estimate an index of breeding site fidelity from the return rates of individually colour-marked waders between the 2 years.

A.R. was the only co-author in the study area in both years. N.M.G. and H.Sp. were briefed before the 1992 expedition by H.Sch. and M.v.R. and took with them detailed maps and descriptions and an earlier draft of this paper. A more subtle difference between the years was that in 1991 the 14-km2 study area was defined a posteriori as the area we were ultimately able to cover: for 1992, this area was defined a priori.

RESULTS

Snow cover, temperature and insect activity

On our arrival on 15 June 1991, snow cover exceeded 95%, and fresh falls of snow had occurred during the preceding days. Snow cover continued to exceed 90% for another week before dropping to below 10% at the beginning of July (Fig.3A). The area between the Good and Bad Rivers was the last to be free of snow, and it was in this area that the latest nesting was observed in 1991 (see below). The first date on which a temperature above 10°C was recorded was

22 June: there was a warm spell from 10 to 18 July, with maximum temperatures approachg 30°C (Fig. 3C). The Good River flowed into the lake for the first time on 23 June, and peak flood occurred on about 27 June. The three rivers in the study area were reduced to a trickle by mid-July. Invertebrate food was available throughout the thaw (Fig. 3E). with peak abundance in mid-July. A dip in abundance towards the end of the month coincided with a period of cold weather from 20 to 25 July (Figs 3C and E), with several rainy days and light falls of snow.

On 23 June 1992, snow cover was 60% (compared with 80% on the same date in 1991) (Fig. 3B). In both years, snow cover fell below 10% for the first time on 3 July. It was colder in 1992 than in 1991, with temperatures above 10°C recorded on 8 days of the study period, compared with 19 days in the same period during 1991 (Fig. 3D). The Good River flowed into the lake on 26 June. Invertebrate food started becoming abundant c. 10 days later in 1992 than in 1991 but did not show the decline exhibited in the earlier year (Fig. 3F).

Lemming and predator densities

Lemming densities were high throughout the study area in 1991. and the observations in the seven 0.25-ha plots suggested densities between 100 and 392 animals per ha. Lem- ming densities were high from the time of our arrival before the thaw until our departure. In 1992, the densities in five plots ranged between 0 and 64 lemmings per ha, with the value of 64 being obtained in the first sample (started on 2 July, during the latter stage of the thaw) and the value of zero in the penultimate sample (16 July). This coincided with the visual assessment that lemming abundance decreased during the study period. In both years. Lemmus sibiricus was dominant (c. 80% of the total number of lemmings trapped).

Lemming predators bred well in 1991. Ten pairs of Snowy Owls Nyctea scandiaca nested within an area of 26 km2, the mean distance of each nest to its nearest neighbour was 2.8 km (s.d. 0.2) and the mean clutch size was 8.6 (s.d. 0.9). One clutch of 12 eggs and three of 11 were observed. The usual range of clutch sizes is 3-9, with extremes of 2-14 (Cramp 1985). There was a high density of skuas, with 14 nests of Pomarine Skuas Stercorarius pomarinus, nine nests of Long-tailed Skuas S. longicaudus and one of the Arctic Skuas S. parasiticus within the 14-km2 study area. There were also many non-breeding skuas. For example, flocks of over 30 Long-tailed Skuas were regularly observed feeding along the lake shore from mid-July: non-breeding Pomarine Skuas tended to be in singles and twos. Both Taimyr Gulls Lams heugZini tairnyrensis (c. 40 pairs) and Glaucous Gulls

+ Figure 3. Time series during the study period at Pronchishcheva Lake, summers 1991 and 1992. (A, B) Percentage snow cover: (C. D) Maximum and minimum temperatures; (E, F) Daily numbers of arthropods in pitfall traps (Collembola. dotted line; other insects. solid line: spiders. dashed line). The heavy lines in A, C and E indicate the period in common between the 2 years.

1993 B R E E D I N G O F W A D E R S A N D B R E N T GEESE A N D A B U N D A N C E O F LEMMINGS 281

100 A SNOW COVER 1991

70 W

30 i

0 1

"C

16 June 26 June 6 July 16 July 26 July 5 August

E ARTHROPODS 1991 350 1 300 I$ 250 1

100 B SNOW COVER1992

70

26 June 6 Julv 16 Julv MAX. AND MIN. 301 D TEMPERATURES 1992

-5

350 - 26 June 6 July 16 July

- F ARTHROPODS 1992

300 -

250 -

200 -

150 -

100 -

26 June 6 July 16 July

DATE 1991 DATE 1992

282 L . G . UNDERHILL E T A L . I B I S 135

Table 1. Numbers, densities and average clutch sizes of wader nests on 14 km2 of Arctic tundra at Pronchishcheva Lake. northeastern Taimyr Peninsula, summers 1991 and 1992

Mobile Deduced Estimated Average

found found behaviour total per km2 size (n) Nests broods from Estimated nest density clutch-

Species 1991 1992 1991 1991 1991 1991 1991 1992’

Ringed Plover 2 3 3 2-5 Pacific Golden Plover 7’ 6 2 7-9 Grey Plover 8 7 - 1 8-9 Knot 1 0 - 2 1-3

-

-

Sanderling 3 4 4 2 7-9 Little Stint 13 24 8-9 2 21-24 Curlew Sandpiper 25 20 6-8 - 31-33 Dunlin 1 0 Turnstone 13 13 - 1 13-14 Grey Phalarope 0 1

1 - -

- - -

Total (1991) 732 18-21 13 91-107

Many nests were taken by predators in 1992 before the clutches were complete. ’ Includes one probable replacement nest.

0.14-0.36 4.0 (2) 0.504.64 3.9 (7)

4 (1) 0.57-0.64 4.1 (9) 0.07-0.21 0.50-0.64 4.0 (3) 1.50-1.71 4.0 (13) 2.21-2.36 3.8 (25)

0.07 4 (1) 0.93-1.00 4.0 (13) - -

6.5-7.6

2.3 (3) 3.7 (6) 3.4 (7)

3.3 (4) 3.5 (24) 3.6 (20)

3.2 (13)

-

-

3 (1)

Larus hyperboreus (c. four pairs) bred successfully on islands in Pronchishcheva Lake (Fig. 2). and their diet consisted largely of lemmings (Filchagov et al. 1992, pers. obs.). No other species of avian predator was seen.

There was one fox den near the boundary of the study area in 1991. and two dens were located nearby, one 3 km east of the study area and one on the east shore of Pron- chishcheva Lake (Fig. 2). The foxes were secretive and were rarely seen in the study area. Moreover, prior to and during the thaw, their tracks were seldom seen in the snow. The foxes at the den closest to the study area had well-grown pups by 16 July. and the three dens raised eight, four and at least two young foxes, respectively. Tracks of a wolf Canis lupus were observed during the thaw on 19 June 1991. but no other mammalian predators were noted.

In 1992, reproduction of avian predators of lemmings was poor. Three or four Snowy Owls were regularly seen on the study area but did not breed. Approximately 70 Pomarine Skuas, 17 Long-tailed Skuas and an occasional Arctic Skua were present. One nest of each skua species was found: Pomarine Skua. one egg on 2 July disappeared by 10 July; Long-tailed Skua, one egg on 27 June disappeared by 29 June: Arctic Skua, one egg at a nest outside the study area on 12 July disappeared by 16 July. A Rough-legged Buzzard Buteo lagopus and a subadult Peregrine Falco peregrinus were both observed over the study area on 17 July. Taimyr and Glaucous Gulls nested in about the same numbers as in 1991 but suffered heavy predation (a fox was observed at one colony on three occasions), and few nests survived to hatching.

In 1992. one den of Arctic foxes with two young was found outside the study area, c. 5 km northeast of the camp near the Kuldima River. In addition, 13 dead foxes were found

in and near the study area after the thaw. The behaviour of the foxes was different in 1992 compared with 1991, and they were regularly seen intensively searching the slopes and valleys for food, sometimes even roaming into the camp. Wolf tracks were observed on 4 July (in a partially destroyed gull colony), on 13 July (200 m from camp) and on 21 July. Stoats Mustela erminea were seen on 27 June and on 9 July.

Breeding density and distribution of waders and Brent Geese

Peak lemming year, 1991

The arrival of the waders and Brent Geese preceded our own. In 1991, all species subsequently found nesting were observed daily from 15 June, mainly at the snow-free patches which accounted for less than 10% of the total area at that stage (Fig. 3A). There was no discernable increase in numbers of waders or geese, nor was there striking evidence of passage through the study area. During the week until the first wader nest was found on 22 June 1991, we frequently observed territorial displays, nest scraping, court- ship behaviour and copulation.

A total of 73 wader nests and an additional 18-21 wader broods, belonging to nine species, were found in 1991 (Table 1). The total number of nests within the 14-kmz section of the study area intensively searched was estimated to be 91- 107, yielding an average density of nests of between 6.5 and 7.6 nests per km.

Two nests of Ringed Plovers Charadrius hiaticula were found, and a further three were suspected, all on the shingle bands of rivers. Seven Pacific Golden Plover Pluvialis fulva nests were located (Fig. 4). of which one was almost certainlv a

1993 B R E E D I N G O F W A D E R S A N D B R E N T G E E S E A N D A B U N D A N C E O F L E M M I N G S 283

M

Figure 4. Distribution of (A) Pacific Golden Plover, (B) Grey Plover, (C) Sanderling. (D) Little Stint, (E) Curlew Sandpiper and (F) Turn- stone nests at Pronchishcheva Lake. The open circles denote broods found within one day of hatching in 1991.

replacement clutch for a pair which lost its eggs early in the incubation period: at least two additional pairs simulta- neously giving alarm calls were present in the study area, so that the overall total was 6-8 pairs. Eight Grey Plover P. squatnrola nests were located in the study area, one more nest near the northeast corner was not found (Fig. 4) and a further nest was found 2 km north of the study area.

One Knot nest was located on a dry, stony area which was about 25% bare, 25% Dryas covered and 50% lichen covered. There were two other similar localities at which Knots were seen displaying and nests were searched for but not found. Three Sanderling nests were found within the study area and another 1 krn west of it (Fig. 4C). Subsequently, four further broods were found in the study area. The number of nests plus broods within the study area was possibly more than the seven recorded, but it is unlikely to have exceeded nine. Only eight Little Stint Calidris minuta nests were found while still in the incubation stage. due to the inconspicuous

behaviour of the adults. After the start of hatching, the adults gave alarm calls, and this resulted in five further nests being found with the chicks still in them (i.e. within a day of hatching). At two of these, hatching was still in progress. Nine further broods were found out of the nest, one of which may have originated outside the study area. We estimate that there were between 21 and 24 nests of this species within the study area.

The Curlew Sandpiper was the most abundant breeding wader, with 25 nests and six to eight broods found (Fig. 4E, Table 1). Almost all nests or broods probably were located, and the total number of nests within the study area was between 31 and 33. One Dunlii Calidris alpina nest was found in typical Curlew Sandpiper nesting habitat and only 150 m from the nearest Curlew Sandpiper nest. This is well north of the limit of Dunlin breeding shown in Cramp & Simmons (1983) Hayward et al. (1986) and Stepanian (1990) and may be the most northerly record in Eurasia, although Dunlins nest as far as 78"N in Greenland (Meltofte 1985). Thirteen Turnstone nests were found within the study area, and there was probably one more nest near the northeast corner of the study area. The density of wader nests within the study area was estimated to lie in the range 6.5-7.6 nests per krnz (Table 1).

The following wader species were observed during the period between our arrival and the end of June, but none remained to breed in the study area: Dotterel Charadrius morinellus, Bar-tailed Godwit Limosa lapponica, Red-necked Phalarope Phalaropus lobatus, Grey Phalarope P. fulicarius. Red-necked Stint Calidris ruficollis. Pectoral Sandpiper C. me- lanotus. and Purple Sandpiper C. maritima.

At least 16 pairs of Brent Geese nested in the area within a radius of 10 krn from camp (Fig. 2). Of these, 15 pairs were in two colonies of seven and eight nests, respectively, in the fox-exclusion zones around Snowy Owl nests, and at least one pair nested singly.

Decreasing lemming year, 1992

Seventy-three nests of eight wader species were found in the study area in 1992 (Table 1). Nests of seven species were found in both years. A Grey Phalarope nest was found during laying, and another pair was seen making a nest cup. No Knot or Dunlin nests were found, but an alarming Knot was observed on 9 July and a male Dunlin was observed displaying on 26 June. Unlike in 1991, Pectoral Sandpipers were present throughout the study period, with up to 14 individuals recorded on a single day, often in pairs, but no proof of nesting was found. Dotterel were observed three times and Red-necked Stint once but, as in 1991, did not breed. Due to high predation on nests (see below), it was not possible to estimate nest density in 1992 because the number of (repeated) replacement clutches was unknown and nests could have disappeared before being found. Two Brent Goose nests, one within the study area, were found within 10 km of camp, both nesting singly.

284 L. G . U N D E R H I L L E T A L . I B I S 1 3 5

Table 2. Indices of breeding site fidelity of waders at Pronchishcheva lake. northeastern Taimyr Peninsula, between summers 1991 and 1992

Percentage Number Percentage observed with Site fidelity

Species marked 1991’ marked 19912 marks in 1992 (n)l index‘

Pacific Golden Plover 4 25 5.6 (36) 22 Grey Plover 13 76 26.1 (36) 35

Sanderling 8 50 11.1 (18) 22 Sanderlings 8 100 11.1 (18) 11 Little Stint 19 42 0.7 (138) 2 Little Stint5 19 84 0.7 (138) 1 Curlew Sandpiper 26 41 1.9 (105) 5 Curlew Sandpiper5 26 81 1.9 (105) 2 Turnstone 17 63 48.8 (43) 77

Knot 2 50 6.7 (15) 13

Number of birds colour-ringed in 1991. Estimated from midpoint of estimated total number of breeding pairs in 1991 (Table 1). assuming two adults per nest (but see footnote 5

Sample sizes (n) are cumulative totals of the numbers of birds checked for colour-rings each day in 1992; some colour-ringed birds will have below).

been observed more than once. * Site fidelity index = (percentage observed with marks in 1992)/(percentage marked in 1991) x 100.

female Curlew Sandpipers were marked in 1991, both males and females would have been checked for marks in 1992. Site fidelity index assuming one adult per nest: probably correct for Little Stint; possibly correct for Sanderling; although it is likely that only

Breeding site fidelity and between-year comparison of nest distribution

The index of breeding site fidelity computed in Table 2 gives an ordering of the return rates to the study area for species which were colour-ringed in 1991. Of the species with at least ten birds colour-ringed in 1991. Turnstone and Grey Plover showed the highest site fidelities and Curlew Sand- pipers and Little Stints the lowest. Our sample sizes for Pa- cific Golden Plovers (three birds out of four colour-ringed in 1991 were observed in 1992), Knots (one out of two) and Sanderlings (two out of eight) were too small for conclusions about site fidelity to be drawn. One colour-ringed Turnstone returned to the same territory in 1992 and had a nest 200 m from the 1991 nest site: another had a nest 1400 m from the previous year’s nest site. One male Pacific Golden Plover nested 200 m from the 1991 breeding site.

There was a striking difference in the distributions of nests for the six most numerous wader species between the 2 years (Fig. 4). A comparison of the overall wader nest distributions (Fig. 5) shows that clusters of nests were in different regions of the study area in 1991 and 1992.

Clutch-sizes

In 1991, 69 of 74 wader nests for which data are available contained four-egg clutches. The exception were Curlew Sandpiper clutches of three, two and one eggs, on Pacific Golden Plover clutch of three eggs and a Grey Plover nest with a clutch of five eggs. Clutch-sizes for Brent Geese in

1991 varied between four and seven eggs, with a mean of 5.0 (n = 12).

Apparent mean clutch sizes in 1992 were smaller than those in 1991 (Table 1); this is an artifact because nests were found before the clutches were complete and taken by predators before the clutch-size could be established. Egg vol- umes did not decrease during the breeding season, indicating that eggs in (presumed) replacement clutches were as large as those in first clutches (N.M. Groen & H. Spiekman, unpubl. data). No Brent Goose clutch-sizes were established in 1992. One nest was taken by a predator before the clutch was complete; the second nest was discovered as predation occurred.

Figure 5. Overall distributions of wader nests at Pronchishcheva Lake in 1991 and 1992.

1993 B R E E D I N G OF W A D E R S A N D B R E N T G E E S E A N D A B U N D A N C E O F L E M M I N G S 285

0.15 i ALL WADER SPECIES 1991 (n = 62)

0.10 - 0.05 - 0.00

I I I I I I I ) ] ] , , I I 1 1 161718192021222324252627282930 1 2 3 4 5 6 7 8 9 101112131415161718192021

JUNE JULY

DATE 1991 O.1° 1 ALL WADER SPECIES 1992 (n = 78)

161718192021222324252627282930 1 2 3 4 5 6 7 8 9 101112131415161718192021 JUNE JULY

DATE 1992 Figure. 6. Estimated probability density functions of the overall start of incubation for all wader species at Pronchishcheva Lake in 1991. using a kernel method (Silverman 1986). and in 1992 (see Appendix). The vertical axes give the values of the probability density functions. Probabilities of incubation starting between two dates are given by the area under the curve between the two dates. The median of each of the probability density functions is indicated by the vertical line.

Incubation periods

Observations on incubation periods relate to 1991 only. The incubation period at one Ringed Plover nest was c. 23 days, compared with a mean incubation period of 24 days (Prater 1974, Pienkowski 1984). At the Knot nest, the incubation period was c. 21 days, within the range of 21-22 days given by Nettleship (1974). P.S. Tornkovich (in litt.) also observed incubation periods of 21 days for two pairs of Knots at Kni- povich Bay (76WN. 98”32’E), in 1990. The incubation period at one Sanderling nest was 20 days, which is shorter than the 24-27-day period given by Cramp & Simmons (1983). This nest was incubated by both the male and female, as also found in Greenland (Pienkowski & Green 1976. cf. Parmelee & Payne 1973). P.S. Tomkovich (in Zitt.) observed a mean incubation period for Sanderlings of 21.0 days (s.d. = 0.7, n = 8) at Knipovich Bay. At two Turnstone nests, incubation periods of 19 and 21 days were observed, shorter than the 22-24-day incubation period given by Bergman (1946). For Brent Geese, two incubation periods of c. 23 days were observed, in the lower part of the range of 21-28 days given by Cramp & Simmons (1977). Barry (1962) gives an average incubation time of 24 days (sample size, standard deviation and range not given).

Nesting phenology

In 1991, the start of incubation at 9096 of the wader nests was estimated to lie between 21 June and 7 July, with median 25 June (Fig. 6). Turnstones had the earliest median starting

date, 22 June (Fig. 7). Apart from one nest at which incubation started on 2 July (and which was deserted 6 days later), Turnstones started incubation between 20 and 26 June. Most Curlew Sandpipers started incubation between 19 and 28 June, with a median date of 25 June (Fig. 7). Three of the nests at which incubation started in July were close to the small lake in the Bad River valley, an area which had remained snow-covered until near the end of June: it seems unlikely that these were replacement clutches. The laying period of Curlew Sandpipers was the most extended of all the wader species, with the last date on which males were heard singing and displaying being 2 July. Little Stints had the latest median starting date, 30 June (Fig. 7). However, the mating system of this species is complex (HildBn 1978, Cramp & Simmons 1983). and the start of incubation may be delayed for several days after the clutch is complete.

In 1992. the start of incubation was more protracted than in 1991, with incubation at 90% of the wader nests estimated to have started between 22 June and 15 July (Fig. 6). This was probably due to (repeated) replacement clutches.

Nest success

In 1991, clutches were lost to predation at only two wader nests, both in the valley of the Ugly River and only 200 m apart, a Ringed Plover nest found on 2 July and a Turnstone nest found on 26 June were both lost between 6 and 9 July. All other nest losses were due to other factors-one to sub- sidence down a lemming burrow, three to anthropogenic

L . G . U N D E R H I L L ET A L . I B I S 135 2 8 6

0.3 - 0.2 -

0.1 -

0.0

PACIFIC GOLDEN PLOVER (n = 7)

1 1 1 1 1 1 1 1 1 1 1 ~ ~ ~ ~ ~ ~ ~ , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ,

0.2 - 0.1 - 0.0

GREY PLOVER (n = 9)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ ~ ~ ~ ~ ,

0.2 -

0.1 -

0.0

161718192021222324252627282930 1 2 3 4 5 6 7 8 9 101112131415161718192021 JUNE JULY

SANDERLING (n = 3)

I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ 1 1 1 1 , 1 1 1 ~ 1 ~ ~ ~ ~ ~ 1 1 1 1

0.3 -

0.2 - 0.1 -

0.0

161718192021222324252627282930 1 2 3 4 5 6 7 8 9 101112131415161718192021 JUNE JULY

n ~ T r 1 ooi

LITTLE STINT (n = 7)

I I I I I I I I I I

0.1 -

0.0 I I I I I I l i l l l l l l l l l l l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 9 9 3 B R E E D I N G O F W A D E R S A N D B R E N T G E E S E A N D A B U N D A N C E O F L E M M I N G S 2 8 7

factors and two or three probably to a late start to incubation; except where accidentally crushed by humans, the eggs were found cold in the nest.

In total. 907 wader nest days were observed in 1991. Thus, by the Mayfield (1975) method, the daily probability of predation on wader nests was 0.0022 (=2/907). and the probability of a nest surviving a 22-day incubation period without predation was 0.95. The overall daily risk of predation or desertion was 0.0066 (=6/907), so that the probability of a nest lasting 22 days was 0.86. No predation on the Brent Geese nests was observed.

In 1992, 347 wader nest days were observed. Siy-eight nests were taken by predators, and three were deserted. Seven nests were still being brooded when the expedition left on 22 July. In one of the latter, a Ringed Plover nest, the eggs were starting to crack. The daily probability of predation on wader nests was 0.1960 (=68/347), and the probability of a nest surviving 22 days without predation was 0.0082. The overall daily risk was 0.2046 (=71/347), and the overall probability of surviving 22 days was 0.0064. i.e. one nest in 154 hatching.

The only Brent Goose nest found on the study area in 1992 had one egg on 24 June, but the egg had disappeared by the next day. On 4 July, the nest of a pair of Brent Geese along the Kuldima River was observed being taken by an Arctic fox. The last Brent Geese on or near the study area left on 8 July, whereas in 1991 clutches hatched from 27 July and a pair of geese were observed with their young on the small lake in the Bad River valley until the departure of the expedition on 8 August.

DISCUSSION

Breeding distribution and densities

In both years, most nests were found in the first areas to be snow-free, so that nests were clustered (Fig. 5). The first snow-free patches were not in the same places in the 2 years. so that the patterns of nest distribution were different (Fig. 5). This is presumably due to differences in wind directions during winter and early spring.

The density of wader nests in 1991 was 7 per km2, which is lower than values reported elsewhere on the Asian tundras. For example, Flint (1974) noted that wader densities decrease from 300 individuals per km2 on the southern tundras on the plains of the Lower Indigirka to 22 individuals per km2 in the northern tundras on Wrangel Island. In the northern Taimyr Peninsula, Flint (1974) reported 63-71 waders per km2, suggesting c. 35 nests per km2. (Meltofte

[1985] quoted Flint 119741 as reporting nest density, whereas in fact he reported density of birds.) Our nest density is near the middle of the range (1-16/km2) reported for northern Greenland (Meltofte 1985), based on study areas comparable in size to ours, and similar to densities of five and seven broods per km2 observed in an area of 3.1 km2 at Ny- Alesund, Svalbard, in two summers (Bengtson 1975). The earlier, larger estimates from Siberia were possibly based on small areas that became snow-free early in the summer.

Based on limited observation, we further gained the impression that the breeding density of waders and Brent Geese within our study area was higher than in the surrounding region, probably because of the later disappearance of snow from these areas. Therefore, using our nest density to ex- trapolate the population size to a larger area, as was done by Meltofte (1985) for high Arctic Greenland, could lead to overestimates. At the other extreme, our study area was not as atypical of the surrounding region as those of Boertmann et al. (1991) in northeast Greenland, where the study sites were “like oases in a barren arctic desert”. In order to estimate breeding population sizes in the Taimyr Peninsula. a large sample of nest densities from both good and poor hab- itats will be required, due to the patchiness of the utilization of the tundra by the birds.

For at least two of the nine breeding species of wader, the number of nests cannot necessarily be equated to the number of breeding pairs. The 24 or so nests of Little Stints may have resulted from only c. 12 females (Cramp & Simmons 1983). For the Sanderling, there is variability in the mating system between areas and possibly years (Cramp & Simmons 1983). so the number of pairs may be less than the estimated 8-9 nests, although at the one nest we observed closely, both the male and female incubated.

The three species showing high indices of breeding site fidelity (Turnstone, Grey Plover and, probably. in spite of the small sample size, Pacific Golden Plover; Table 2) are monogamous, have strong pair bonds and vigorously defend their territories (Cramp & Simmons 1983). Of the species with low fidelity indices, the mating system of Little Stints involves successive bigamy by both sexes, and Curlew Sand- piper males leave the breeding grounds soon after the start of incubation (Cramp & Simmons 1983, Portenko 1959); for these species the probability of between-year mate fidelity is likely to be low. Both Little Stints and Curlew Sandpipers had small territories, which were used mostly for display and were not defended once incubation was underway. In contrast, both these species are known to be site faithful to their non-breeding sites (for example, Middlemiss 1961, El- liott et al. 1976. but see also Martin et al. 1992).

e

Figure 7. Estimated probability density functions, using a kernel method (Silverman 1986). of the start of incubation at Pronchishcheva Lake for six species of waders for which data are available for at least three nests. The vertical axes give the values of the probability density functions. Probabilities of incubation startiig between two dates are given by the area under the curve between the two dates. The median of each of the probability density functions is indicated by the vertical line.

L . G. U N D E R H I L L ET A L . IBIS 135 2 8 8

Incubation periods

For the wader species for which observations were available, incubation periods tended to be shorter than those recorded elsewhere or at the lower end of known ranges. The length of the incubation periods of tundra birds was commented on by Uspenskii (1984, especially pp. 174-175), who suggested that in high Arctic species with small numbers of eggs the incubation time is shorter than it is further south. This phenomenon should be investigated further as an adaptation to the intensive utilization of the short Arctic summer.

Nest success

We here place the two study years in the context of predator and lemming abundance and observed waderlgoose nest predation in the previous 3 years on the Taimyr Peninsula. In 1988, there were moderate to high lemming numbers and waders had a good breeding season (Tomkovich 1989). In 1989, there were low lemming and high fox abundances, and wader nest losses were as high as %95% because of predator activity (Prokosch & Hotker 1990, Kondratyev 1992). In 1990, a year without lemmings. predators were scarce and breeding success good (YCsou 1991, B.S. Ebbinge & P.S. Tomkovich, pers. comm.. A. Rybkin. unpubl. obs.). In terms of the c. 3-year cycles in lemming abundance. it was anticipated that lemmings would be abundant during summer 1991 on the Taimyr Peninsula, that predators would breed successfully but feed almost exclusively on lemmings and that nest success of waders and geese would be high (Summers & Underhill 1991). Likewise, 1992 was anticipated to be a year of low lemming but high predator abundance, with poor bird breeding success.

In 1991, the daily risk of nest predation in the study area was about one in 450 at wader nests, and we observed no predation at Brent Geese nests. Other common breeding birds present. notably Ptarmigan Lagopus mutus and Lapland Bunting Calcarius lapponicus, also had high rates of nest success. In addition, we had the impression that predation was very limited during the fledging period. Only two (unsuc- cessful) attacks by predators (Pomarine Skuas) on young birds (Ptarmigan broods in both cases) were seen. Chick survival of Curlew Sandpipers was also high (M.J.W. van Roomen & H.S. Schekkerman. unpubl. data). In 1992. the daily risk of nest predation was about one in five at wader nests, and the only Brent Goose that nested in the study area lost its egg within one day. Thus, at this particular locality, the scenarios predicted by the Roselaar-Summers hypothesis for low and high predation years were enacted perfectly.

In summer 1991, lemming population dynamics showed some variation across the Taimyr Peninsula. Besides the quantitative data from Pronchishchev Lake, lemming densities were estimated qualitatively at three well-spaced mainland localities and at Bolshevik Island (78"40'N, 102O3O'E) in the Severnaya Zemlya Archipelago (Fig. 1). Along the lower reaches of the Bolshaya Balakhnya River (73"36'N, 106030fE), near the southern limit of breeding dis-

tribution for high Arctic birds, Iemmings were abundant (Filchagov et al. 1992). Arctic foxes were present, there was negligible predation and breeding productivity was high (P. YCsou, pers. comm.). At Knipovich Bay, at the mouth of the Nizhnyaya Taimyra River (76"04", 9S032'E), lemmings were present but in relatively low numbers, Arctic fox numbers were very low, predation on waders was negligible and breeding productivity was good (P.S. Tomkovich, pers. comm.). At the mouth of the Pyasina River (74"08'N, 86"45'E), lemmings were extremely numerous in mid-June but decreased rapidly in early July so that there were few lemmings alive in mid-July (B.S. Ebbinge, W. Kania & P. Chylarecki, pers. comm.). Most wader nests and Brent Geese nests which were not on islands were taken by Arctic foxes, and broods of geese on islands were taken by Taimyr Gulls (B.S. Ebbinge, pers. mmm.). To the north of the Taimyr Peninsula, on Bolshevik Island in the polar desert, both lemmings and foxes were absent, and Brent Geese and waders had good breeding productivity (Vokov & Pridatko in press). It appears, therefore, that despite good lemming numbers over large parts of the Taimyr Peninsula in 1991, there was some variation in the timing of the peak lemming abundance.

Throughout the Taimyr Peninsula, the thaw in 1991 was later than normal: in the west at the Pyasina River mouth, 50% snow cover persisted until 25 June, and in the east at Pronchishcheva Lake, this value was not reached until 2 days later. These geographical limits encompass most of the breeding range of the Dark-bellied Brent Goose (Uspenski 1960).

In summer 1992, at the localities visited on the Taimyr Peninsula (Sibiriakov Island, Pyasina River, Nizhnyaya Tai- myra River, shrub tundra of central Taimyr and Malaja Lo- gota, and Sverdrup Island and the Izvestiya Tsik Archipelago in the Kara Sea), lemmings were present but in low to very low numbers, and at the Nizhnyaya Taimyra River, it was reported that they decreased during the summer. At all localities, there were daily observations of hunting foxes, with few active dens. Long-tailed and Pomarine Skuas were abundant, with only a few nests. Snowy Owls were present at most localities. but no nests were found, and many left the study areas after the thaw. Predations levels on wader, geese and gull nests were high at all localities. In addition, at many localities the thaw was even later than in 1991 and the summer was very cold (B.S. Ebbinge, A. Lindstrom. J.H. Mooij, G. T. de Roos & P.S. Tomkovich, pers. comm. to H.Sch. or L.G.U.: E.E. Syroechkovski, Jr., pers. obs).

The percentages of first-year Brent Geese in winter 1991/ 1992 were 3 5 4 0 % in The Netherlands (B.S. Ebbinge & C. Berrovoets, pers. comm. to H.Sch.) and 31.2% in Britain (Kirby 1992), in the top quartile of percentages observed since 1954 (Summers & Underhill 1991, Kirby 1992). In 1992/1993, the percentage was 0% (H. Schekkerman, pers. observ.: C. Berrevoets. pers. comm. to H.Sch.). The percentage of first-year geese in flocks in the wintering areas integrated breeding productivity over the whole breeding range and includes losses due to non-breeding and during incubation, fledging and migration (Syroechkovskiy et aZ.

1993 B R E E D I N G OF W A D E R S A N D B R E N T G E E S E A N D A B U N D A N C E O F L E M M I N G S 289

1991). Thus, in spite of the late thaw and predation losses at the western edge of the range, the Dark-bellied Brent Goose had a good breeding year in 1991 and a poor one in 1992. A similar pattern was observed for Curlew Sandpipers. In autumn 1991.258 of 372 Curlew Sandpipers trapped on passage through the Vistula Mouth, Gulf of Gdansk, Poland, were first-year birds (69%); whereas, in 1992. there were only 16 first-year birds of 96 Curlew Sandpipers trapped (17%) (Gromadzka 1988, 1992, in litt.).

Weather and insect abundance

Newton (1977) reviewed factors influencing the breeding productivity of Arctic-nesting geese. First, there is a large dependence, during incubation, on body reserves accumu- lated in winter and spring, with limited opportunity for feeding until 2-3 weeks after the thaw. Second, inter-year variability of arrival dates is less than that of the thaw. If the thaw is delayed, both reserves are consumed and less energy is available for the production of eggs when the thaw even- tually arrives. Thus, failure of Brent Geese in the Canadian Arctic was interpreted as being due to the lateness of the thaw (Barry 1962). Similarly, the breeding success of Bar- nacle Geese Branta leucopsis was correlated with the timing of the thaw (Owen & Norderhaug 1977).

Unlike geese, which are grazers, waders eat invertebrates, which are available throughout the thaw (Fig. 3). and seed food (Nettleship 1974. Kistchmski 1982. Morrison & David- son 1990). We frequently observed waders feeding at this time, especially on areas of newly exposed tundra (as shown in Pienkowski & Green 1976: plate 20). Thus, waders are unlikely to be as dependent as geese on body reserves ac- cumulated before arrival, and body condition and timing of thaw may be less important in determining the breeding productivity of waders. Knots and Turnstones arrive on the breeding grounds with substantial fat and protein loads, and these are thought to have survival value if conditions on arrival are bad (Morrison & Davidson 1990, Evans 1991, Davidson & Morrison 1992). Davidson & Morrison (1992) speculated that these reserves may even influence breeding productivity, but there is no evidence for this at present. In extreme situations, if there are no snow-free areas by the end of June or late snowfalls cause nests to be abandoned, breeding fails for waders because there is too little time for the breeding cycle to be completed (Green et ul. 1977, Evans 1991).

Our observations of both good and poor breeding success for waders and Brent Geese occurred after late thaws. In the Arctic tundra subzone of the Taimyr Peninsula, the time of arrival of waders usually takes place about 1-2 weeks later than in the southern part of the Taimyr Peninsula (E.E. Syroechkovski. Jr. & H.G. Lappo, pers. obs.). Similarly, Brent Geese appear in the Arctic tundra 2-3 weeks later than Bean Geese Anserfabulis and White-fronted Geese A. albgrons appear in the southern tundra. In 1991. we found that waders and geese had arrived prior to 15 June. Despite the cold weather in both years (Fig. 3), the waders remained on the

largest available snow-free patches and nested there. The late thaws did not influence the initiation of breeding nor, in 1991, the clutch-sizes of the Brent Geese.

Holmes (1966) found that the hatching date of Dunlins in Alaska coincided with the emergence of adult Diptera and suggested that weather conditions at this critical time could s ihcan t ly alter the food supply for chicks and cause a food shortage. At Pronchishcheva Lake, the insect food supply in 1991 was sufficient for successful raising of the chicks in spite of a cold spell that appeared to depress insect availability a few days after many chicks hatched, a situation that did not arise in 1992. Further, the poor breeding success of waders at Pronchischcheva Lake in 1992 was due to losses of clutches rather than losses of chicks.

Interrelationships between lemmings, predators and high-Arctic breeding birds

The observations made in the Taimyr Peninsula in 1991 and 1992 show that the interrelationships of breeding productivity with lemming and predator abundances on the Taimyr Peninsula are complex. In particular, the Summers-Rose- laar hypothesis was based on the assumption that the crash in the lemming population takes place in autumn, after the peak of the birds’ breeding season (Summers 1986, Summers & Underhill 1987). Progress in understanding these relationships requires future fieldwork to quantiy lemming abundance throughout the breeding season. The qualitative observations used in earlier papers (lemmings ”high”, “me- dium”, “low” or “absent”) are too subjective to contribute much more to our knowledge.

The observations at the Pyasina River of predation on Brent Geese and waders by two lemming predators, Arctic foxes and Taimyr Gulls, during a period of decreasing lemming abundance in midsummer 1991 (and similar observations made in 1989 [Konratyev 19921) are pertinent to Model 2 of Summers & Underhill (1987). This model stated that it is not lemming abundance per se which is the critical factor determining predation levels, but the change in lemming abundance. However, this model referred to between- year changes, not within-year changes.

During a peak lemming year, foxes in Siberia are strongly territorial and breed successfully, but in the following year there is an increased number of non-territorial foxes (Syr- oechlovskiy et al. 1991). There are similar, but quantitative, results from the Canadian tundra, where McPherson (1969) found that 33% of 1-2-year-old foxes were reproductively active and the remainder were wandering. This may explain the marked change in fox behaviour between the 2 years, with a low frequency of fox sightings in the study area in 1991 and repeated sightings in 1992.

An additional insight into the prey-switching hypothesis is that, when lemming densities are high, almost all predators breed and raise their young almost exclusively on the lemmings found in the area immediately surrounding their den or nest. When lemming densities are low or falling, foxes (and probably other predators) are mostly non-territorial

I B I S 1 3 5 290 L . G . U N D E R H I L L E T A L .

and adopt a wide-ranging search strategy that leads to all areas of the tundra being visited regularly.

McPherson’s observations would result in large numbers of non-territorial foxes in the 2 years following a lemming peak, especially in the first year. Such foxes can cover great distances in search of food, so that fox predation could ra- diate to encompass a large area from a localized lemming peak which lasted until autumn. The movements and re- distribution of lemming predators after a lemming peak any- where in the Taimyr Peninsula may be sufficient to induce a general breeding failure in the following year. The presence of stoats at Lake Pronchishcheva in 1992 is of significance in this respect. Stoats were observed at Chelyuskin (77”41’N, 104”oS’E) in polar desert in summer 1989, the year following the previous lemming peak U.H. Mooij, pers. mmm. to H.Sch.). Thus, the Summers & Underhill (1987) prey-switching hypothesis may not depend on a synchronized peak in lemming abundance across the entire Taimyr Peninsula.

The prey-switching hypothesis as applied to Brent Geese had an additional twist. We found Brent Geese nesting close to Snowy Owls, which aggressively defend their territories and create a fox-exclusion zone around their nests. This has been described previously for the subspecies of Brent Geese Branta b. nigricans nesting on Wrangel Island in association with Snowy Owls and other birds of prey (Portenko 1972, Dorogoi 1990). Ellenberg & Dreiike (1992) coined the term “abrition” to describe the ecological relationship between at least three species, whereby one species offers indirect protection from a predator to other species, and also gave further examples of this phenomenon. Thus, if abrition is widespread for Brent Geese, if forms part of the explanation for their cyclic pattern of breeding productivity, because Snowy Owls only breed in years of peak lemming abundance, such as 1991. However, it does not explain the good breeding productivity of Dark-bellied Brent Geese in years such as 1990 (Kirby 1991) with few lemmings and presumably little breeding by Snowy Owls. Some Brent Geese ap- parently nest in isolation in all years, taking the chance of being more difficult to detect. Further fieldwork is required in order to determine how individual geese adapt to colonial, isolated or non-breeding strategies in different years, in ac- cordance with the lemming and predator conditions they discover on arrival.

Newton (1977) ascribed little importance to predation as a factor influencing breeding productivity of geese, although he noted that nest sites chosen by geese frequently give protection from mammalian predators. Jehl(l971) also dem- onstrated the importance of predation as an influential factor in the breeding success of Arctic birds. In the Taimyr Pen- insula, it appears to be the key factor.

Our fieldwork during two breeding seasons provided direct observational evidence in support of the correlational studies that lead to the development of the Roselaar-Summers prey- switching hypothesis. In particular, Model 2 of Summers & Underhill (1987). modified to include within-year changes of lemming abundance, was supported. Additionally, we suggest that, in the year following a lemming peak in which

lemming populations have declined or are declining, predation is intensified by (i) a change in the strategy of foxes and other lemming predators from being strongly territorial to being wide-ranging nomadic hunters, (ii) a movement of predators (both mammalian and avian) to areas which may not have experienced a lemming peak in the previous year and (iii) an abstention from breeding by Snowy Owls, de- priving the geese of the fox-exclusion zone around their nests.

We express our gratitude to Academician Professor E. E. Syroech- kovski and Dr Helena Rogacheva of the Institute of Evolutionary Morphology and Animal Ecology. Russian Academy of Sciences, for the invitations to participate in the International Arctic Expeditions of the institute, and to their staff. especially Dr N. Vronsky. Dr M. Sinitsyn and M. Zabelin for logistical support in difficult circum- stances. E. Pospelova, M. Orlov, D. Mil‘ko and F. Romanenko provided information on the geobotany and geomorphology of the study area. L. G. Yemelyanova and M. Gale were also involved in lemming studies. C. Berrevoets, P. Chylarecki. B. S. Ebbige. J. Gromadzka, W. Kania. A. Lidstrom, J. H. Mooij, G. T. de Roos, P. S. Tomkovich and P. Ybou contributed unpublished observations. L.G.U. ac- knowledges support from the Foundation for Research Develop- ment, Pretoria, and the University of Cape Town Research Com- mittee. R.P.-J. and R.W.S. participated under the joint scientific agreement between The Royal Society and then U.S.S.R. Academy of Sciences. The Royal Society for the Protection of Birds allowed R.W.S. special leave to take part in the work. Sir Herbert Bonar, Margaret Bett. British Airways Assisting Nature Conservation. Brit- ish Ecological Society. W. H. Brown, Iris Darnton Foundation and M. A. Johnston provided Financial and logistical support. N.M.G., H.Sch.. H.Sp. and M.v.R. acknowledge the Ministry of Agriculture. Nature Management and Fisheries, The Netherlands, and Bever Zwerfsport for Financial support. and G. C. Boere and B. S. Ebbinge for logistic support. R. E. Green and L. Jenni commented on drafts.

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Submitted 29 May 1992; revision accepted 31 December 1992

APPENDIX

Estimating the start of incubation with known rate of nest disappearance and uniform clutch sizes

Assume the daily rate of nest disappearance is p and that a complete clutch was found on day t. Let I be the maximum number of days of incubation on day f. If the nest is visited once only, then I is equal to the incubation period for the species. If the clutch is last seen on days t*, then I = t* - t . Then Pr(t - j ) , the probability that incubation started on day t - j , for j = 0, 1, . . . , I is given by

Pr(t - j ) = c(1 - p)*-f

where c-' = 2&,, (1 - p)". For nests which were found with incomplete clutches, we

assumed that one egg would be laid daily until the clutch would be complete and that incubation was equally likely to start on that day or the next.

These probabilities were computed for each nest, assuming an incubation period of 22 days, a clutch size of four and p = 0.20, the observed daily rate of nest disappearance in 1992. Probabilities for each cdendar day were cumulated and standardized to generate a probability function (Fig. 6).

Documents

Breeding of waders (Charadrii) and Brent Geese Branta bernicla bernicla at Pronchishcheva Lake, northeastern Taimyr, Russia, in a peak and a decreasing lemming year