IBIS 130: 153-163

Distribution and movements of Barrow's Goldeneye Bucephala islandica young in relation to food

A R N I EINARSSON University of Aberdeen, Culterty Field Station, Newburgh, Aberdeenshire

A B 4 OAA €9 Institute of Biology, University of Iceland, Grensasvegur 12, 108 Reykjavik, Iceland

Accepted 30 July 1987

Barrow's Goldeneye females with broods in the Lake Myvatn district, Iceland, concentrated in a small area in the lake's outlet where the density of blackfly Simulium oittatum larvae, the principal food of the young, was highest. A downstream migration of broods took place as predicted from the growth pattern and emergence of blackflies in the river.

In altricial land birds, the main factor limiting brood size and thus reproductive output is food density combined with the ability of the parents to gather food for the young (Lack 1947, 1948). For precocial birds, whose young feed themselves, the situation is different. Instead of bringing food to the young, the parent(s) take the young to the food. The growth and survival of precocial young may therefore depend heavily on their parents' ability to locate the best feeding areas and lead them there.

Waterfowl (Anatidae) broods often travel a considerable distance, overland or on water, from their nests to certain brood-rearing habitats (see Bengtson 1971). Siren (1952), for example, reported a movement of 2.1 km by a newly hatched brood of Common Goldeneye Bucephala clangula. Velvet Scoters Melanitta fusca have been found to move about 0.8 km (Koskimies 1957). Shelducks Tadorna tadorna moved up to 4.8 km (Hori 1969), and Eider Somateria mollissima broods between 3 and 14 km (Bedard & Munro 1977, Minot 1980).

Within the brood-rearing areas the broods appear to be fairly mobile. Newly hatched young of Red-breasted Merganser Mergus serrator and Goosander M . merganser moved 2-5 km per day within an area of about 4 km2 (Bergman 1956). Common Goldeneye broods move up to 1.5 km between adjacent ponds (Siritn 1952) and distances between 4.8 and 8 km travelled by Mallard Anasplatyrhynchos broods have been reported (Dzubin & Gollop 1972, see also Evans et al. 1952, Berg 1956, Beard 1964, Young 1967).

It has sometimes been assumed that movements of waterfowl broods reflect gradients or patchiness in food abundance (Bergman 1956, Mendall 1958, Beard 1964, Bengtson 1971), but apart from Eriksson's (1978) study on Common Goldeneyes, direct evidence is lacking.

In this paper I test the hypothesis that movements of Barrow's Goldeneye broods can be predicted by the dispersion of food.

Study area

The study area included Lake Mjrvatn and the upper stretches (about 1.5 km) of its effluent river, the Laxa. Lake Mjrvatn (65"35'N, 17"00'W, 37 km2, 277 m above sea

ARNI EINARSSON I B I S 1 3 0 I54 level) lies in the neovolcanic zone of northern Iceland. The lake is eutrophic and divided into three basins. Physical and chemical characteristics of Mjrvatn are described by 6lafsson (1979a,b) and other aspects of its limnology were summarized by J6nasson (1 979).

The River Lax6 flows out of Mjtvatn in three branches, Ystakvisl, Midkvisl and Sydstakvisl, here collectively named Kvislar. The river branches are about 100 m wide or less. They have numerous small islets. The current velocity is about 0.5 m/s in the broader stretches but reaches 1 m/s in the rapids. In slow flowing stretches the bottom substrate is mostly of sand and gravel but in the rapids it is coarse rubble or bare bedrock. The river banks are overgrown with grass and there is hardly any emergent vegetation.

The invertebrate fauna of the river is dominated by blackfly Simulium vittatum larvae. The overwintering generation of blackfly larvae uses boulders and stones for substrate. This generation emerges in early June. Later in June, when a new generation appears, the riverbed has normally become overgrown with green algae Cladophora cf. glomerata which are then used as a substrate by the larvae.

The production of Simulium in Laxa shows a downstream gradient (Gislason 1985). In 1977-78 the estimated production close to the outlet was 1231 g (wet weight)/m2/year. Four km downstream from the outlet it was 11 11 g, and 756 g 18 km further downstream. In 1978-79 the corresponding figures were 403,353 and 264 (Gislason 1985).

The Barrow's Goldeneye population in the Mjrvatn-Laxa area numbers about 2000 birds and is mostly confined to and resident in the area (Gardarsson 1978).

Methods

Ringing methoda Females were caught on the nest, usually in the last week of incubation. Each female was ringed with a metal ring with a serial number and plastic (PVC) colour rings. Individual colour combinations enabled identification through a 20-45 x telescope at up to 100 m, when the birds lifted their legs out of the water, which they did frequently.

In 1981-1983 147 females were caught and ringed on their nests. A total of 85 females from moulting flocks caught in funnel traps erected in shallow water was colour-ringed in 1981 and 1982.

Brood census methoda In 1981 the whole Kvislar area and Ytri Breida were censused every second or third day during the brood season (July-August) and the locations and sizes of all broods plotted on a map. In 1982 the work was concentrated on Midkvisl and censuses were carried out daily or every other day. Weekly censuses were made in other parts of the Kvislar area, and occasional visits were made to the lake and other parts of the river to assess the total number and distribution of young. All adult females with broods were checked for leg rings.

Females seen arriving with young on the river were followed and observed continuously until they had either settled or lost their broods. The term 'brood' is used here for any cohesive group of young.

Sampling of Simulium larvae In the brood season Cfadophoro with the attached Simulium larvae was sampled from the riverbed of Laxa at six stations (Fig. 1). This was done by wading halfway across the river, picking up tufts of Cladophora 4-6 times on the way, taking care not to alter the current, and thus to cause the larvae to fall off the algae. Each tuft of Cladophora was put into a plastic bag and deep-frozen the same day. In the laboratory samples were thawed and the larvae picked out and counted. The length of about 100 larvae, taken at random from each sample was measured to the nearest mm. The length distribution of larvae was then used to convert larval counta into volume by means of a regression equation Y = 0.006X3'3"7, where Y = volume

I 988 FOOD O F YOUNG BARROW’S GOLDENEYE I55

Figure 1 . The Laxa outlet area. M1-6 are sampling stations of blackfly larvae. See Figure 2 for location.

b 0 1 2 3km - Figure 2. Movements of brood-attending Barrow’s Goldeneye females ringed on nests (filled circles) outside the main brood-rearing habitat to brood-rearing sites (open circles) in 1981 and 1982. Main study area in frame.

A R N I EINARSSON I B I S 130 156

in mm3 and X = length in mm. The volume of a larva used in the regression analysis was estimated by taking its average diameter based on six measurements along the length of the animal. The volume of a cylinder was then used as an approximation to the larval volume. The total volume of each sample of larvae was then multiplied by 1.05 (Hynes 1961) in order to convert it into wet weight. Cladophora was dried at 70°C for three days and weighed. Simulium density is expressed as g larvae (wet weight) per g Cladophora (dry weight). This is a semiquantitative method and the figures can be taken as indices of food abundance.

Results

Numbers and movements of broods Most females took their broods to the Kvislar area and six of them were found to move across the lake (up to 9.5 km) to get there (Fig. 2). Two marked females with broods moved in an opposite direction (Fig. 2).

In 1981 and 1982 60% and 70%, respectively, of the young were seen on the river (Kvislar). In the period 18 July-19 August 1981,80 young were seen in Nordurvogar and the remainder (about 130) mainly along the eastern shore of the lake and off the western shore. A census on 6-10 August 1982 yielded 145 (32%) young outside the river, almost all in the southeast corner of the lake.

The total number of young on Midkvisl in 1982 increased linearly from zero on 3 July to a peak of about 500 in the last week of July (Fig. 3). In the first half of August the numbers declined again at a similar rate to that of the earlier increase. The total number of young on the whole Kvislar area in 1981 fluctuated in a similar manner as that on Midkvisl in 1982 (Fig. 3) but the total numbers were much lower in 1981.

Three females were seen leaving their young temporarily. All three flew downstream and out of sight of their young. Two of the females stayed away 20 and 109 minutes, respectively. The one female that stayed away for 109 minutes flew 550 m downstream, passing the territories of four other females. It fed most of the time and gradually worked its way back to its brood. Females with young retained full capability of flight until at least 20 August.

In both 1981 and 1982 there was a downstream movement of young on the river. In 1981 the earliest broods, or 82% of the total number of young, gathered just above the outlet and the other 18% on the uppermost part of Midkvisl. In the first week of

600-

500-

p 400-

's 300-

5 10 15 20 25 30 5 10 15 20 25 July August

Figure 3. Total numbers of Barrow's Goldeneye young on Midkvisl in 1982 (black dots) and on Kvislar in 1981 (rectangles).

1988 FOOD OF Y O U N G BARROW’S GOLDENEYE

2.0-

1.5- - c

2 E 0 c 5 1.0-

0.5 -

I57 2.4 -I

L

5 0.8 0.6 0.4 0.2

I24 T

408 466 111 ~ l l l l l l r , l l : l l l l l l l l l l l l l l l l l l l 1 1 1 1 1 1 1 1 1 1 1 1

&

10 15 20 25 30 5 10 15 July August

Figure 4. Distribution of Barrow’s Goldeneye young on Midkvisl in July and August 1982. Thin bars show total distribution. Black bars show the two middle quartiles. The line joins median positions of young. Sample sizes are also shown. All distributions are significantly ( P < 0.001, Kruskal-Wallis tests) different from the previous one unless otherwise indicated.

August, i.e., a month after the first broods arrived, these areas had been deserted and the young occupied areas farther downstream.

In 1982 this movement was followed in detail on Midkvisl. In contrast to the previous year no broods settled just above the outlet but all the females gathered on the uppermost 1.2 km of the river (Fig. 4). Later, the lower stretches of Midkvisl were also settled, but the highest densities of young were found 700-800 m

-F . -4

0’ - 1 l l 1 l l l l l l I I I I I I I I I I I I I I I I I I I I I I I I I I J I I I

10 20 301 10 July August

Figure 5 . Positions of individually marked Barrow’s Goldeneye females with young on Midkvisl(l982). Observed movements from the river to the lake are indicated with an x.

A R N I EINARSSON I B I S 130

downstream from the outlet (Fig. 4). At this time about 30-40 broods were seen on Midkvisl. This distribution remained stable until about 28 July when the uppermost stretches gradually became devoid of young (Fig. 4), and the total number of females on the river decreased. At the same time new areas farther downstream were colonized. On 15 August only five broods remained on Midkvisl, four of which were more than 1.9 km from the outlet (Fig. 4).

The contribution of individual movements to the general downstream migration of females was studied in detail on Midkvisl in 1982 (Fig. 5). Nineteen ringed females seen more than once apparently did not move their territories on Midkvisl. Seven females moved, but not until about 26 July. Two females moved in two stages (Fig. 5). The others moved in one step (0.5-1.7 km). The uppermost females were the first to move, those in the middle part staying about a week longer before moving. Three of the females which moved established territories in a previously unoccupied part of the river.

158

Distribution of Simulium larvae in time and space The biomass of Simulium wittatum larvae in Midkvisl declined through July and early August and decreased with distance from the outlet (Fig. 6, Table 1). In early July the largest biomass of Simulium larvae occurred at 0.65 km from the outlet (9.2 g ww larvae per g dw Cladophora). This was the maximum biomass found during the brood season. Other sampling sites in Midkvisl yielded 1.9-3.5 g Simulium per g Cladophora. In mid-July the largest biomass was still found on the same site but had declined to 4.2 g. The biomass on other sampling sites was 1.5-2.5 g. On 20 July the maximum biomass, 2.7 g, was found 1.4 km from the outlet and the other sites varied from 0.8 to 2.2 g. On 3-4 August the maximum biomass was 1.5 g on the lowermost station, 2.3 km from the outlet. At this time the biomass at the uppermost two stations had dropped to almost zero. On 17 August biomass was still negligible on the uppermost two stations and had become less than 1.0 g on the other stations.

The size distribution of Simulium larvae showed a similar systematic change with time and with distance from the outlet (Fig. 7, Table 1). The greatest change in larval

Distance from oullel

Figure 6. Biomass of Simulium larvae (g wet weight per g dry weight Cladophora) in Midkvisl o. time and distance from the outlet (1982).

1988 FOOD OF Y O U N G BARROW’S GOLDENEYE 159

Table 1. Results of statistical tests on ( a ) : downstreamgradients in Sirnulium biomass indices (F-tests) and size distribution (X2-tests) of larvae and on ( b ) : seasonal changes in Sirnuliurn biomass indices and size distribution of larvae; F-tests were performed on log-transformed data

d.f. P

(a) Date 8 July F 14.47 5, 26

X 2 113.21 15 14 July F 1.91 5, 18

X 2 261.11 30 20 July F 3.94 5, 19

X 2 198.98 30 3Aug F 35.39 5, 18

X 2 183.41 10 17Aug F 31.33 5, 18

9 330.41 5

M1 F 285.50 4, 17 X 2 472.15 12

M2 F 59.03 4, 16 X 2 425.15 12

M3 F 12.85 4, 17 X 2 209.25 12

M4 F 9.37 4, 16 203.58 12

M5 F 16.73 4. 17 X 2 163.37 12

M6 F 3.09 4, 16 X 2 193.72 8

(b) Station

< 0~0001 < 0~0001

NS < 0~0001 < 0.01 < 0~0001 < 0~0001 < 0~0001 < 0~0001 < 0~0001

< 0~000 1 < 0~0001 < 0~0001 < 0~0001 < 0~0001 < 0~0001 < 0.001 < 0~0001 < 0~0001 < 0~0001 < 0.05 < 0.0001

size and biomass occurred at the uppermost two sampling sites. From late July to middle of August the modal larval length dropped from 6-8 mm to less than 1 mm (Fig. 7). At the next station farther downstream (M3) the modal length of larvae dropped from 2-3 mm to less than 1 mm over the same period (Fig. 7). At the lowermost station, 2.3 km from the outlet, the modal length increased from 2-3 mm to 3-4 mm.

Food of young on the river The stomach contents of eight medium to large downy young collected on Sydstakvisl in early August 1983 showed that they had fed mainly on Simulium larvae and pupae. Between 5 5 and 100% of the items eaten were Simulium larvae and pupae. The remainder was mainly larvae of orthocladiin chironomids. The Simulium larvae were almost fully grown (88% of them were 7-9 mm long) and the larvae and pupae comprised 84-100% by volume of the stomach contents (Table 2).

Discussion

Most of the Barrow’s Goldeneye young were led by the females to the lake’s outlet. This ‘focal area’ had a high concentration of blackfly larvae, the main food of the ducklings there. The larvae belonged to a summer generation which hatched from

I 60 A R N I EINARSSON I B I S 1 3 0

Sampling stations:

MI M2 M3 M4 M5 M6

";: :p 7 p rryr J'u4 :p-J 7 p pu n-122 147 137 98 120 70

95 115 95 152 149 143

20-21 Jul

3-4 Aug

ib 7

84 Ll7 104 98 EP 130 75 111

5 7 PFP fp7~lvj ,

7 147 60 76 149 118 130

Figure 7. Size distribution (mm) of Simulium larvae on the Midkvisl sampling stations (Ml-6) on various dates in 1982. M1 is the uppermost station. For locations of sampling stations see Figure 1. Numbers on the leftmost histograms indicate every second boundary (in mm) between size classes.

Table 2. Stomach (prwentrinrlus) contents of Barrow's Goldeneye young from Kvislar 7 August 1982. Volume ojSimulium pupae was estimated equal to volume ojfully grown larvae. Volume ojrmall chironomid larvae estimated as 1.5 mm3

Mean s.e. n

Size of young (9) 190.8 22.06 8 Stomach contents (% of volume)

S. vittatum 92.91 2.36 783

Chironomidae 6.83 2.28 342

Other 0.26 0.16 18

(larvae & pupae)

(larvae)

eggs laid in early June. The uppermost part of the river has been shown (Gislason 1985) to be more productive than any parts of the lake investigated so far. The annual production of Simulium in Midkvisl (station M2) was about 75 g a.f. d.w./m2/yr (Gislason 1985) but the highest record for benthic invertebrate production on a sampling site in the lake is about 60 g/m2/yr (Lindegaard & Jonasson 1979). This

1988 FOOD O F YOUNG BARROW’S G O L D E N E Y E 1 6 1

Table 3. Distribution of Barrow’s Goldeneye young in the MJivatn area (Gardarsson 1978, A . Gardarsson and A . Einarsson unpublished data)

Number of young ( O h )

Year Dates

1976 21-24A~g 1977 6-16Aug 1978 25-27 Aug 1979 10-15 Aug 1980 7-11 Aug 1981 13-19Aug 1982 10-15 Aug 1983 1 1 Aug 1984 15-18Aug 1985 21-26 Aug 1986 10-12Aug Mean percent

River

678 (100) 525 (94) 221 (84) 350 (97) 326 (65) 163 (52) 308 (67) 191 (100) 736 (100) 900 (98) 800 (93)

86.4

Total

678 557 262 361 498 3 1 5 46 1 191 736 918 863

difference is reflected in about ten times more intensive utilization of the upper Laxa than Lake Mjrvatn by diving ducks (Gardarsson 1979).

Total censuses of Barrow’s Goldeneye young in the Mjwatn area in the period 1976-1985 (Table 3, see also Gardarsson 1978) indicate that the uppermost part of the river Laxa is normally the main brood-rearing area of this species. Selection of the richest feeding areas by newly hatched ducklings has also been reported in the Common Goldeneye (Eriksson 1978).

In the beginning of the brood season the river area nearest to the lake outlet had the highest abundance of larvae, most of which were full grown and ready to pupate and emerge. A short distance farther downstream the larvae were smaller, their total biomass smaller and they emerged later in the season or overwintered in the river.

The reason for this gradient in larval growth rate is probably a gradient in the suspended organic particles the larvae feed on, which are flushed from the lake (blafsson 1979b, Gislason, pers. comm.). This has been well documented in other rivers (see McCullough et al. 1979 and references therein) and is believed to result mainly from the filter-feeding of the blackfly larvae. The upstream larvae reduce the amount of food available for larvae further downstream.

Emergence of blackflies close to the outlet was the main reason for the decline in numbers of larvae there. There was a considerable drift of Cladophora in the river. The drift was not quantified, but because these algae were the main substrate for blackfly larvae it may have contributed to the depletion of larvae in the uppermost part of the river. Gislason 8z Jbhannsson (1985) estimated that in Midkvisl, 600 m from the outlet, about 0-003-0*006% of the blackfly population (estimated as 10 000- 690000 larvae per m2) was adrift at any instant. At the lower end of the brood rearing area any depletion due to drift or mortality may have been compensated for by colonization of larvae drifting from upstream stretches of the river. At the outlet, obviously, no such drift could compensate for the loss of larvae.

The Barrow’s Goldeneye young followed the pattern in the distribution of Simulium larvae. At the beginning of the brood season the young settled in the uppermost part of the river where food was most abundant. When the food situation there deteriorated below that which could be found farther downstream, at least

I 62 LRNI EINARSSON I B I S 1 3 0

some of the broods travelled downstream, as expected. Their downstream neigh- bours had no reason to leave their sites, because the food situation there was now the best in the river. The moving broods, therefore, had to pass a number of brood territories and establish new ones at the bottom of the brood-rearing area. This resulted in a leap-frog type of movement down the river.

Because of the rapidly changing food conditions, it is important for adult females to monitor the situation. It is perhaps significant that they do not moult their flight feathers while attending broods and were seen flying away from the young for short periods. Delayed moult of brood-attending females is a feature common to most ducks (Weller 1964, Klint 1982). Alison (1976) described ‘exploratory’ flights by female Long-tailed Ducks Clangula hyemalis before they took their broods to new lakes or ponds. Temporary desertion of broods has been observed in several other duck species (Bergman 1956, Beard 1964). See HQland (1983) for an alternative explanation.

Individual Barrow’s Goldeneye females with broods seem to move in response to two counteracting forces. They are attracted to the rich food supplies but repelled by agonistic behaviour of the other females present. Where exactly new broods settle in the brood-rearing habitat may depend on a series of stochastic events and probably cannot be predicted. The general timing, movements and distribution of Barrow’s Goldeneye young in the Mqvatn-Laxii area can, however, be predicted by the distribution and life cycle of the most abundant food, the Simulium larvae.

I wish to thank Mr Ivar H. Stefansson, Mrs Birna Bjornsdottir, Mr Einar fsfeldsson, Mrs Holmfridur Stefansdottir, Miss Audur fsfeldsdottir, Mr Eysteinn Sigurdsson, Mr Arni Gislason and Mrs fda Thorgeirsdottir for their hospitality at Lake Mjvatn. Messrs Gaukur Hjartarson, Kristjan Lilliendahl, Sverrir Thorstensen, Hjorleifur Sigurdarson, Kristinn H. Skarphedinsson, Dr Ian J. Patterson, Mrs Sigrun Jonsdottir and Miss Elin Einarsdottir assisted with the ringing operations. Mr Kristinn H. Skarphedinsson participated in the brood censuses. I thank Dr Ian J. Patterson and Professor Arnthor Gardarsson for their tireless effort in discussing this study and making the work successful. Financial aid was received from the Icelandic Science Foundation and the Icelandic Student Loan Fund. Finally 1 thank Mr Magnus Magnusson and my wife, Sigrun Jonsd6ttir, for their support. Dr Gisli Mar Gislason and Professor Arnthor Gardarsson read the manuscript and made many valuable suggestions.

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