Aquatic Ecology 38: 297–307, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Moulting diving ducks and their food supply

Arni Einarsson1,∗ and Arnthor Gardarsson2

1Myvatn Research Station, 660 Myvatn, Iceland; 2Institute of Biology, University of Iceland, 101 Reykjavik,Iceland ∗Author for correspondence (e-mail: arnie@hi.is)

Key words: Aythya fuligula, Aythya marila, Bucephala islandica, Chironomidae, Clangula hyemalis, Mergusserrator, Moult, Myvatn Simulium, Waterfowl

Abstract

Numbers of moulting diving ducks at Myvatn, north Iceland, were monitored over a period of 25 years; aquaticinsects (Chironomidae and Simuliidae), a major food resource, were monitored with window traps for 23 years.The response of the duck populations to changes in the food situation during the moulting period varied betweenspecies. Histrionicus histrionicus and Melanitta nigra invariably go to the sea to moult. Three species (Aythyafuligula, Clangula hyemalis and Mergus serrator) have shown long term variation in moult migration habits.Aythya marila and Bucephala islandica stay for moulting, their numbers supplemented by birds from elsewhere.A. marila is a generalist feeder and therefore at low risk of starving, the strategy of Bucephala islandica is tomove within the water system where the food situation tends to alternate between the lake and the river. Numbersof Bucephala islandica males moulting on Lake Myvatn were strongly and positively correlated with chironomidnumbers and those moulting on the river Laxá with Simulium vittatum, the main food resource there. We did notfind convincing evidence that numbers of moulting A. marila and C. hyemalis responded to variation in the foodsupply during the study period. This applied also to A. fuligula in the South Basin of Myvatn, but numbers inthe North Basin were positively associated with chironomid numbers. M. serrator moulting on Myvatn showednegative correlations with the chironomids, perhaps reflecting a negative association between chironomids and itsmain food, Gasterosteus aculeatus. Apart from safety considerations for a flightless bird, the choice of a moultingsite is apparently influenced by the local food conditions on one hand and by the opportunities and risks involvedin migrating to distant moulting sites with an unknown food situation on the other hand.

Introduction

Moulting, or the periodical renewal of plumage inbirds, takes place during certain times of the year. Re-generation of feathers is costly in terms of energy andnutrients and the moulting birds may respond by redu-cing their locomotor activity, by storage of nutrientsused for feather growth, by foraging selectively forspecific nutrients or by seeking out rich feeding habit-ats (Murphy 1996; Fox and Kahlert 2003). The timingand sequence of feather replacement, or the moultcycle, varies with bird taxa. Waterfowl (Anatidae)moult their flight feathers once a year, in late summerafter breeding, and many of them migrate to com-munal moulting sites, often far away (102–103 km)

from the breeding grounds (Salomonsen 1968). Be-cause waterfowl lose all their flight feathers simultan-eously and remain flightless for about a month, safetyfrom predators undoubtedly plays an important role inthe choice of a moulting site. Although the security ofmoulting sites has apparently not been demonstratedby observation or experiment, Fox and Kahlert (2000)showed that flightless moulting greylag geese Anseranser (L.) stayed close to the water’s edge despite thebetter grazing grounds further landwards. Before los-ing their flight feathers, the geese used inland sites,suggesting that predator avoidance during the flight-less period constrained their movements. Apart fromsafety from predation, the main requirement of amoulting site is sufficient food (Salomonsen 1968).

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Fidelity to moulting sites has been shown by differentworkers (Hohman et al. 1992; Flint et al. 2000). Also,long-term tradition (sensu Hochbaum 1955) wouldseem to be of potential value in the choice of a moult-ing site, though this is hard to confirm scientifically.Annual variation in food abundance constitutes a riskfor flightless birds. Moulting sites should preferablyhave a reasonably stable food resource, and it seemsadvantageous for moulting waterfowl to assess foodprospects before becoming flightless. Observations in-dicating a relationship between food availability anddensity of moulting birds seem, however, to be lacking(but see Fox and Kahlert 2003).

In this paper we present data on the numbers andlocal distribution of moulting ducks on Lake Myvatnand its outlet river, the Laxá, gathered in a monitoringstudy over a period of 25 years, 1975–1999. Breed-ing numbers and the production of young were alsomonitored. Aquatic insects, a major food source, weremonitored for 23 years, 1977–1999, using windowtraps on the lake and the river. The monitoring thatis still continuing has been extended to include fishand crustaceans, and we expect that more informationrelating to this study will become available in the nearfuture.

Here we focus on numbers of moulting divingducks in relation to aquatic insects. The large quasi-cyclic changes in the insect populations in LakeMyvatn and the River Laxá (Gardarsson et al. 2004)have enabled us to estimate the importance of foodresources to the five diving duck species that moultregularly in the area. The deviation of moulting num-bers from the numbers breeding in the area each yearis of interest, i.e., if local breeders stay to moult in thearea or leave before the moult, and if there is furtherseasonal immigration for moulting. We address thequestion whether the difference in numbers of ducksbreeding and moulting at Lake Myvatn annually isrelated to the food supply, which might explain themoult migration strategy. Studies in this regard showthat the numbers of aquatic insects and the productionand numbers of several duck species are positivelycorrelated (Gardarsson and Einarsson 1994, 1997a,2004b). An earlier study of the numbers of divingducks moulting at Lake Myvatn, based on the first13 years, and using Chironomidae collectively as anindependent variable, showed only a weak relation-ship in one case (Gardarsson and Einarsson 1994). Thepresent analysis is based on much longer time-series(23 years) and species determination of the Chironom-idae. We examine the total numbers of moulting birds

Figure 1. Lake Myvatn, Iceland, showing place names mentionedin the text and the location of window traps.

in the area, and for two diving duck species, tuftedduck Aythya fuligula (L.), and Barrow’s goldeneye Bu-cephala islandica (Gmelin), we inquire how variationin the local distribution is related to the food situation.

Methods

Study area

The main study area is the shallow eutrophic lakeMyvatn and the upper parts of its outlet river, theLaxá, in north Iceland (65◦40′ N, 17◦00′ W). Forinformation on other moulting areas in Iceland, wealso occasionally surveyed other fresh waters andcoastal marine waters, and assessed the distributionand numbers of moulting waterfowl (Gardarsson 1979and unpublished). Detailed descriptions of the Myvatnecosystem are given by, for instance, Jónasson (1979),Gardarsson and Einarsson (1994, 2000) and Gíslason(1994).

Water enters Lake Myvatn mainly by subter-ranean flow, originating from a volcanic area of about1400 km2. The study area of permanent surface watersis about 50 km2, of which Lake Myvatn is 37 km2

and other lakes and permanent ponds about 9 km2.The river Laxá is divided into 3 main sections ofwhich the upper two (3.5 km2) were surveyed annu-

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ally. Lake Myvatn has a long indented shoreline withmany islands, and is often divided into natural sub-areas, the South and North Basins (Figure 1, see alsoGardarsson, 1979). The South Basin, 29 km2, has amean depth of 2.3 m and a maximum of 4.2 m. Mostof the area below the 2.5-m contour is covered byfilamentous chlorophytes of the order Cladophorales:Cladophora glomerata (L.) Kütz. and Aegagropila lin-naei Kütz. The algae are an important food of adultscaup Aythya marila (L.), and also form a substratewith which important epifaunal invertebrates are as-sociated, notably the chydorid Eurycercus lamellatus(Müller), the orthocladiin chironomid Psectrocladiusbarbimanus (Edwards) and Orthocladius consobrinus(Holmgren), and the chironomin Chironomus island-icus Kieffer. The infauna, as in much of Myvatn, isdominated by chironomid larvae, mainly Tanytarsusgracilentus (Holmgren).

The eastern part (Bolir, 8.5 km2) of the SouthBasin is cut off from the main part by a row of largeislands. Much of the area is influenced by cold springsthat line the southern and eastern shorelines. Thesouth part of Bolir is dominated by the macrophytesRanunculus trichophyllus Chaix and Myriophyllumalterniflorum DC. The north part of Bolir forms atransition between the Cladophora community of theSouth Basin proper and the Potamogeton filiformisPers. stands of the North Basin. The spring-dominatedlittoral parts of Bolir are one of the two main habitatsof Barrow’s goldeneye at all times of the year. Theyappear to feed extensively on tanytarsin chironomids,including both T. gracilentus and Micropsectra spp.(unpublished data).

The North Basin, 8 km2, is mostly very shallow(mean depth 1.05 m) although diatomite mining oper-ations which began in 1967, have resulted in a deeperarea of some 3.5 km2. The North Basin is character-ized by large stands of P. filiformis and Myriophyllumspicatum L. Warm springs (7◦–26 ◦C) enter the NorthBasin from the east. Three-spined sticklebacks (Gas-terosteus aculeatus L.) are usually more abundant inthe North Basin than elsewhere in Myvatn.

A large number of ponds and small lakes are foundin the Myvatn basin of which Lake Sandvatn (3.6 km2)is the largest, and the only one, in adition to LakeMyvatn, where moulting drakes, mainly scaup andtufted duck, are sometimes found. As in many of thesmall waterbodies, the main macrophyte of Sandvatnis Myriophyllum alterniflorum.

The uppermost part of the Laxá river (river sec-tion LM, 1.5 km2, Figure 1) at first forms a wide,

slowly flowing pool which then forms three rapidlyflowing main channels that again join to form a singlemain channel with numerous islands, eventually flow-ing from the Myvatn area into the Laxá valley. Thechannels support high densities of Simulium vittatumZett. (blackfly) larvae, which contribute about two-thirds of the total benthic production of this part ofthe river, usually between 40 and 150 g ash-free dryweight m−2 year−1 and exceptionally reaching 900 g(Gíslason 1994). In the headwater channels, Simuliumemerges twice annually, in June and August. Furtherdownstream, Simulium densities and production de-crease, and about 5 km downstream from the lake,only a single annual generation of Simulium is pro-duced. The uppermost reaches of the River Laxá areof exceptional importance to the Barrow’s goldeneyeat all times of the year. In the Laxá valley (river sectionLL, 2.2 km2), benthic densities are lower than at theoutlet.

Censuses of ducks

The programme of annual censuses was begun in 1975and is an ongoing project. Ducks in the Myvatn-Laxáarea are monitored by annual censuses conducted byexperienced observers, to estimate breeding numbersof each species in spring, production of several speciesin mid-summer (July and August) and the numbers ofadult diving ducks moulting in the area. For detaileddescriptions of census methods see Gardarsson (1979)and Gardarsson and Einarsson (1994).

Moulting males, an overview

Eight species of diving ducks (tribes Aythiini andMergini) breed regularly in the Myvatn area. Theirnumbers vary from a few (goosander, Mergus mer-ganser L.) to up to some thousands, the most abundantbeing tufted duck (average about 4000 males) andscaup (about 2000 males). Two common breeding spe-cies, the common scoter, Melanitta nigra (L.), and theharlequin duck, Histrionicus histrionicus (L.), moveto the sea before the moult. The scoter moults in west-European waters and the harlequin on the Icelandiccoast.

Five diving duck species moult in the Myvatn area.The South Basin of Myvatn holds the main populationof moulting scaup in Iceland, small numbers some-times moult on nearby Lake Sandvatn. From 1000to 2000 scaup have been found moulting in Icelandoutside Myvatn, mostly on shallow lakes (area, 3 to9 km2). About 2500 tufted ducks, 60% of the spring

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total, moult on Myvatn which seems to be the onlyimportant moulting locality in Iceland. We presumemost of the tufted ducks emigrating before moult go tolakes abroad, perhaps in Ireland where Lough Neaghis known to support a large population of moultingtufted ducks (Hutchinson 1989). The long-tailed duckClangula hyemalis (L.) moulted in large numbers onLake Myvatn in 1960 (Gardarsson 1991), and pre-sumably in earlier times, but only small numbers havedone so from 1975 on. Small groups have been foundmoulting, both on the sea and on lakes. Moulting ofBarrow’s goldeneye is restricted to Myvatn and the up-permost Laxá, although a few birds have been reportedmoulting on other freshwaters. Moulting red-breastedmergansers are numerous in certain coastal localitiesand on a few inland lakes, including Myvatn, wheretheir numbers tend to vary considerably.

Food habits of the ducks

The food habits of diving ducks at Myvatn have beendescribed by Bengtson (1971), Gardarsson (1979,1991) and Gardarsson and Einarsson (2002). Asbackground information, we used the food composi-tion in stomachs (oesophagus-proventriculus) of adultdiving ducks collected in summer (June–September)at Myvatn over several years, mainly from fishing nets(Gardarsson and Einarsson 2002).

In Lake Myvatn, red-breasted mergansers fed ex-clusively on three-spined sticklebacks, the other duckspecies fed mainly on chironomid larvae and thechydorid Eurycercus lamellatus. They also fed onmolluscs (Radix peregra (Müller) and Pisidium cf.casertanum (Poli)) as well as plant material, espe-cially Cladophorales and Potamogeton filiformis. Theducks also feed on numerous emerging chironomids;however, this is not evident from the stomach con-tents. Scaup ate significantly more vegetation (nearlyall Cladophorales) than other species. Scaup, tuftedduck and common scoter consumed more molluscsthan long-tailed duck and Barrow’s goldeneye. Tuftedduck and Barrow’s goldeneye fed significantly moreon insects than the other species, mainly chironomidlarvae (Gardarsson and Einarsson 2002). In the RiverLaxá, Barrow’s goldeneyes fed extensively on blackflylarvae and pupae (unpublished data).

Food availability

Aquatic insects, mainly Chironomidae (midges) andSimuliidae (blackflies) have been monitored in the

Myvatn area since 1977, using window traps, perman-ently located at four sites, which catch flying insects(Jónsson et al. 1986; Gardarsson et al. 1995). Thetraps are in use from May to September. We use thegeometric averages of two window traps to expressthe abundance of insects at the two main subdivisionsof the study area, traps KS and SN for Lake Myvatn,traps DR and HE for the upper Laxá (Figure 1).

Catches of chironomids and simuliids are identi-fied to species and are separated into two periods:before 20 July (period A) and after 20 July (periodB), corresponding to two emergences of many midgespecies and distinguishing between possible effects onadults and young ducks. For species that emerge onlyonce a year we use annual sums (S).

Data handling

This analysis is based mainly on correlating relativenumbers of moulting ducks with indices of the abund-ance of their food resources. The relative number ofmoulting ducks each year is expressed as the log pro-portional change, log (Nm/Nt ), from the spring census(Nt – usually in May) until the moult census (Nm

– late July or early August). Only males were con-sidered, except for red-breasted merganser becauseof difficulties in assigning correct sex to moulters. Ifmoulting males occupied clearly demarkated subareaswe examined the number of moulters on this subarealbasis: Barrow’s goldeneye on Lake Myvatn and theuppermost River Laxá, tufted duck on the two mainbasins of Lake Myvatn.

For comparison with the relative numbers ofmoulting ducks, the numbers (log n + 1) of the morecommon chironomid midge species and Simulium vit-tatum caught during different flight periods (A, B andS) are used, periods A and B for species with twoannual flight periods, and period S for those with asingle flight period. The correlations presented com-pare insect catches and duck censuses made in thesame year.

Results

Numbers of aquatic insects

Annual abundance indices of the main aquatic insectsare summarised in Figure 2. The most important insectof the Laxá river is Simulium vittatum. This speciesfluctuated in the early years of the study, decreased in1977–1979, and then peaked in 1984–1985. The dom-inant insect in Lake Myvatn, Tanytarsus gracilentus,

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Figure 2. Abundance of aquatic insects as log numbers caught in window traps at Laxa and Myvatn each year in 1977–1999.

was on the average about 70% of the catches in win-dow traps KS and SN on the lakeshore. It oscillated,with high numbers in 1978–1982, 1986–1987 and1991–1995, and low numbers in 1983–1985, 1988–1989 and 1997–1998 (Figure 2; Einarsson et al. 2002).Larvae of Tanytarsus are important in moulding thesediment surface of Myvatn (Gardarsson and Snorra-son 1993) but apparently of little importance as duckfood. A number of other chironomid species fluctuatedin step with Tanytarsus (Gardarsson et al. 1995, 2004),including Chironomus islandicus, Procladius island-icus (Goetghebuer), Orthocladius consobrinus andPsectrocladius barbimanus, all relatively abundantspecies commonly recorded as food and often showingcorrelations with population variables of the ducks.Cricotopus tibialis (Meigen) and Cricotopus sylvestris(Fabricius), both of which are abundant, show weak ornon-significant correlations with Tanytarsus and ducksat Myvatn (Gardarsson et al. 2004).

Numbers moulting

During 1975–1999, the mean total of moulting malescaup was 2429±147 (SE), ranging from 1140 (1975)to 3893 (1985) (Figure 3). The numbers of scaupmoulting were somewhat higher than spring numbersin most years, mean 1.31 ± 0.07, range 0.82–2.30,as a proportion of spring numbers. The lowest pro-

portions, 0.82 in 1975 and 0.86 in 1990, are hardlyoutside the range of observation error. High propor-tions occurred for instance in 1980, 1985–1987 and1993. Scaup moulted mostly on the more exposedwaters of the South Basin, mean 2354 ± 145, range1135–3878; small and variable numbers were foundelsewhere, mainly on the North Basin (26 ± 9, range0–163) and Lake Sandvatn (mean 12 ± 9, up to 219in 1982). The total moulting outside the South Basinaveraged 49 ± 15, or about 2% of the total numbers ofmoulting scaup in the area.

The mean total number of moulting male tuftedduck (Figure 3) was 2484 ± 166, range 864 (in1990) to 4215 (in 1982). The numbers moulting weremarkedly lower than spring numbers, mean proportion0.61 ± 0.05, range 0.24–1.13. In the first 4 years ofstudy, 1975–1978, moulting numbers were almost thesame as spring numbers, ranging as a proportion from0.90 to 1.12, but in 20 years after that the proportionaveraged 0.52 ± 0.04 and did not exceed 0.82.

Tufted ducks mainly moulted in mixed flocks withthe scaup on the South Basin, where their numbers av-eraged 1760 ± 200, range 65–3640; the proportion ofmoulters on the South Basin was 0.73 ± 0.06, range0.02–1.00. They also moulted regularly and some-times in large numbers on the North Basin, mean509 ± 167, range 0–3252, proportion 0.19 ± 0.05,

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Figure 3. Numbers of ducks breeding and moulting in the Myvatn study area and its subregions, 1975–1999. Males only, except red-breastedmerganser, males and females.

range 0–0.77. An average of 215 ± 50 (range 0–548)tufted ducks moulted on other waterbodies, of whichSandvatn was the most important, corresponding to amean proportion of 0.08 ± 0.02, range 0–0.32.

In most years during 1975–1999, only a few long-tailed ducks (Figure 3) moulted in the Myvatn area,nearly all on the South Basin, except in 1994 when 28of 48 were found on the North Basin. The mean totalof moulters was 39 ± 8, range 0–204, and the mean

proportion of moulting birds (as compared with springnumbers) was 0.24±0.05, range 0–1.04. Only in 1979did the moulting total (204) resemble the number ofbreeders; in 1993 the proportion was 0.72 or a total of114 moulters.

The Myvatn study area is practically the onlymoulting site for the Icelandic population of Barrow’sgoldeneye (Figure 3). The mean total of moultingmales was 976 ± 46, range 601–1402. As a proportion

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Table 1. Pearson’s correlation coefficients of log relative numbers, log(Nm/Nt ), of moulting ducks vs. log-numbers of chironomidscaught in flytraps at Lake Myvatn in 23 years, 1977–1999. The numbers of moulting ducks are total males on Lake Myvatn, exceptMergus serrator where both sexes are included. For Aythya fuligula, numbers moulting on the South and North Basins are shownseparately. Insect catches are divided into two periods, before (A) and after (B) 20 July each year, S denotes whole year’s catch.

Duck species A. marila A. fuligula A. fuligula A. fuligula C. hyemalis B. islandica M. serrator

Moulting site Myvatn Myvatn S. Basin N. Basin Myvatn Myvatn Myvatn

Chironomid species

Tanytarsus gracilentus A 0.04 −0.04 −0.36 0.56∗∗ 0.44∗ 0.79∗∗∗∗ −0.41

Tanytarsus gracilentus B 0.30 0.16 −0.23 0.64∗∗ 0.24 0.88∗∗∗∗ −0.44∗Cricotopus tibialis A 0.35 0.07 0.11 0.15 −0.02 0.46∗ −0.20

Cricotopus tibialis B 0.45∗ 0.16 0.13 0.28 −0.11 0.48∗ 0.00

Cricotopus sylvestris S 0.29 0.50∗ 0.17 −0.06 −0.25 0.04 −0.06

Orthocladius consobrinus A 0.03 0.13 −0.32 0.58∗∗ 0.32 0.73∗∗∗∗ −0.57∗∗Orthocladius consobrinus B 0.27 0.37 0.07 0.46∗ 0.13 0.48∗ −0.49

Micropsectra lindrothi A 0.33 0.23 0.14 0.35 0.37 0.52∗∗ −0.21

Micropsectra lindrothi B 0.42∗ 0.30 0.30 0.23 −0.11 0.42∗ −0.11

Chironomus spp. S −0.20 −0.23 −0.41 0.35 0.14 0.60∗∗ −0.43∗Procladius islandicus S 0.08 0.06 −0.27 0.67∗∗∗ 0.32 0.88∗∗∗∗ −0.46∗Psectrocladius barbimanus S 0.04 0.01 −0.51∗ 0.56∗∗ 0.11 0.77∗∗∗∗ −0.55∗∗Total Chironomidae males A 0.27 0.18 0.06 0.43∗ 0.32 0.68∗∗∗ −0.18

Total Chironomidae males B 0.34 0.30 −0.09 0.37 0.25 0.76∗∗∗∗ −0.38

Total Chironomidae males S 0.16 0.19 −0.18 0.61∗∗ 0.43∗ 0.86∗∗∗∗ −0.40

∗P < 0.05, ∗∗P < 0.01, ∗∗∗P < 0.001, ∗∗∗∗P < 0.0001.

of the numbers of adult and yearling males counted inspring (Myvatn study area plus the Laxá valley, n = 24years), this amounts to 1.19 ± 0.04, range 0.61–1.55.The lowest proportion, 0.61, occurred in 1983; in 1995we observed 0.90 of the number expected from thespring census, but in all other years the proportion wasabove 1.0.

As at other times of the year, Barrow’s goldeneyesoccupied Lake Myvatn and the upper Laxá in vary-ing proportions. On the lake the mean proportionwas 0.59 ± 0.06, range 0.04–1.00; nearly all moult-ing males were found on the east shores (Bolir) andalong the southwest shores, small numbers were some-times observed on the North Basin, just north of itsboundary with the South Basin, and exceptionallyon the spring fed Lake Graenavatn south-east of theSouth Basin. The highest proportions of Barrow’sgoldeneyes moulting on Myvatn occurred in 1979–1982, 1987 and 1990–1996. On the Laxá, the meanproportion was 0.41 ± 0.06, range 0–0.96; the moult-ing males were especially abundant on slow flowingpools. Proportionally high numbers moulted on theriver in 1976, 1983–1985, 1988–1989 and 1997.

Because of difficulties in identifying the sexesduring moult, data for both sexes of red-breasted mer-ganser are pooled (Figure 3). A mean of 405 ± 42

mergansers moulted in the area, with a range of 34–850. As a proportion of the spring total this represents0.56±0.06, range 0.07–1.23. The total number moult-ing exceeded the estimated spring total in five years,1975, 1976, 1988, 1997 and 1998. On the average 76%of the total moulted on the South Basin, 21% on theNorth Basin and the rest on other waters in the vicin-ity. However, the large moulting flocks (mainly males)were mostly on the South Basin; the North Basin wasused mainly by females and broods.

The relationship between moulting numbers and thefood supply

The relationships between the abundance of the maininsect species and the relative numbers of moultingducks are summarized as Pearson’s correlation coef-ficients in Tables 1 and 2, and scattergrams of someof the more significant associations are shown in Fig-ure 4. The spring numbers of ducks at Myvatn areknown to be related to feeding conditions in the previ-ous summer (Gardarsson and Einarsson 1994, 1997a,2004). We found no significant relationship betweenmoulting birds and their feeding conditions in theprevious summer (not shown in Table 1), nor anyindication that relative moulting numbers were cor-

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related with spring numbers (correlated coefficients oflog(Nm/Nt ) on log Nt were not statistically significantexcept for tufted duck, r = −0.41, P = 0.045).

We did not find convincing evidence at Myvatn thatrelative numbers of moulting scaup and long-tailedduck responded to the food supply, although a few re-lationships are significant at the 5% level (Table 1) andthe same can be said of tufted duck on the South Basin.However, numbers of tufted duck on the North Basinwere clearly associated with chironomid numbers, es-pecially Procladius islandicus (r = 0.67, P < 0.001),Psectrocladius barbimanus (r = 0.56, P < 0.01),Tanytarsus gracilentus in early summer (period A,r = 0.56, P < 0.01) and late summer (period B,r = 0.64, P < 0.01), and Orthocladius consobrinusin early summer (r = 0.58, P < 0.01).

The relative numbers of red-breasted mergansermoulting on Myvatn were negatively correlated withall chironomids (Table 1): the strongest negative asso-ciations recorded were with Orthocladius consobrinusin early summer (r = −0.57, P < 0.01) andPsectrocladius barbimanus (r = −0.55, P < 0.01).

Barrow’s goldeneyes moulting on Lake Myvatnshowed positive correlations with all the abundantchironomid species there, except Cricotopus sylvestris(Table 1). The relationship was particularly strong(P < 0.0001) with Procladius (r = 0.88), Ortho-cladius consobrinus in early summer (r = 0.73),Psectrocladius barbimanus (r = 0.77) and Tanytarsusgracilentus in early (r = 0.79) and late summer(r = 0.88). A highly significant, negative associ-ation was found with Simulium vittatum (r = −0.76,P < 0.0001). On the Laxá, the situation was re-versed (Table 2), the Barrow’s goldeneye showed astrong positive relationship with Simulium vittatum(r = 0.70, P < 0.001) and negative associationswith chironomids of the lake, particularly Psectro-cladius barbimanus (r = −0.83, P < 0.0001),Procladius (r = −0.72, P < 0.001), Tanytarsus inlate summer (r = −0.73, P < 0.001) and Ortho-cladius consobrinus in early summer (r = −0.66,P < 0.001).

Discussion

In his pioneering review, Salomonsen (1968) stressedthat safety and an adequate food supply should mainlydetermine the choice of moulting sites by waterfowl.Postbreeding waterfowl can choose several potentialmoulting sites that differ in safety, food resources and

Table 2. Pearson’s correlation coefficients of logrelative numbers of moulting Bucephala island-ica (males) on the River Laxa and Lake Myvatnvs. numbers of simuliids and chironomids caughtin window traps on the River Laxa in 23 years,1977–1999. Change in numbers of moulting ducksis expressed as log proportion of spring total,log(Nm/Nt ). Insect catches are divided into twoperiods, before (A) and after (B) 20 July each year,S denotes whole year’s catch.

Bucephala islandica

Laxa Myvatn

Insects:

Simulium vittatum A 0.25 −0.18

Simulium vittatum B 0.70∗∗∗ −0.76∗∗∗∗Simulium vittatum S 0.63∗∗ −0.67∗∗∗Total Chironomids A 0.00 0.00

Total Chironomids B 0.20 −0.32

Total Chironomids S 0.06 −0.06

∗∗P < 0.01, ∗∗∗P < 0.001, ∗∗∗∗P < 0.0001.

distance from the breeding site. One may assume thatthe levels of protection from predators and food re-sources at a remote moulting site have to outweigh thetravel costs to and from the site. Such costs involvenot only energy expenditure but also risk; as distanceincreases less information on the available food situta-tion is perceived by the bird. A bird has a choice to stayin a familiar area or leave for a remote site with poorlyknown food prospects. Evolutionary constraints, suchas the seabound moulting of the harlequin duck andcommon scoter, may play a role.

Given the large variation in the food resources ofLake Myvatn and its overriding regional importanceas a productive breeding area, a relationship betweenthe food situation and the influx/outflux of moultersis to be expected. That the relative moulting numberswere neither related to food resources in the year be-fore nor to densities in the breeding season suggest thatthe determination of moulting numbers is independentof the food-related processes determining variation innumbers of breeding birds (Gardarsson and Einarsson,1994, 1997a, b, 2004). Gardarsson and Einarsson(1997a) showed that the number of chironomids in thepreceding autumn was the best predictor of spring re-turn rates of a breeding population of wigeon (Anaspenelope L.). If the numbers of moulting birds aredetermined neither by the number of breeders northe past food situation one would assume that thebirds used current information on the food situationto determine their moulting site.

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Figure 4. Scattergrams of some of the more significant associations between relative numbers of moulting ducks and the abundance of aquaticinsects at Myvatn and Laxa.

The clearest response to varying food abundanceis shown by the Barrow’s goldeneye. This populationis very isolated and is almost restricted to our studyarea both for breeding, moulting and, to a lesser de-gree, overwintering (Gardarsson, 1978). The Barrow’sgoldeneye divides its dispersion between the lake andthe river, feeding largely on chironomids in the lakeand simuliids in the river. The positive and highlysignificant correlations between aquatic insects andthe numbers of Barrow’s goldeneye suggest a directfeeding relationship. Negative relationships shown bythe Barrow’s goldeneye are less easy to understand,and do not indicate a direct, cause-and-effect, feedingrelationship. Instead, the negative correlations havetheir origin in an inverse biotic relationship in the lakewhere years of low secondary productivity (periodsof low chironomid numbers) coincide with years ofcyanobacterial blooms, Anabaena flos-aquae, whichserve as food for Simulium in the river (Gíslason,1994).

The short distance (< 10 km) movements ofmainly local breeding birds to their moulting sitesseem to reflect a direct, immediate response to spatio-temporal variation in resource availability. For thelong-distance migrants, there is a striking absence of aresponse to food availability at Myvatn. Long-distance

migrants should moult in localities with stable andpredictable food resources, although within such a loc-ality, the fine-tuning to the resource would presumablyproceed by a process identical to the local responsedescribed here. Habitat diversity within a moultinglocality thus is important in maintaining predictabil-ity and reducing the risk associated with long-distancemoult migration. Provided there is little year to yearvariation in safety (predation, disturbance) over thestudy period, variation in moulting numbers at Myvatnshould be determined by variable resources, i.e., food.Perhaps we are not measuring the appropriate resourceor perhaps variation in resources at alternative, dis-tant moulting sites is more important than we haveassumed. We have shown elsewhere (Gardarsson andEinarsson 1994, 1997a, 2004) that changes in numbersof breeding ducks are related to the resources availableon the breeding grounds at departure in the previ-ous autumn. Similar transfer of information betweeenyears could also be of importance in long-distancemoult migration.

For the scaup, stomach contents suggest thatinsects are of relatively little importance, and thesimplest explanation would be that adult scaup areindeed unspecialised and are in any case not depend-ent on chironomids to meet their protein or other

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dietary requirements for plumage growth. For themainly herbivorous canvasback, Aythya valisineria(Wilson), Thompson and Drobney (1997) concludedthat changes in diet composition of moulting maleswere related to trends in food availability rather thanspecific nutrient requirements for the moult. This typeof explanation is difficult to apply to the tufted duckat Myvatn, where birds in the South Basin behaved‘unselectively’ like the scaup and were not correlatedwith chironomid levels, and could yet track the supplyof chironomids in the nearby North Basin. The scaupfeeds on a mixed diet, to a large extent on bottomvegetation, which enables it to moult regularly on itsnorthern breeding grounds, seemingly without regardto a variable food situation.

The moulting dispersal shown by tufted duck andlong-tailed duck suggests that food may be import-ant. In 1960, moulting long-tailed ducks at Myvatnfed mainly on Eurycercus lamellatus which becamea scarce food in the late 1960s (Gardarsson 1979) andhas remained so since then. Although the few long-tailed ducks moulting at Myvatn in recent decades failto show a clear-cut association with the availability ofaquatic insects, this may simply indicate low prefer-ence for this food and does not invalidate the potentialimportance of crustaceans which were not monitoredover the whole period. It is also likely that the numbersof long-tailed duck moulting at Myvatn are too low tobe of use in a study based on broad scale monitoring.

In the first four years of this study, the numbers ofmoulting tufted ducks were about the same as springnumbers, suggesting that local breeders remained atMyvatn to moult. After 1978, the numbers of moultingmales at Myvatn have remained low relative to breed-ing season numbers. Surveys revealed that only smallnumbers of tufted duck moulted elsewhere in Ice-land and the large numbers of males leaving Myvatnbefore the moult are presumably emigrating to moult-ing localities abroad (Gardarsson, unpublished data).As a total, the tufted ducks remaining to moult donot show close associations with the chironomids inthe Myvatn area. However, for the North Basin, cor-relations between moulters and several chironomidspecies are highly significant, suggesting that there isa direct trophic relationship. In the South Basin, thecorrelations with midges remained low and mostly atnon-significant levels.

Being a fish eater feeding on three-spinedsticklebacks, the red-breasted merganser unexpecedlyshowed significant inverse responses to the abund-ance of certain chironomids, especially two common,

epibenthic, orthocladiin species. This indirectly indic-ates that numbers of orthocladiin chironomids are lowin years of high stickleback numbers. The responseof breeding mergansers to low chironomid numbersbegins in spring (Gardarsson and Einarsson 1997b)and high moulting numbers are a continuation of aresponse started earlier in the year.

Conclusions

We can conclude that moulting dispersion and move-ments amongst the diving duck species studied arepartly related to food resources, although there arespecies-specific differences, with some species leav-ing completely, others staying and sometimes showingfood-related patterns of dispersion. The large geo-graphical scale and the long temporal scale neces-sary to make meaningful estimates of resources ofmigratory populations, constitute major obstacles tounderstanding the role of food as a proximate factordetermining the distribution and numbers of moultingdiving ducks, both on a local and flyway scale. Forsome species, information on the current food supplyis sufficient for the ducks to determine the numbersmoulting at a site. Birds which migrate to remotesites may have little information about current foodprospects there, and may rely more on indirect inform-ation or experience; stable and abundant food suppliesmay be a prerequisite for regular long distance moultmigration.

Acknowledgements

This paper emanated from a long-term study on theMyvatn–Laxá ecosystem and its splendid waterfowl.Numerous scientists and students, too many to listhere, assisted in the field and laboratory. We areespecially indebted to Thóra Hrafnsdóttir, ErlendurJónsson, Haraldur R. Ingvason and Jón S. Ólafsson foridentifying the chironomids, and to Yann Kolbeinssonand Thorkell L. Thórarinsson for assisting in censuses.Research grants from the Iceland Science Fund andUniversity of Iceland Research Fund are gratefully ac-knowledged. We thank Anthony D. Fox and Ólafur K.Nielsen for critical reading of the manuscript.

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