Large numbers of birds breed each summer on the tundra of the northern hemisphere. A prominent group of the Arctic bird fauna are the shorebirds. Breeding, which is an energetically costly activity irrespective of climate (Drent and Daan 1980), is especially so in the High Arctic. This is mainly due to low temperatures and high wind speeds in an open landscape (Piersma and Morrison 1994, Wiersma and Piersma 1994). In addition, the summer is short.

This leaves little time for activities before, during or after breeding. Thus, costs are high and available time is short! Not only is breeding alone costly, most birds migrate vast distances between their Arctic breeding sites and their temperate or tropical wintering grounds. Migration is also an energetically costly event. This intense rate of living places great demands on the birds and we can expect them to have evolved a wide range of physiological and behavioural adaptations. Given the inaccessibility of most tundra areas and the necessity for relatively advanced techniques, ecological energetic studies of waders and waterfowl are relatively scarce (with the notable exception of tundra breeding Nearctic geese).

We have studied same of the important energy turnover processes of waders during the hectic Arctic summer with the aim of evaluating these in an evolutionary context. Of particular interest are the importance of energy and nutrient stores for breeding, the energy turnover of breeding birds in relation to species and microclimate, and the fat deposition and basal metabolism of birds preparing for autumn migration. The project is partly a continuation of work carried out during the 1994 Swedish Russian Tundra Ecology Expedition (Lindström and Piersma 1995).

Raising a family on capital or income?

Due to the short summer period, Arctic birds are probably hard pressed to start egg laying soon after arrival at the breeding grounds. However, upon arrival food availability is often low. Female birds may therefore be forced to produce a clutch using, to some degree, body stores accumulated during their migratory journey. Birds using such a strategy are called ”capital breeders”, in contrast to ”income breeders”, that only use resources obtained at the breeding grounds (Drent and Daan 1980). Our objective was to investigate how common the ”capital breeder” strategy is on the Nearctic tundra. We expect the capital breeding strategy to be more common:

1. In larger species because they may be more time-stressed.
2. At northern sites where breeding has to start immediately upon arrival.
3. In relatively early arriving species that arrive when food is not yet available at the breeding sites.

The fact that we visited many different habitats with different climates, foraging conditions and phenology suited this project extremely well.

One way to study capital and income breeding strategies is to collect tiny pieces of downy feathers from chicks and look at their chemical ”fingerprints”! The chemical fingerprints analysed by us are the ratios of stable isotopes of carbon and nitrogen (C¹²/C¹³ and N¹⁴/ N¹⁵).

Shorebird chicks hatch with a completely downy plumage. Thus, the down is built from nutrients the mother put in the egg when it was produced. These nutrients may have their origin at feeding sites along the migration route and arrive in the egg via the mother’s body stores (capital breeding). Alternatively, they may originate from food eaten at the site where the eggs were produced (income breeding). The chemical fingerprints of tissues often reflect the nature of the food sources and the location at which food was ingested. In this way, the isotopic sign of the nutrients picked up along the migration route or at the breeding site may find its expression in the chemical composition of the eggs and ultimately in the chick down. Thus, distinct differences may be expected in the down of newly hatched young from capital and non-capital breeders. Such differences may also appear within the clutch in cases where the female has used a mixed strategy. Comparing the isotope ratios in down samples within broads with isotope ratios in potential food sources at the breeding ground thus provides a clue to the extent the mother has made use of the capital breeding strategy.

We collected down from the chicks of 13 shorebird clutches: white-rumped sandpiper (3 clutches), Baird’s sandpiper (3), red knot (2), red phalarope (2), semi-palmated sandpiper (1), ruddy turnstone (1) and black-bellied plover (1). The chicks were trapped in July when they were still being attended by their parents. In most cases we also collected feathers and a small blood sample from the parent(s), which will provide additional information about the isotope ratios of the food at the breeding and wintering grounds. We also sampled the insect food of waders in the likely foraging areas of the chicks and parents studied.

Traces of down are also present on juvenile birds sometime after they have become independent of the parents and have grown their first real plumage. Such down was collected from 39 juveniles of 8 species in August. These birds were then in migration. It is an exciting thought that the down traces on birds migrating southwards can provide us with information about the food their parents ingested on their northward migration a few months earlier. The samples are presently being analysed for C¹²/C¹³ and N¹⁴/ N¹⁵ ratios using mass spectrometry at the Centre for Limnology at the Netherlands Institute of Ecology. Hence, we cannot yet say whether the family planning of shorebirds is built on capital or income.

Costly breeding

The few available measurements of energy expenditure in incubating shorebirds in tundra regions have shown that breeding in the Arctic is indeed energetically very costly (Piersma and Morrison 1994, Piersma et al. in preparation). The high levels of energy expenditure stem from the combined effects of low temperatures and high wind speeds in an open landscape, but may also be affected by the birds’ own intense foraging activities. However, the measurements that have become available up till now do not cover the whole ”climate space” that Arctic breeding waders encounter, due to the bias in study sites and the particularities of weather conditions during the few studies that have been carried out.

Field measurements of energy expenditure levels involve capturing a bird on the nest, injecting a tiny amount of water with a particular ratio of stable isotopes of H and O, and taking a blood sample. The bird is then released. After 24h it is recaptured and a new blood sample is taken. From the difference in stable isotope concentrations between the samples it is possible to estimate the CO₂-production, and thus the energy expenditure of the bird.

A large sample of measurements (35 individuals of 7 species) was collected on the 1994 Swedish-Russian expedition (Lindström and Piersma 1995). In Canada it was our intention to increase the number of species measured, and to obtain additional data from more environmental conditions. However, the time schedule of the expedition was less favourable this time. The expedition did not start until early July (early June in Russia), and in addition, the average start of breeding is at least a week earlier in Canada than in Russia. We therefore expected to find only few birds still incubating eggs. Unfortunately our misgivings were justified. Only for two sanderlings and one white-rumped sandpiper could a successful sample be collected. These data points will, however, make a nice addition to the data collected in Russia.

Migrating shorebirds in the Arctic – the racing cars among birds?

Waders need high-performing bodies to cope with their energetically intense rate of living. This is reflected in their basal metabolic rate (BMR). The BMR of an animal is the energy it spends at rest at night (for day active animals), in thermo-neutral conditions, without processing food, and when it is not involved in productive activities like reproduction, moult or growth. The BMR of a bird may be compared with the fuel consumption of an idling ear engine. A racing ear that operates at an incredibly high rate also has a high cost of idling. A standard ear with a less impressive engine requires less fuel to keep running. Just as the cost of idling reflects the potential power of an engine, the BMR reflects the potential work rate of an animal body.

Waders have a comparatively high BMR compared to other non-passerine birds (Kersten and Piersma 1987). Moreover, studies of captive knots have shown that they vary their BMR over the year (Piersma et al. 1995). In addition, waders trapped during the first part of their autumn migration in Arctic Eurasia were found to have a higher BMR than their conspecifics at tropical wintering grounds in Africa (Lindström 1997, Kersten et al.1998). This suggests that waders can adjust the size of their ”engine” which makes sense, since the best solution would be to have a strong ”engine” when circumstances so demand, and a smaller ”engine” during more relaxed parts of the year (for example at wintering grounds in warmer latitudes).

We are actually more interested in the long-term maximum rate of energy expenditure as a measure of adaptations to a high rate of living. However, this is very difficult to measure, and especially so in a comparable way. Instead, BMR, which is supposed to reflect the maximum energy turnover potential, is fairly easy to measure, and figures from different investigations can be compared.

During the 1994 expedition to the Eurasian Arctic, the BMR of 24 juvenile waders of five different species was measured in a respirometer (Lindström 1997). In Canada we aimed at checking the generality of the pattern found by measuring the BMR of a new set of shorebird species.

Juvenile shorebirds were caught ashore in August in portable walk-in traps. From the different sites, between 1 and 12 birds were carried on board for over-night metabolic measurements in a ”respirometer”. The birds were kept in small perspex cages and their O₂-consumption and CO₂-production were measured. In this way, BMR was estimated for 45 birds of 10 species. Using the average mass and BMR for each species, an allometric relationship of species-specific BMR against body mass (BM, g) was estimatedas BMR (W)=0.042·BM^0.68(R²=0.93, P<0.001). This is remarkably close to the corresponding relationship found for the Eurasian Arctic sample, BMR=0.041·BM^0.68 (R²=0.97, P=0.002, Lindström 1997), the difference being on average only 3% for a given body mass. Only a single species, ruddy turnstone, occurred in both samples. This shows that setting out on autumn migration with high-performing machinery may be universal among Arctic shorebirds.

Only a few fat birds

Although it is well-known that most shorebird species put on huge energy reserves prior to migration to the Arctic, very little is known about the size of reserves carried by shorebirds prior to departure from the Arctic. This is necessary to know in order to understand the migratory strategies adopted (Alerstam and Lindström 1990) and when analysing migration routes. The data collected in the Eurasian Arctic 1994 indicated that most shorebirds leave the tundra with only small stores, but that substantial stores can be put on closer to the temperate regions (Lindström 1998). Whereas much is known about the size of the energy stores of migration waders further south in America (for example, McNeill and Cadieux 1972, Harrington et al. 1991, Driedzic et al. 1993), very little is known about the fuel stores put on by migrant shorebirds starting their autumn migration from their Arctic breeding grounds.

During the latter half of the expedition we concentrated on catching waders at coastal sites. In total, 214 waders of 11 species were trapped in the post-breeding period. The birds were trapped in mist-nets or in walk-in traps set along the shorelines of rivers, ponds and the sea.

Only three birds were adult: a male semi-palmated plover on southern Banks Island (site 9), a female pectoral sandpiper at Ivvavik (site 10) and a purple sandpiper on Ellesmere Island (site 15). The purple sandpiper Had just started its primary moult. The reasons why only few adults were caught are twofold. Most adult waders set out on autumn migration as early as July, well before the migration of juveniles, and well before our trapping started. In addition, they probably put on the necessary fuel stores at the breeding grounds while attending their young. Hence, they did not show up at the coastal stop-over sites we visited. The same dominance of juveniles in the catches was found on the 1994 expedition to the Eurasian Arctic (Lindström and Piersma 1995).

In late July and early August, many of the juveniles trapped still had some downy feathers on their necks and around their bills. Nevertheless, the y had started to flock and most likely had already commenced autumn migration. Some birds with down were also trapped in late August, which shows that some birds also breed late in the season.

The fat put on for migration can be seen through the transparent skin by blowing throat feathers aside. By scoring the amount of fat and weighing the birds, we found that most of them carried only small fat loads, even those trapped in late August. As many as 146 out of 189 juveniles (77%) scored for fat, had fat scores between 0 and 2 (on a scale from 0 to 9). Fat scores of 0-2 probably correspond to a fuel load of less than 10% above lean mass. This means that the initial part of migration out of the Canadian Arctic must be carried out in small hops, with regular stop-over for refuelling. This is in contrast to the substantial amounts of fuel put on by these birds in southern Canada and northern USA before making long over-water flights to wintering grounds in Central and South America (Page and Middleton 1972, Harrington et al. 1991, Driedzic et al. 1993).

There were, however, some examples of higher fat scores among the migrants. The eleven buff-breasted sandpipers trapped on northern Banks Island (site 12, 10-12 August) had an average fat score of 4.4, four had a fat score of 5 and one a score of 6. These were the only birds of any species trapped with substantial fat stores. The average mass of the buff-breasted sandpiper, 58 g, is similar to what has been recorded during autumn migration in south Canada and in Surinam, but much lower than the 85 g recorded during spring migration (Lanctot and Laredo 1994). Whereas all 59 white-rumped sandpipers had fat scores of 0-1 at Ivvavik (site 10, 3-5 August), 19 out of 29 had fat scores of 2-4 on Devon Island (site 16, 25-26 August). Since most white-rumped sandpipers seem to leave North America via the south-eastern parts of Canada, the two sites may be situated along the general migration route of the species. Whether the higher fat scores in the east are due to the difference in time or geographic position is impossible to tell. A similar pattern, where migrants were fatter later in the season and further along the migration route was found in little stints in Arctic Russia (Lindström 1998). We conclude that most juvenile shorebirds leave the Canadian tundra with small to moderate fat stores, which is in contrast to the huge amounts stored at other sites and during other parts of the year.