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During winter and migration, most Arctic-breeding waders eat estuarine invertebrates, shifting to terrestrial and limnic invertebrates on tundra breeding grounds. Invertebrates from estuarine and tundra habitats have distinctly different carbon-isotope ratios4. Diet-based differences in carbon-isotope ratios are expressed in bird tissues, including eggs and feathers4,5. We investigated whether Arctic-breeding wader species that use estuarine habitats during the non-breeding season are capital breeders, producing eggs and hatchlings with carbon-isotope ratios that are typical of estuaries.

As expected, the flight feathers produced during early winter show ratios that are typical of estuarine systems (Fig. 1). The same is true for shoulder feathers, which are among the last to be moulted before migration to breeding grounds6. However, the carbon-isotope signatures of eggs and natal down are typical of ter-restrial and limnic systems. The eggs and resultant hatchlings therefore seem to be produced from local nutrients. This conclusion is supported by the strong resemblance of the carbon-isotope signatures of natal down and juvenile flight feathers, showing that eggs and post-hatch tissues have the same nutritional source.

Figure 1: Carbon stable-isotope ratios (parts per thousand difference from 13C/12C ratio in PeeDee limestone4; δC) of eggs, natal down and feathers of different species of Arctic-breeding waders at different times during the year.
figure 1

All samples were collected in northeast Greenland and Arctic Canada in 1999 and 2000. Feathers collected from nest-attending adults were grown either in winter (adult flight feathers) or during spring migration (adult shoulder feathers); data are averages across individuals ± s.d. Eggs were collected from deserted nests (egg content); natal down was collected from hatchlings; data are averages across clutches ± s.d. per species. Data for natal down still attached to the tips of head and neck feathers of independent young8, and for secondary flight feathers from independent young are averages across individuals ± s.d.). Values of δC were determined using a Carlo Erba model 1106 elemental analyser which was coupled online, through a Finnigan conflow 2 interface, to a Finnigan Delta S mass spectrometer. Feathers were washed in chloroform before carbon-isotope analysis. Using a hierarchically nested design in a mixed model ANOVA, in which species was nested within tissue type, which in turn was nested within the tissue groups 'adult feathers' and 'egg, natal down and juvenile feathers', we found significant effects of tissue group (F1,152 = 805.3, P = 0.001), tissue type (F4,152 = 6.7, P < 0.001) and species (F9,152 = 2.8, P < 0.005) on δC.

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Our results also indicate that the use of a mixed capital/income breeding strategy by these waders is unlikely. Egg-laying females use a single, local nutrient source, as indicated by the low within-clutch variation compared with the among-clutch variation in carbon-isotope ratios of natal down. (We used a hierarchically nested design in a mixed-model ANOVA, in which brood is nested within species as a random factor, and found significant effects of species (F6,30 = 4.8, P < 0.002) and brood (F29,93 = 16.8, P < 0.001) on the carbon stable-isotope ratios of natal down.)

We conclude that, with respect to egg production, the Arctic-breeding waders investigated here are income, rather than capital, breeders. For these waders, the fitness costs of transporting extra nutrient stores to breeding grounds7 outweigh the potential benefits1,2,3. The capital strategy may still be used by large species of Arctic breeding migrants, such as geese1,2 — it is likely that larger species need relatively smaller body stores for egg production. Also, larger species, although constrained by the same fixed period of opportunity as smaller birds, require longer to complete their breeding.