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HøYE, TOKE T  Department of Population Biology, Biological Institute, University of Copenhagen, Copenhagen, Denmark.
Schmidt, Niels M  Department of Ecology, Royal Danish Agricultural University, Frederiksberg, Denmark.
Meltofte, Hans  Department of Arctic Environment, National Environmental Research Institute, Roskilde, Denmark.
Forchhammer, Mads C  Department of Population Biology, Biological Institute, University of Copenhagen, Copenhagen, Denmark.

Waders migrating to High Arctic breeding grounds have extraordinary energy demands during migration. Previous work has shown that High Arctic waders in Greenland are “income breeders”, that is, the energy source used for reproduction is almost solely based on biomass collected on the breeding grounds. Thus, waders arriving on the breeding grounds are strongly dependent on rich food supply, and their breeding phenology is expected to respond to food availability. At the same time, mean temperatures and the onset of snow melt may also influence timing of reproduction either directly or mediated through the food resources (trophic cascades).

Here, we report the intra- and inter-annual variation in nest initiation (1st egg dates) of three wader species breeding in High Arctic Greenland (dunlin, ruddy turnstone and sanderling). Systematic field work during eight complete breeding seasons in a geographically restricted area has enabled us to quantify the relative importance of abiotic and biotic factors on variation in breeding phenology. Dates of nest initiation have been recorded in parallel with food abundance (invertebrates), temperature, and snow cover within the study area. Since snow cover and temperature may both influence the abundance of invertebrates, we have modelled the direct effects of food abundance, snow cover and temperature along with the indirect effects of snow cover and temperature on nest initiation through their influence on food abundance. Food abundance had the strongest direct effect on date of nest initiation with important indirect added effects of snow cover through influence on food abundance for dunlin and ruddy turnstone. Likewise, food abundance was the most important direct factor for nest initiation in the sanderling. However, the indirect effect of snow was negligible in this species. The sanderling breeds later than dunlin and ruddy turnstone and prefers dry heath with early snow melt as breeding habitat, explaining this result. Thus, low food abundance and late snow melt may delay the onset of dunlin and ruddy turnstone breeding, whereas low food abundance may delay sanderling breeding. Our data also suggest that delayed breeding may in turn negatively affect mean clutch size, thus, contributing to reduced long-term reproductive output.

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