Resource use for reproduction depends on spring arrival time and wintering area in an arctic breeding shorebird

Resources for egg production may come from body reserves stored before breeding (‘‘capital breeders’’) or from food acquired at the breeding site (‘‘income breeders’’). Arctic migrants were long thought to be capital breeders, because they often arrive at a time when local food availability is still limited. However, later evidence suggested that arctic breeding shorebirds are primarily income breeders, or that they use a mixed strategy depending on laying date. We explored the relationship between laying date and resource use for reproduction in the pectoral sandpiper Calidris melanotos breeding in the Alaskan arctic by contrasting carbon isotope (dC) values of the local diet and of maternal plasma, cellular blood, feather and claw with those of the eggs produced. Our results revealed that early breeding females utilize resources for egg production that were acquired recently at staging areas, whereas later breeding females mostly relied on nutrients derived from local food sources. These findings suggest that the resource allocation strategy used for reproduction differs among females, and varies depending on the timing of arrival and the start of reproduction. The arrival date at the breeding ground and laying date may critically depend on non breeding season events such as winter habitat choice, staging areas or migration routes. By comparing maternal feather dC, claw dC and feather dD, we examined whether non breeding season events influenced the use of resources for egg production through variation in capture date or clutch initiation date. Female pectoral sandpipers originating from moulting areas characterized by higher (more positive) dD signatures were caught earlier and started laying earlier, and they used stored resources for reproduction. Using regional maps of dD values for precipitation in the wintering sites in South America, we compared the spatial variation in the observed feather dD signatures. This analysis indicated that female pectoral sandpipers with higher dD signatures, presumably coming from more north easterly wintering sites in southern America, started laying earlier and used mostly stored resources for egg production, compared to females that wintered (or at least moulted) further south. Our results thus show that winter moulting habitat is linked to breeding resource allocation strategy in this high arctic breeding shorebird.