Do arctic-breeding Red Knots (Calidris canutus) accumulate skeletal calcium before egg laying?

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Abstract
Earlier studies have indicated that the diet of egg-laying female birds which eat only terrestrial arthropods has to be supplemented with calcium if they are to produce high-quality eggshells without interruption. During egg laying, females of tundra-breeding shorebird species may supplement their diet with fragments of mammalian skeletons, but as an alternative strategy they might store skeletal calcium before egg formation. We examine the possibility of calcium storage on the basis of temporal changes in the ash mass (a good indicator of skeletal mass) of male and female Red Knots (Calidris canutus islandica) collected during their stopover in Iceland in May and July, and after arrival on the breeding grounds in northernmost Ellesmere Island, Canada, in late May and early June. Significantly higher ash masses of females than of males, an increase in ash mass of females before the period of egg formation in mid-June in combination with a subsequent decrease, and the notable absence of temporal changes in ash mass of males, lead us to propose that female Red Knots do store skeletal calcium before egg laying. The rate of calcium storage would be 2.3 times higher after arrival on Ellesmere Island than during the stopover in Iceland, but the dietary components through which storage is achieved remain unclear. With an almost 50% change in the skeletal mass of females, Red Knots currently hold the record with respect to skeletal calcium dynamics in free-living egg-laying birds. The stored skeletal mass would allow them to produce at least half the clutch without further calcium intake.