4.1 Age-related dietary specialization
Differences in morphology, social status, and acquired skills among shorebirds of different ages can facilitate resource partitioning (Alves et al., 2013; Catry et al., 2012; Recher, 1966; Stein et al., 2008). Although morphological differences between juveniles and adults may influence prey selection in some shorebird species (Durell, 2000), we did not observe significant differences in bill length or body mass between juvenile and adult western sandpipers within each sex Therefore, an age-related difference in morphology is an unlikely explanation for the differences that we observed in diet composition between juveniles and adults. However, differences in social status between juvenile and adult western sandpipers could lead to differences in diet composition between age classes. Direct evidence of adult dominance in western sandpipers is lacking, but social status could play a role in the differential use of winter foraging habitats between juveniles and adults (Buenrostra, Warnock, & De la Cueva, 1999; Fernández & Lank, 2006; Warnock & Takekawa, 1995). In SF Bay during spring, juveniles may focus their foraging efforts on biofilm and microphytobenthos because they are excluded by adults from sites with more profitable invertebrate prey. Age-related differences in acquired foraging skills may also play a role in prey selection in western sandpipers. Juvenile birds tend to forage less efficiently than adults, and evidence from several species of shorebirds including Eurasian oystercatchers, ruddy turnstones, and black-necked stilts suggests that foraging ability improves with age (Burger, 1980; Durell, 2000; Goss-Custard & Durell, 1987; Groves, 1978). If juvenile western sandpipers are less adept foragers than adults, juveniles may consume more biofilm and microphytobenthos in spring as a bet-hedging strategy; biofilm and microphytobenthos offer a readily available and highly abundant energy source that could be used by juveniles to secure adequate nutrition to prepare for migration (Schnurr, Drever, Kling, Elner, & Arts, 2019; Stal, 2003; Underwood & Paterson, 2003).