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).