4.3 Seasonal resource partitioning
Coincident with previous studies, we did not detect differences in diet
composition among demographic groups of western sandpipers during
mid-winter (Franks et al., 2013). However, in spring, diet composition
differed among demographic groups, which could have been driven by
several factors. Seasonal shifts in prey abundance resulting from
changing environmental conditions and prey population dynamics could
influence diet composition (Rowan, 2012). For example, in SF Bay
polychaetes (> 20 mm) and small bivalves (≤ 6 mm) within
the size classes consumed by western sandpipers are present in greater
densities in January than in April (Rowan, 2012). In contrast, biofilm
biomass is greater in spring than in winter due factors such as a longer
photoperiod, greater photon flux density, and greater temperature
(Guarini et al., 1998; Schnurr et al., 2019). Furthermore, biofilm fatty
acid concentrations peak in spring (Schnurr, Drever, Elner, Harper, &
Arts, 2020). Accordingly, we observed that western sandpipers from all
demographic groups focused their foraging efforts on invertebrates
during mid-winter, whereas, in spring, biofilm and microphytobenthos
consumption increased in all demographic groups, but especially in
juveniles.
Increased inter- and intraspecific competition caused by
density-dependent prey depletion could also contribute to seasonal
differences in western sandpiper diets. SF Bay is one of the most
important foraging areas for migratory shorebirds along the Pacific
coast, supporting an average of 52.3% of all migratory shorebirds in
the region during spring (Page et al., 1999). High densities of foraging
shorebirds could increase interspecific competition for prey in spring.
For example, competitive interactions with dunlin at migratory stopover
sites are known to influence western sandpiper foraging behavior and may
cause western sandpipers to increase their biofilm consumption to reduce
competition (Jiménez, Elner, Favaro, Rickards, & Ydenberg, 2015; Mathot
et al., 2010; Senner, Norton, & West, 1989). Non-avian competitors
including fishes and crabs could also reduce the available prey biomass
in spring (Lovvorn, De La Cruz, Takekawa, Shaskey, & Richman, 2013). In
addition, intraspecific competition could influence western sandpiper
diets. In spring, western sandpipers migrating from southern latitudes
begin their migration earlier than those wintering farther north
(Bishop, Warnock, & Takekawa, 2004). This pattern of differential
migration leads to an increased abundance of western sandpipers in SF
Bay in April relative to mid-winter (Rowan, 2012). At our study site in
April, sandpiper abundance can exceed carrying capacity based on
invertebrate energy content, and it is likely that other foraging areas
in SF Bay become similarly depleted in spring (Rowan, 2012). Our
observation that western sandpipers increased their biofilm and
microphytobenthos consumption in April could indicate that sandpipers
shift their diets in spring in response to density-dependent increases
in inter- and intraspecific competition.
As they prepare for their spring migration, western sandpipers undergo
changes in physiology and organ morphology that facilitate rapid
fattening and help condition birds to endure long-distance migratory
flights (Egeler & Williams, 2000; Guglielmo & Williams, 2003; Stein et
al., 2005; Williams et al., 2007). Coincident with the increase in
biofilm and microphytobenthos consumption that we observed in all
demographic groups in spring, these physiological changes may help
western sandpipers to assimilate energy from biofilm and
microphytobenthos during migration. Biofilm and microphytobenthos could
provide sandpipers with performance enhancing fatty acids to sustain
long-distance migratory flights. Several studies have documented the
importance of n-3 polyunsaturated fatty acids in exercise performance
(Maillet & Weber, 2006, 2007). Although biofilm contains low
concentrations of these fatty acids, their concentrations peak in
spring, and sandpipers could consume a sufficient quantity of biofilm to
enhance their migratory performance (Quinn, Hamilton, & Hebert, 2017;
Schnurr et al., 2020, 2019).