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).