Abstract
For migratory birds that specialize on particular benthic macroinvertebrate species, the timing of migration is critical since prey availability may be temporally limited and a function of local ambient temperature. Hence, variation in local ambient temperature can influence the diet composition of migrant birds, and, consequently, they may be constrained by which stopover and wintering sites they are able to utilize during periods of colder temperatures. Here, we use fecal analysis, observer‐based population counts, digital video recordings, and temperature data to test five predictions regarding the influence of local ambient temperature on the activity and availability of mudflat crabs—a key prey resource at three staging/wintering sites in eastern China, for migratory Red‐crowned cranes (Grus japonensis) and how this subsequently influences crane diet and use of wetland sites. Pearson's correlations and generalized linear models revealed that mudflat crabs became significantly more surface active with increasing burrow ambient temperature. Piecewise regression analysis revealed that crab surface activity was largely limited to a burrow ambient temperature threshold between 12 and 13℃ after which activity significantly increased. Crab activity declining temporally during the crane's autumn migration period but increased during spring migration. Crabs accounted for a significant proportion of crane diet at two of three sites; however, the frequency of crab remains was significantly different between sites, and between autumn and spring migration. Analyses of crane count data revealed a degree of congruence between the migration timing of Red‐crowned cranes with periods of warmer ambient temperature, and a significant, positive correlation between the percentage of crab remains in crane feces and site ambient temperature. Collectively, our data suggest that temperature‐related mudflat crab activity may provide an important time window for migratory Red‐crowned cranes to utilize critical stopover sites and the crabs’ food resources.
Highlights
Benthic macroinvertebrates serve as critical nutrient-rich prey resources for many migratory waterbird species at coastal saltmarsh and intertidal mudflat stopover or wintering sites (Anders, Churchyard, & Hiddink, 2009; Castro & Myers, 1993; Choi et al, 2017; Piersma, 1987; Yang et al, 2013)
We have shown how variation in burrow ambient temperature influences diet composition of the migratory Red-crowned crane population across three wintering/stopover sites and different migration seasons through its influence on crab prey availability
Mudflat crab activity was largely influenced by burrow ambient temperature and crab activity significantly increased at a burrow ambient temperature threshold of 12 ~ 13°C
Summary
Benthic macroinvertebrates (e.g., polychaetas, bivalves, and crustaceans) serve as critical nutrient-rich prey resources for many migratory waterbird species at coastal saltmarsh and intertidal mudflat stopover or wintering sites (Anders, Churchyard, & Hiddink, 2009; Castro & Myers, 1993; Choi et al, 2017; Piersma, 1987; Yang et al, 2013). Crab prey availability may be a function of ambient temperature at stopover or wintering areas for crab specialist migratory bird species during narrow temporal windows for migration. These bird species may be more adversely affected and more constrained by which stopover and wintering sites they are able to utilize during periods of colder, more severe winter weather. Li et al (2014) found that cranes increase their intake of mudflat crabs during late February to mid-March, prior to their northward spring migration This period coincides with increasing regional ambient temperature and the emergence of mudflat crabs from their winter hibernation burrows (Figure 1).
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