Burrow ambient temperature influences Helice crab activity and availability for migratory Red-crowned Cranes Grus japonensis

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 faecal 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 generalised 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 12~13℃ 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 faeces and site ambient temperature. Collectively our data
suggests that temperature-related mudflat crab activity may
provide an important time window for migratory Red-crowned cranes
to utilise critical stop-over sites and the crabs’ food
resources.