Foraging success of an animal can be explained by habitat quality, group size of the forager, time of foraging, and distribution of prey. If these conditions are met, animals should sustain a regular foraging regime over time, however, this is not the case in reality. Large-sized shorebird species (Eastern Curlew Numenius madagascariensis), without predators in their intertidal mudflat habitats, have both abundant prey and foraging time, but have been found to have varying foraging success across time and among individuals. We hypothesized that the benthic prey invertebrate (crab species Macrophthalmus japonicus) shifts its vertical distribution under the mudflat across time and that this affects the foraging success of Eastern Curlews due to phenotypic limitations. Prey distance from the surface (PDS) was modeled by measuring crab individuals’ PDS (n=45) across time along with its physical characteristics and environmental variables (temperature, flooded hours, solar altitude etc.) from 2014 to 2015. Eastern Curlew foraging activity was filmed, with sex (sexually dimorphic bill length) and foraging rate measured and confirmed in situ. Mudflat characteristics, prey density, season, sex and environmental variables were used as independent variables to analyze the effects on the foraging rate of Eastern Curlews.
Results/Conclusions
Based on the AIC values, for individuals measured in fall, both female crabs and males with larger carapace width remained deeper in the mudflat. As air temperature rises and as the mudflats are exposed to air for longer periods due to ebbing tide, crabs tended to distribute closer to the surface. Foraging rate of the Eastern Curlews (n=353) were negatively affected by the PDS and females tended to have higher foraging rates compared to males. Results of our study corroborate our hypothesis that the foraging rate of the Eastern Curlews are dependent on bill morphology and PDS. Our findings could explain the sex specific distribution in stopover sites and the phenotypic evolution of longer bills in females which have to forage in stopover sites where early arrived males have already preyed on the near-surface individuals. Moreover, our results alert us that functional responses or foraging rates measured without considering the prey behavior, could lead to biased results. This could lead to erroneous estimates of the carrying capacity of the habitat as well as parameters to be used in estimating the survival rates of the species e.g., Individual-based models.