4.3 Optimal foraging theory
Predatory animals may have a series of alternative foraging modes (Kuwae, Miyoshi, Sassa, & Watabe, 2010). The optimal foraging theory (Pyke, 1984) predicts that decisions on foraging strategy, including what, where and how to hunt, are based on a maximization of currencies (Bautista, Tinbergen, & Kacelnik, 2001). Therefore, in their natural habitat, predators are expected to take different foraging strategies in response to both prey abundance and vulnerability such that they attain the highest energy intake rate.
As predicted by OPT, M. squamatus showed high behavioural flexibility by adjusting its foraging methods and behavior profiles in shallow and deep areas. In deep waters, where prey abundance and vulnerability are lower, M. squamatus allocated most of the time to repeated pursuit diving to minimize the foraging energy cost. This strategy was awarded with bigger gains and resulting in comparable energy gain as in shallow waters, where smaller preys are more abundant and easier to catch. The wintering grounds of M. squamatus are mainly mountainous streams (Zhao & Pao, 1998), which resources are patchily distributed (Taylor & Warren Jr, 2001). The behavioural flexibility is critical for predators foraging in heterogeneous environment (Abrahms et al., 2020), where prey is patchily distributed and may be unpredictable for diving birds (Tessier & Bost, 2020).