1. Introduction
Wild animals have been constantly subjected to environmental fluctuations during their evolutionary history, and have evolved to match physiology and behaviour to the predictable environmental variations in their natural habitats (Schmidt-Nielsen, 1997). Adjusting behaviour is often the first response when environmental conditions are altered (Snell-Rood, 2013). Behavioural responses are not always adaptive, but the ability of wildlife to behave appropriately in response to environmental changes is crucial for their survival (Sih, Ferrari, & Harris, 2011; Wong & Candolin, 2015). Behavioural plasticity could allow species to adjust behaviour to suit the conditions of its immediate environment and, in so doing, increase its fitness (Van Buskirk, Candolin, & Wong, 2012).
Among all adaptive tactics, foraging behaviours are most advantageous and allow the species to navigate diverse landscapes while efficiently searching for food (Schoener, 1971), and gain physiological, survivorship, and fitness benefits (Hadfield & Strathmann, 1996). For example, Great knots (Calidris tenuirostris ) extended their feeding time and adjusted their food selection to adapt the suddenly decline of original main food (Zhang et al., 2019).
Some young salmonid fishes clearly decrease their foraging behaviours from still water habitats to slow-running waters to fast-running waters (Tunney & Steingrímsson, 2012). To assess animals’ foraging plasticity in a changing environment, habitat–behaviour interactions within the wide array of environmental conditions are to be characterized (Gilmour et al., 2018).
Flexible and specialized foraging strategies are dependent on the stability of available resources in habitats (West-Eberhard, 1989). For most waterbirds, water depth might be the primary of habitat quality (Zeng, Lu, et al., 2018). Different water depths mean different species, sizes and quantities of food and therefore a key factor to affect their foraging behaviours and techniques (Giraldo et al., 2017; Harvey & Stewart, 1991).
Scaly-sided Merganser (Mergus squamatus ) is endemic to east Asia and is listed as globally endangered (BirdLife & International, 2017). It’s a habitat specialist, which occurs on clear, flowing rivers in the mountainous regions (Zeng, Wei, & Lei, 2018). It mainly breeds in southeastern Russia, and northeast China, and winters in the central and southern China, especially middle and lower Yangtze River (Zeng, Wei, et al., 2018; Zhao & Pao, 1998). M. squamatus is an opportunist piscivore who selects the most abundant food source (Zhao & Pao, 1998), such as fish and macroinvertebrates. It has long thin hooked bill with many serrations at the tip (Zhao & Pao, 1998) and is adapted to active hunting of moving prey in water column. M. squamatus mainly takes two foraging modes: diving and head-dipping (Solovieva, 2013; Zhao & Pao, 1998). In breeding areas, M. squamatus usually immerses its head into shallow waters to pick aquatic insects and use diving techniques to catch fish in deep waters (Solovieva, 2013; Zhao & Pao, 1998). Foraging strategy and energy budgets of M. squamatus were unknown in the wintering grounds (Solovieva, 2013).
The loss of lateral and longitudinal connectivity of free-flowing rivers through flow regulations such as damming presents a major threat to global freshwater biodiversity (Barbarossa et al., 2020; Dudgeon, 2019; Tonkin, Heino, & Altermatt, 2018) including the obligated freshwater megafauan (He et al., 2019) and foragers such as waterbirds (Zeng, Lu, et al., 2018). Knowledge of animal behaviour is fundamental in understanding and mitigating the effects of habitat loss and fragmentation. In this study, using three years (from 2018 to 2020) of video footages of M. squamatus foraging at the wintering habitats with different water depths, we aim to evaluate the extent to which foraging behaviour flexibility and plasticity exist. We also compare the foraging energetics in different habitats by estimating the behavior energy costs, fish catch rates, and energy intakes. We use these findings to highlight the risk of the rapid anthropogenic environmental changes to habitat specialists (Sih et al., 2011).