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).