2.2 Behaviour recordings and identification
Video recording was often used to monitor the foraging behaviour in a
natural complex environment (Bruijn, Vosteen, Vet, Smid, & Boer, 2021).
In this study, video footages (Nikon P900S) were taken from six fixed
locations (3 for each habitat types, Fig. 1) to record the foraging
activities of M. squamatus in three winters (2018-2020).
Recording was initiated when a bird started a foraging bout (see below)
and the camera then followed the bird till it finished the foraging or
disappeared from the range of view. Based on field observation and
examining the videos, we defined the duration of a foraging bout as the
time interval encompassing the descending, swimming, diving, and the
subsequent resting on the water surface and taking off (Table 1). Each
foraging bout could last for 3-15 minutes and contained multiple
activities.
We focused on the foraging behaviours of individuals, which were
identified as 8 activities from 3 related types: fishing, preparing and
other social activities (Table 1). Here we distinguished two main
fishing behaviours: diving and head-dipping (Fig. 2). While diving is
mainly performed in deep waters, head-dipping is the main fishing tactic
in shallow waters. The length of time that a M. squamatus spent
in each behaviour was divided by the length of the foraging bout to give
the percent of time spent in each behaviour.
Table 1. Description of identified behaviours of Scaly-sided Merganser