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