River width and depth as key factors of diurnal activity energy expenditure allocation for wintering Spot-billed Ducks in the Xin'an River Basin

Rivers are important habitats for wintering waterbirds. However, they are easily influenced by natural and human activities. An important approach for waterbirds to adapt to habitats is adjusting the activity time and energy expenditure allocation of diurnal behavior. The compensatory foraging hypothesis predicts that increased energy expenditure leads to longer foraging time, which in turn increases food intake and helps maintain a constant energy balance. However, it is unclear whether human-disturbed habitats result in increased energy expenditure related to safety or foraging. In this study, the scan sample method was used to observe the diurnal behavior of the wintering Spot-billed Duck (Anas poecilorhyncha) in two rivers in the Xin'an River Basin from October 2021 to March 2022. The allocation of time and energy expenditure for activity in both normal and disturbed environments was calculated. The results showed that foraging accounted for the highest percentage of time and energy expenditure. Additionally, foraging decreased in the disturbed environment than that in the normal environment. Resting behavior showed the opposite trend, while other behaviors were similar in both environments. The total diurnal energy expenditure of ducks in the disturbed environment was greater than that in the normal environment, with decreased foraging and resting time percentage and increased behaviors related to immediate safety (swimming and alert) and comfort. These results oppose the compensatory foraging hypothesis in favor of increased security. The optimal diurnal energy expenditure model included river width and water depth, which had a positive relationship; an increase in either of these two factors resulted in an increase in energy expenditure. This study provides a better understanding of energy allocation strategies underlying the superficial time allocation of wintering waterbirds according to environmental conditions. Exploring these changes can help understand the maximum fitness of wintering waterbirds in response to nature and human influences.