Foraging behavior of wintering Bewick’s Swans

By Chao Yu, Lizhi Zhou, Anhui University, China

The Influence of Food Density, Flock Size, and Disturbance on the Functional Response of Bewick’s Swans in Wintering Habitats

Perceiving how animals adjust their feeding rate under a variety of environmental conditions and understanding the tradeoffs in their foraging strategies are necessary for conservation. Feeding behavior components [feeding rate (number of prey consumed per unit time of active foraging), handling time (the time taken to masticate a food item), searching rate (searching area per unit time of active foraging, derived from the data of harvest rate against food density by fitting a Type II Holling disc equation)] are influenced by factors such as prey density, disturbance and flock size for migrating waterbirds in wintering habitat, especially Bewick’s Swans.

The shallow lakes in the middle and lower Yangtze River floodplain are traditionally important wintering refuges for Bewick’s Swans, however, aquatic plants have declined and even disappeared under pressure from purse seine fisheries and increased water levels. Besides tubers, swans foraged foxnuts seeds and rice grains in foxnut ponds and paddy fields around lakes, these two habitats are more disturbed by human beings due to pedestrians and electric bicycles passing by.

We collected focal sampling data on the foraging behavior of swans that foraged rice grains, foxnut seeds, and tubers in paddy field, foxnut pond, and lake habitats, respectively, in Shengjin and Huangpi lakes during winter from 2016 to 2018. The feeding rate was not correlated with food density and displayed a positive relationship with searching rate but negative relationships with handling time, flock size, and disturbance time. Handling time was negatively correlated with food density and flock size, yet it increased with disturbance time. Searching rate was negatively correlated with food density, flock size, and disturbance time. Feeding rate was affected by the combined effects of handling time and searching rate. The shape of the observed functional response could not be fitted to Holling’s disc equation.

This provides insight into how wintering waterbirds adopt appropriate foraging strategies in response to complicated environmental factors, which has implications for wildlife conservation and habitat management.