The effects of food abundance and disturbance on foraging flock patterns of the wintering Hooded Crane (Grus monacha)

Background Food abundance and availability affect flock patterns of foraging birds. Cost and risk tradeoffs are especially critical for flocks of wintering waterbirds foraging in lake wetlands. Waterbirds losing suitable habitats face insufficient food supplies and high levels of disturbance, affecting their foraging activities. Our Objective was to study the effects of food abundance and disturbances on flock size and the structure of Hooded Crane flocks wintering at Shengjin Lake and, as well, to understand the response of wintering waterbirds to habitat degradation for future management decisions and protection of the population.

Methods We investigated food abundance, disturbances and flock foraging activities of the wintering Hooded Crane in several foraging habitats of Shengjin Lake from November 2013 to April 2014. Flock size and structure were observed by scan sampling. Data on food abundance and disturbances were collected by sampling. Flock size and structure were compared among three wintering stages. The relationship between food resources, disturbances and flock size were illustrated using a generalized linear model.

Results In the early and middle wintering periods, the Hooded Crane used paddy fields as its major foraging habitat, where the number of foraging birds and flocks were the highest. During the late period, the cranes took to meadows as their major foraging habitat. The variation among foraging flock was mainly embodied in the size of the flocks, while the age composition of these flocks did not change perceptibly. Family flocks were notably different from flock groups in size and age composition. The Results of a generalized linear model showed that the food abundance had a marked effect on foraging flock size and age composition, while disturbances had a significant effect only on flock size. From our analysis, it appeared that the combined effect of the two variables was significant on the size of the foraging flock, but had less impact on age composition.

Conclusions Food abundance and disturbances affected the flock size of the Hooded Crane. With abundant food and high disturbances, flock sizes increased owing to cooperation in foraging. To avoid competition and maximize foraging benefits, flock size reduces with an abundance of food but low disturbance. By trading off risks and costs, the cranes showed flexible flock distributions and a variety of foraging strategies to maximize benefits and to improve their fitness.