Fitness consequences of diet choice in waterbirds of agricultural and natural landscapes

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Recent agricultural practices lead to a decrease in habitat diversity and thereby changed food availability for wild animals. High-quality food is a major factor for reproduction, which in turn drives population development. Although food abundance is generally high in agricultural habitats, food diversity tends to be low. How food availability jointly affects fitness on the individual level has rarely been assessed, mainly due to difficulties in estimating individual diet choice. The proposed study will for the first time provide empirical evidence for how an individual¿s fitness varies with its diet choice and how food availability in different habitats affects diet-choice width. The target species will be Mute Swan Cygnus olor, a waterfowl that has apparently profited from farming. The project will investigate swan populations of natural and agricultural habitats, where individuals may rely on specific habitats for foraging. In these populations, individual diet choice will be assessed by comparing stable-isotope ratios in potential food with stable-isotope ratios in blood and tissues of chicks and subadults. Thus, individual diet choice will be measured with respect to habitat-specific food availability, and compared between populations. Furthermore, the level of diet selection can be directly related to fitness by simultaneously measuring chick growth trajectories, immunocompetence and survival of chicks. Additionally, involving longterm data of reproductive success allows generalisations of how diet choice affects fitness and how populations cope with food availabilities in agricultural landscapes.