Search paths of swans foraging on spatially autocorrelated tubers

Abstract

1. Tundra swans forage on below-ground pondweed tubers that are heterogeneously distributed in space. The swans have no visual cues to delineate patches. It was tested whether swans employ an area-restricted search tactic. Theory predicts that swans should alternate between an intensive (low-speed, sinuous) search mode in high tuber density areas and an extensive (high-speed, directed) search mode between these areas. 2. A quantitative analysis of movement paths recorded over short time frames (15 min) revealed that the sequential step lengths were strongly autocorrelated. After partitioning the data in low-speed paths and high-speed paths, this autocorrelation was very much reduced. 3. Movement paths with low speed were non-directional and could well be described as random walks. In contrast, high-speed paths were directed forward, and were better described as correlated (i.e. directional) random walks. 4. Movement paths recorded over longer time frames (1-4 h) provided empirical evidence that an alternation of low-speed, sinuous and high-speed, directed searches occurred. 5. There was a spatial autocorrelation in tuber biomass density, being significantly positively correlated until c. 10 m distance. The scale of the food clump size and step length of high-speed paths matched, suggesting that they were causally linked. 6. Computer simulations confirmed that swans using the observed search tactic achieved a higher energy gain than swans using only an intensive search mode, provided that the tuber biomass density occurred in clumps. They also achieved a higher gain than swans that alternated between intensive and extensive search mode, but always moved in a random direction.