Journal Article

Movement of foraging Tundra Swans explained by spatial pattern in cryptic food densities

Bankert, Daniëlle, Nolet, Bart A., Klaassen, Raymond H. G.

2006

Overview

We tested whether Tundra Swans use information on the spatial distribution of cryptic food items (belowground Sago pondweed tubers) to shape their movement paths. In a continuous environment, swans create their own food patches by digging craters, which they exploit in several feeding bouts. Series of short (<1 m) intra-patch movements alternate with longer inter-patch movements (>1 m). Tuber biomass densities showed a positive spatial auto-correlation at a short distance (<3 m), but not at a larger distance (3-8 m). Based on the spatial pattern of the food distribution (which is assumed to be pre-harvest information for the swan) and the energy costs and benefits for different food densities at various distances, we calculated the optimal length of an inter-patch movement. A swan that moves to the patch with the highest gain rate was predicted to move to the adjacent patch (at 1 m) if the food density in the current patch had been high (>25 g/m⁲) and to a more distant patch (at 7-8 m) if the food density in the current patch had been low (<25 g/m⁲). This prediction was tested by measuring the response of swans to manipulated tuber densities. In accordance with our predictions, swans moved a long distance (>3 m) from a low-density patch and a short distance (<3 m) from a high-density patch. The quantitative agreement between prediction and observation was greater for swans feeding in pairs than for solitary swans. The result of this movement strategy is that swans visit high-density patches at a higher frequency than on offer and, consequently, achieve a 38% higher long-term gain rate. Swans also take advantage of spatial variance in food abundance by regulating the time in patches, staying longer and consuming more food from rich than from poor patches. We can conclude that the shape of the foraging path is a reflection of the spatial pattern in the distribution of tuber densities and can be understood from an optimal foraging perspective.

• Publisher: Ecological Society of America
• Subject: Analysis of Variance / Animal and plant ecology / Animal behavior / Animal feeding behavior / animal movement / Animal, plant and microbial ecology / Animals / Anseriformes / Anseriformes - physiology / Aquatic plants / area-restricted search / Autoecology / Aves / bayesian foragers / bewicks swans / Biological and medical sciences / Biomass / Bird migration / Birds / correlated random-walk / Current density / Cygnus / Cygnus columbianus / Cygnus columbianus bewickii / Cygnus columbianus columbianus / Demography / Density / depression / Ecosystem / energy gain / field / Food / Food movements / Foraging / Fresh Water / Fundamental and applied biological sciences. Psychology / Geography / heterogeneity / information / inter-patch distance / lakes / mathematical models / Models, Biological / Movement / Movement - physiology / Netherlands / Normal Distribution / optimal foraging / Ornithology / patchy environment / physiology / plant density / Potamogeton pectinatus / Potamogetonaceae / Potamogetonaceae - physiology / prediction / Sago / searching behavior / Spatial distribution / spatial heterogeneity / step length / Stuckenia pectinata / Swans / Tubers / Tundra / Tundra Swan / Tundras / turning angle / Vertebrata / Vertebrates: general zoology, morphology, phylogeny, systematics, cytogenetics, geographical distribution / wildlife food habits