Abstracts & Contributions

Physiological conditions predict nocturnal restlessness at a stopover site in wild migratory passerines

Leonida Fusani1, Wolfgang Goymann2

1Department of Biology and Evolution, University of Ferrara, 44100 Ferrara, Italy, leofusani@gmail.com;  2Max-Planck-Institut for Ornithology, D-82319 Seewiesen, Germany, goymann@orn.mpg.de.

During migration a number of bird species rely on stopover sites for recovering and replacing their energy reserves before and after crossing ecological barriers such as desert regions or seas. The duration of the stopover depends on a series of factors including environmental and physiological ones. Previous studies indicated that lean birds prolong their refuelling stopover compared to fat birds, however the quantitative relationship between physiological conditions and stopover behaviour had not been studied yet. We studied whether the amount of migratory restlessness (Zugunruhe) shown at a stopover site depends on physiological conditions in three passerine species, the whinchat Saxicola rubetra, the garden warbler Sylvia borin, and the whitethroat Sylvia communis). A large sample of migrating birds of these three species was caught at a ringing station on the island of Ponza, Italy. After registering a series of morphological and physiological variables, the birds were set in custom-made individual cages with water and food. The cages were equipped with infra-red locomotor activity recorders. The birds were released the following morning. An integrated measure of condition based on body mass, amount of subcutaneous fat, and thickness of pectoral muscles, strongly predicted the intensity of Zugunruhe shown in the recording cages in the night following capture. These results provide novel and robust quantitative evidence in support of the hypothesis that the amount of energy reserves determines the stopover duration in migratory birds.