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Biol Lett. Jun 23, 2009; 5(3): 302–305.
Published online Mar 18, 2009. doi:  10.1098/rsbl.2008.0755
PMCID: PMC2679912
Stopover decision during migration: physiological conditions predict nocturnal restlessness in wild passerines
Leonida Fusani,1* Massimiliano Cardinale,2 Claudio Carere,3 and Wolfgang Goymann4
1Dipartimento di Biologia ed Evoluzione, Università di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
2Institute for Marine Research, Swedish Board of Fisheries, 453 21 Lysekil, Sweden
3Department of Theoretical Biology, University of Groningen, 9751 NN Haren, The Netherlands
4Max-Planck-Institut für Ornithologie, Abteilung für Verhaltensneurobiologie, 82319 Seewiesen, Germany
*Author for correspondence (leofusani/at/
Received December 12, 2008; Accepted February 20, 2009.
During migration, a number of bird species rely on stopover sites for resting and feeding before and after crossing ecological barriers such as deserts or seas. The duration of a stopover depends on the combined effects of environmental factors, endogenous programmes and physiological conditions. Previous studies indicated that lean birds prolong their refuelling stopover compared with fat birds; however, the quantitative relationship between physiological conditions and stopover behaviour has not been studied yet. Here, we tested in a large sample of free-living birds of three European passerines (whinchats, Saxicola rubetra, garden warblers, Sylvia borin and whitethroats, Sylvia communis) whether the amount of migratory restlessness (Zugunruhe) shown at a stopover site depends on physiological conditions. 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 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 plays a major role in determining the stopover duration in migratory birds.
Keywords: bird, migration, stopover, migratory restlessness, Zugunruhe
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