Food availability and fuel loss predict Zugunruhe

Eikenaar, Cas; Bairlein, Franz
January 2014
Journal of Ornithology;Jan2014, Vol. 155 Issue 1, p65
Academic Journal
Migrating birds spend most of their time at stopover sites where they replenish the fuel used during flight, termed refueling. The overall time of migration thus largely depends on the duration of stopovers, and factors shaping stopover duration therefore are of interest. A handful of field studies have shown that the likelihood of departure from stopover sites increases with poor feeding conditions. However, food availability and stopover duration are generally difficult to quantify accurately in the field. Results of fasting-refueling experiments on captive birds using migratory restlessness (Zugunruhe) as a proxy for departure likelihood are mixed. Although Zugunruhe usually decreased with refueling, fasting often failed to increase Zugunruhe. In addition, some experiments lacked randomization. In a fasting-refueling experiment on Northern Wheatears ( Oenanthe oenanthe), using birds as their own control in a randomized fashion, we found that fasting increased Zugunruhe, whereas refueling decreased Zugunruhe. These results show that the motivation to migrate, quantified by Zugunruhe, is affected by changes in food availability. Furthermore, Zugunruhe during refueling did not depend on fuel reserves left after fasting, but tended to decrease with the amount of fuel lost during fasting. We discuss why extent of fuel loss may be a better predictor of stopover duration than fuel reserves.


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