Body reserves and unpredictable breeding conditions in the Eurasian kestrel, Falco tinnunculus

Organisms exposed to unpredictable feeding or thermal conditions are expected to carry higher levels of body reserves than organisms facing more predictable conditions. To test this prediction we studied patterns of body mass and mass changes of Eurasian kestrels, Falco tinnunculus, breeding under unpredictable conditions in western Finland, and compared our results with data from kestrel populations that breed under more predictable conditions in central and western Europe. In contrast to expected patterns, Finnish female kestrels were found to be significantly lighter during the incubation and nestling periods than conspecifics in central and western Europe, suggesting lower body reserve levels during breeding. Similarly, male kestrels in our study area were lighter during the nestling time compared to the other populations. Opportunities for obtaining body reserves may be limited in our kestrel population due to its migratory habit, harsh spring climate, and a short vegetative season within which to complete breeding. No obvious relationship between female body reserves and hatching or fledging success was found.