Parental effort of kestrels (Falco tinnunculus) in nest defense: effects of laying time, brood size, and varying survival prospects of offspring

We studied the nest defense behavior of Eurasian kestrels (Falco tinnunculus) towards a stuffed pine marten (Martes martes) through a 3-year vole cycle (1990–92) in western Finland. Survival probability of offspring decreases with a later start of breeding, and, therefore, early breeders should protect their offspring more than late ones. We found this true for males during the incubation period, but not for females. In addition, we expected the nest defense intensity to increase with offspring number. During the incubation period, this was true for females, but not for males. During the nestling phase, parents did not adjust their defense effort to natural or manipulated (by one to two young) brood size. Survival prospects of kestrel offspring were highest in the increasing vole year 1991 and lowest in the decreasing vole year 1992, and, therefore, we expected the defense activity of kestrels to follow the same trend. However, the opposite result appeared true for females with a similar tendency for males. Most hypotheses predicting avian nest defense behavior were not supported by our data. Temporally heterogeneous environment and low degree of nest-site tenacity of migratory kestrels may make them unfamiliar with environmental variation and survival prospects of their offspring. Therefore, fitness benefits of parental care are not predictable, and kestrels may thus adjust their parental effort to their own future reproductive potential (i.e., number of future breeding attempts), rather than to some current investment indicator, like offspring age and number.