INCREASED DAILY WORK PRECIPITATES NATURAL DEATH IN THE KESTREL

1. Costs of reproduction have been assessed experimentally by measurin g subsequent survival and reproduction of parent animals raising enlar ged and reduced numbers of offspring. Reported effects on survival hav e so far always referred to local survival of marked individuals in th e study population. They do not provide definitive proof of a cost of reproduction, since reduced local survival may be due either to reduce d survival or to an increased tendency to emigrate from the study area . Therefore, it is important to assess mortality rates in connection w ith brood size experiments. 2. We report an analysis of the time of de ath in 63 cases where kestrels, Falco tinnunculus L. had raised broods of manipulated size and were subsequently reported freshly dead. 60% of the parents raising two extra nestlings were reported dead before t he end of the first winter, compared to 29% of those raising control o r reduced broods. This result confirms our interpretation of the manip ulation effects on local survival as due to mortality rather than emig ration. The extra mortality occurred in the winter following the brood enlargement. 3. Kestrel parents in these experiments have been shown to adjust their daily energy expenditure to the modified brood size. I ncreased parental effort in this species thus entails an increased ris k of death half a year later.