Reduced adult survival and increased reproduction in Swedish kestrels

The decline of the Swedish kestrel population during the 1950s and 1960s is probably caused by a decrease in yearly adult survival rate, from 0.66±0.03 S.E. to 0.48±0.03 S.E.Contrary to what would be expected from generally harmful effects of pesticides on reproduction, brood size increased from 3.4±0.4 S.E. to 4.2±0.2 S.E. during the same period.We found no relationship between brood size and breeding density. The increased reproduction therefore does not seem to be a density-dependent effect of reduced population size. The increase in brood size may rather be an evolved adaptation, selected for by increased optimal reproduction as a consequence of higher adult mortality.