Breeding Lifespan and Nest Predation Determine Lifetime Production of Fledglings by Male Merlins Falco columbarius

Components of variance in lifetime production of fledglings were determined for 92 male merlins Falco columbarius that bred in north Sweden up until 1994. Nest predation and breeding lifespan were the most important components of variance in lifetime production of fledglings. Owing to nest predation, 20% of the males failed to produce any fledglings at all. The nest predation frequency was higher before than after the eggs had hatched, and higher among pairs that commenced laying late than among other pairs. Breeding experience did not reduce the nest predation risk but inexperienced as well as experienced breeders could fail due to predation on nest contents. Males with a long breeding lifespan produced more fledglings because a long lifespan increased the number of successful breedings. These results agree with previous observations that nest defence by the parents improves annual offspring production in merlins.