Strong evidence for selection for larger brood size in a great tit population

We measured the selection pressure on brood size in a recently established population of great tits (Parus major L.) in the northern Netherlands by manipulating brood size in three years (1995: n = 51, 1997: n = 66, 1998: n = 51), and we estimated fitness consequences in terms of local survival of both offspring and parents. Enlarged broods had highest fitness; the offspring fitness component was positively affected by manipulation and the parental fitness component was unaffected. Parental survival and the probability that parents produced a second clutch were not affected by the treatment. However, parents that had raised enlarged broods produced their second clutch later in the season. Clutch size, brood size, and laying date of birds recaptured in the next breeding season were largely independent of the treatment. We conclude that there is strong evidence for selection for larger brood size and reject the individual optimization hypothesis for this population because the number of young in the nest predicts fitness independently of the manipulation history.