JOURNAL OF AVIANBIOLOGY 27: 164-170. Copenhagen 1996 Age, arrival date and reproductive success of male American Redstarts Setophaga ruticilla G. A. Lozano, S. Perreault and R. E. Lemon Lozano, G. A., Perreault, S. and Lemon, R. E. 1995. Age, arrival date and reproductive success of maleAmerican Redstarts Setophaga ruticilla. - J. AvianBiol. 27: 164-170. We testedwhether the age-related differences in reproductive success in male Ameri- can Redstarts arecaused by differences in date of arrival on the breeding grounds. By relating arrival date to threemeasures of reproductive success we determined what stage of the breeding cycle is affected by date of arrival. Adult male redstarts were moresuccessful than subadults at obtaining mates. Adultmales were also more likely than subadults to have nests built in their territories, and to fledgeyoung. Among mated males, neither age class was significantly more likely to obtainnests. However, among males that nested, there were significant age-related differences in fledging success. Adult malesarrived on the breeding grounds beforesubadults. Within both age classes, malesthat mated had arrived significantly earlier thanthose that did not mate. Similarly, malesthat nested had arrived significantly earlier thanthose that did not. Therewereno significant differences in arrival date between malesthat fledged young and those that did not. Finally, adultmales were more likely to renestafter nest failure. Therefore, we concludethat reproductive successin male redstarts is independently affected by both arrival date and age, and that the lower successof subadults may not be a consequence solely of their age and late arrival, but also of their lack of experience in selecting territories early in the breeding season. G. A. Lozano (correspondence), S. Perreault and R. E. Lemon, Department of Biology, McGill University, 1205 Dr. Penfield Ave., Montreal, Quebec, Canada H3A JBI. E-mail: Lozano@biol.lan.McGill.ca In many birds reproductive success increases with age (e.g., Perrins and Moss 1974, Afton 1984, Smith 1993, Black and Owen 1995, Pugesek 1995). Three general explanations have been proposed to interpret this phe- nomenon. First, the correlation between age and repro- ductive success may be a statistical consequence of poor breeders also having lower survivorship (Curio 1983). This explanation is refuted by the finding in several species that individuals increase their reproductive suc- cess in successive reproductive bouts (e.g., Pyle et al. 1991, Forslund and Part 1995). Second, the restraint hypothesis, based on the theoretical trade-off between current and future reproduction (Williams 1966, Pianka and Parker 1975), predicts that current reproduction is more valuable for older individuals because they have a lower residual reproductive value (Curio 1983). Older individuals are therefore more apt to put greater effort into reproduction. Finally, the constraint hypothesis argues that younger individuals cannot reproduce as well as older ones because they are at a disadvantage in obtaining breeding opportunities or raising offspring. The two latter hypotheses are not mutually exclusive, and separating them can be difficult because it requires finding out whether younger individuals are unable or unwilling to reproduce as well as older ones. The restraint hypothesis is based on life-history the- ory; thereby, age per se is used to explain age-related differences in reproductive success. On the other hand the constraint hypothesis predicts that younger individ- uals are disadvantaged when compared to older indi- viduals at one or several stages of the breeding cycle. Therefore, when testing the constraint hypothesis it is not sufficient to show that younger birds are disadvan- taged, but we must also determine the ecological vari- C JOURNAL OF AVIAN BIOLOGY 164 JOURNAL OF AVIAN BIOLOGY 27:2 (1996)