Age-Specific Survival in Breeding Black-Capped Chickadees (Parus atricapillus)

-I followed the survival of 153 Black-capped Chickadees (Parus atricapillus; 68 males, 85 females), which had bred at least once in or near my study area. All were of known age, and had been banded during a 10-year period, (1980-1981 through 1989-1990). Male breeders lived an average of 3.19 ? 1.85 years (n = 68), while female breeders averaged 2.53 ? 1.98 years (n = 85). Most of this difference was due to differential mortality during and immediately after their first breeding season; female breeders suffered a significant peak in mortality during this period, while males showed no particular peaks in mortality associated with any given season or age class. Two factors had a significant effect on female survival in their first breeding season: high initial rank in the previous winter's nonbreeding flocks; and being mated to a male with prior breeding experience. Moreover, oversummer mortality of young females mated to inexperienced males was significantly higher in years with cold spring weather (mean low temperature in May 7?C or below) than in years with warmer spring weather (mean low in May > 7C). This suggests that inexperienced males may provide less food for their mates during egg laying and incubation than do experienced males, and that the effect on females of lower provisioning by inexperienced males is worsened by harsh weather conditions. The relationship between provisioning level and male experience also may carry over to the nestling period. The cost to a female of producing a particular brood size will, therefore, vary both with her own status and with the degree of experience of her mate. Hence, the relative cost of increased fecundity should vary from population to population, depending on the local proportion of young females and experienced breeding males. Received 18 July 1994, accepted 4 January 1995. AN ENORMOUS literature exists on the possible costs of avian reproduction (e.g. Partridge and Harvey 1985, Gustafsson and Sutherland 1988, Pettifor et al. 1988, Linden and M0ller 1989, Nur 1990). Evidence of reproductive cost can be derived from several approaches. One is foodsupplementation studies; for example, female Mountain Bluebirds (Sialia currucoides) that received no supplemental food during the nestling period lost an average of 3 g during that period, while supplemented females showed no such loss (Garcia et al. 1993). Another very effective approach involves clutch or brood manipulation (Hegner and Wingfield 1987, Korpimaki 1988, Dijkstra et al. 1990). For example, European Kestrels (Falco tinnunculus) that received artificially increased broods showed lower nestling and parental survivorship than those with unchanged or smaller broods (Dijkstra et al. 1990). However, these studies are not easily interpreted and can lead to conflicting results. Nur (1984, 1988) published extensively on reproductive costs as shown by brood manipulations on Blue Tits (Parus caeruleus) and concluded that reproduction imposes significant costs, especially on females. However, Pettifor (1993) reanalyzed Nur's data, along with three additional years of data on brood manipulations, and found no strong evidence of a cost of reproduction in Blue Tits, measured either as an increase in mortality or a decrease in fecundity of parents given additional young to rear. This apparent contradiction can be explained by the fact that various factors can serve to mask reproductive costs. One such factor is seasonal variation; in studies conducted under particularly favorable conditions, reproductive costs might well be masked. This is best countered by long-term studies (e.g. Perrins and McCleery 1989), or by comparing responses in moreversus less-favorable habitats (e.g. Kluyver 1961). However, if certain age classes within a breeding population are less capable of bearing reproductive costs, the age structure of the breeding population itself might affect whether a study of that population would reveal such costs. Reproductive costs also can be masked by phenotypic optimization (Perrins and Moss 1975, Pettifor et al. 1988, Pettifor 1993). Finally, an-