AGE AND REPRODUCTION IN SAVANNA SPARROWS AND TREE SWALLOWS

1. Compared to older females, 1-year-old Savannah sparrows (Passerculus sandwichensis) and tree swallows (Tachycineta bicolor), studied over seven breeding seasons on Kent Island, New Brunswick, Canada, laid eggs later in the season, had smaller clutches, and produced fewer surviving offspring. 2. To determine why young birds have lower reproductive success than older birds, we induced birds of different ages to replace clutches under the same conditions by removing clutches in an experiment simulating nest predation. 3. In both species, yearlings produced eggs similar in size to those of older females, but they laid fewer eggs per clutch in both first and replacement clutches than older birds. Yearling Savannah sparrows took more time to replace their clutches and lost more mass than older females. Differences were not significant in tree swallows because only three 1-year-old experimental females replaced their clutches. 4. Replacement clutches were larger than first clutches in Savannah sparrows and mean egg size increased between clutches, outcomes not expected had there been a major physiological cost of reproduction. Tree swallows showed a decline in clutch size and no change in mean egg size between clutches. Possibly Savannah sparrows lay their first clutch earlier than optimal in terms of clutch and egg size in order to leave time to replace failed clutches, to lay a second clutch after their first brood fledges, or to coordinate fledging (rather than egg-laying) with periods of food abundance. 5. The results of this experiment suggest that the higher reproductive success of older birds is due to improvement of reproductive performance with age and experience, rather than higher survivorship of successful breeders or increased reproductive effort. Age-specific reproduction was not an artefact of differential mortality of inferior breeders: birds that laid early in the season or produced large clutches were no more likely to survive than less successful breeders. Yearlings did not appear to withhold reproductive effort nor did older birds seem to invest more in reproduction, although the failure of some yearling tree swallows to replace their lost clutches provided some support for age-specific differences in reproductive effort. Constraint, rather than restraint, most probably explains the relatively poor reproductive success of yearlings.