Age and Reproduction in Savannah Sparrows and Tree Swallows

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 demales, by they laid fewer eggs per clutch in both first an replacement clutches than older birds