Ross, Howard A. 1980. The reproductive rates of yearling and older Ipswich Sparrows, Passerculus sandwichensis princeps. Canadian J of Zoology, 58: 1557-1563.
Summary (AGH): Songbirds breeding in their first year often don’t reproduce as well as older birds, perhaps because they are less experienced or because they put more effort into survival rather than reproduction. In Ipswich Sparrows, older adults produce more nestlings than first-time breeders. Although they were slightly better at successfully rearing young from the egg stage, the main reason older adults produced more young is that they nested more often each year and laid more eggs in each nest. Moreover, they started to breed sooner in the season and produced subsequent nests in more rapid succession. Along with Ross’s other studies of reproduction in Ipswich sparrows, this study highlights important differences in how younger and older birds reproduce, which, when combined with knowledge of their overwinter survival rates, help us understand how well the population can survive setbacks, such as harsh winters.