Joseph Burant

In birds, rearing multiple broods per season can substantially increase the annual number of fledglings produced. However, the contribution of double-brooding to lifetime fitness is unclear because the number of recruits arising from single- and double-brooded females is rarely measured. Poor estimates of fitness also make it challenging to document potential trade-offs between double-brooding and survival or future reproductive output. To understand the contribution of double-brooding to lifetime fitness and whether double-brooding was associated with life-history trade-offs, we used 30 years of reproductive data on female Savannah sparrows (Passerculus sandwichensis) breeding on Kent Island, New Brunswick. Estimates of fitness included an analysis of recruitment of both F1 (first generation) and F2 (second generation) offspring from females that did and did not raise a second brood. We detected no net costs of double-brooding. Double-brooded females had higher annual apparent survival rates than single-brooded females and F1 offspring from first broods of double-brooded females were more likely to recruit into the population than F1 offspring from single-brooded females. Double-brooding also improved lifetime fitness. Recruitment of F1 offspring was positively related to the number of seasons that a female double-brooded and, as a result, there was a higher number of F2 recruits from F1 offspring arising from double-brooded females than from F1 offspring arising from single-brooded females. Our results provide strong evidence that double-brooding is a beneficial reproductive strategy for Savannah sparrows and suggests that double-brooding females are likely high-quality individuals capable of rearing two broods a season with no net fitness costs.

The size and shape of an animal’s breeding territory are dynamic features influenced by multiple intrinsic and extrinsic factors and can have important implications for survival and reproduction. Quantitative studies of variation in these territory features can generate deeper insights into animal ecology and behavior. We explored the effect of age, breeding strategy, population density, and number of neighbors on the size and shape of breeding territories in an island population of Passerculus sandwichensis (Savannah Sparrow). Our dataset consisted of 407 breeding territories belonging to 225 males sampled over 11 years. We compared territory sizes to the age of the male territorial holder, the male’s reproductive strategy (monogamy vs. polygyny), the number of birds in the study population (population density), and the number of immediate territorial neighbors (local density). We found substantial variation in territory size, with territories ranging over two orders of magnitude from 57 to 5,727 m² (0.0057–0.57 ha). Older males had larger territories, polygynous males had larger territories, territories were smaller in years with higher population density, and larger territories were associated with more immediate territorial neighbors. We also found substantial variation in territory shape, from near-circular to irregularly shaped territories. Males with more neighbors had irregularly shaped territories, but shape did not vary with male age, breeding strategy, or population density. For males that lived 2 years or longer, we found strong consistent individual differences in territory size across years, but weaker individual differences in territory shape, suggesting that size has high repeatability whereas shape has low repeatability. Our work provides evidence that songbird territories are highly dynamic, and that their size and shape reflect both intrinsic factors (age and number of breeding partners) and extrinsic factors (population density and number of territorial neighbors).