by Raleigh J. Robertson Department of Biology, Queen’s University, Kingston, Ontario K7L 3N6 and Jason Jones
Spatial and demographic effects on Tree Swallow nest quality and reproductive success
The reproductive success of Tree Swallows (Tachycineta bicolor) is affected by a number of factors. For populations nesting in natural cavities, ecological factors such as cavity volume, predation and nest usurpation are often the primary determinants of reproductive success (Rendell and Robertson 1989; Robertson and Rendell 1990; Robertson et al. 1992). For Tree Swallows occupying nest-boxes, these factors are usually not important because the researcher’s use of predator guards almost eliminates predation, use of uniformly-sized boxes controls for effects of cavity volume on clutch size, and boxes with carefully designed entrance dimensions reduce nest usurpation by competitor species such as European Starlings (Sturnus vulgaris), Common Grackles (Quiscalus quiscula) and Northern Flickers (Colaptes auratus) (Rendell and Robertson 1991). However, in nest-box studies, other factors can influence reproductive success either directly or potentially through an effect on nest quality. Some researchers (e.g., McCarty and Secord 1999a, b) have evaluated the potential effects of exposure to chemicals in the environment on nest quality and/or reproductive success in Tree Swallows. However, there are other variables that can have a major impact on these endpoints. Inter-nest spacing, proximity to forest edge, timing of settlement and nest-building, availability of nesting material, history of the nest-box or grid, and age of the breeding pair could all affect nest quality and/or reproductive success: (i) Inter-nest spacing may affect nest-building and productivity since competitive interactions become more intense when boxes are close together (Muldal et al. 1985;
Tree Swallow nest quality & reproductive success page 3 Mitchell and Robertson 1996), so birds may spend more time in nest-box defense, and less time in other nesting activities. (ii) Proximity to edge of an open area, where shrubs or trees are close to the nest-box, may have an effect since House Wrens (Troglodytes aedon), which occupy shrub habitat, may interfere with Tree Swallow nesting (Rendell and Robertson 1990). (iii) A trend of reduced breeding success as the season progresses has been established (Stutchbury and Robertson 1988); hence, settlement date and/or the time of nestbuilding may also affect quality of the nest, and reproductive success. (iv) The type and availability of nesting material may also affect nest quality, hatching success and fledging success. The type of nesting material will likely vary from site to site, as will the availability of feathers to use as nest lining (usually waterfowl feathers). The type and number of feathers probably vary as a function of wind exposure, whether there is flowing or standing water, and the local population of waterfowl or other birds. Hence, availability of nesting materials may affect nest quality and in turn, reproductive success. (v) The history of an individual nest-box within a grid, or of the entire nest-box grid, may influence the quality of nests and potentially reproductive success. Since birds often return to the same box to breed, the history of a given box (i.e. old or new) may be related to the likelihood of being occupied by an experienced female or pair, which in turn may affect nest quality and/or reproductive success. Also, a grid with a long history is likely to have a stable population with many returning birds, compared with a newly established grid, which would likely have a younger age distribution, plus more breeders new to the study site. (vi) Finally, female age may have an effect on nest quality and/or reproductive success, with younger, inexperienced females expected to build less substantial nests, and also to have reduced reproductive success (Stutchbury and Robertson 1988). Similarly, male age could have an effect through the male’s ability to compete for nest-lining feathers, or his ability to assist with feeding nestlings. Unless all of these factors are carefully controlled, it is difficult to pinpoint a single causal factor as being responsible for any differences between study sites in either nest quality or reproductive success. Moreover, it is not clear that nest quality affects reproductive success in Tree Swallows. Several studies that have evaluated this issue (e.g., Winkler 1993; Lombardo et al. 1995; McCarty and Secord 1999a) have reached varying results. The purpose of this study was to determine the effects of (i) inter-nest spacing, (ii) proximity to forest edge, (iii) settlement and nest-building date, (iv) availability of nesting material, (v) history of the nest-box, and nest-box grid and (vi) female and male age, on both nest quality and reproductive success of the Tree Swallow. In addition, this study evaluated the impact of nest quality on reproductive success. METHODS We conducted this study using existing grids of nest-boxes, as well as adjacent, newly established grids, at the Queen’s University Biological Station (QUBS), 50 km north of Kingston,
