Population decline and the threat of extinction are realities currently facing many species. Yet, in most cases, the detailed demographic data necessary to identify causes of population decline are unavailable. Using 43 years (1975−2017) of data from a box-nesting population of tree swallows (Tachycineta bicolor), we identified reduced survival of offspring as a probable demographic cause of population decline. Poor fledging success was associated with increased predation and poor weather conditions during early nestling development. Low juvenile survival and subsequent recruitment was linked to poor weather conditions during the post-fledging period and may also be linked to conditions on the wintering grounds. Regional weather conditions during critical stages of breeding (early nestling and post-fledging) have become progressively worse over the 43-year study period. None of the other factors linked to offspring survival have similarly deteriorated. Overall, our results suggest tree swallows should be added to the growing list of species challenged by climate change, and that other species of aerial insect specialists may face similar impacts of climate change.
Tree swallows breed across much of North America in secondary cavities pre-existing holes in trees but preferentially breed in artificial nest boxes when available (Robertson and Rendell 1990). Highly philopatric, tree swallows return year after year to breed in the same sites (Winkler et al. 2004, 2005), with adult females breeding an average of 1.4 times (range 1−9) in our study population. During the breeding season, they exclusively forage on and provision their offspring with aerial insects (Winkler and Allen 1996). After breeding, tree swallows migrate to the southern U.S., Mexico, and Central America, where they spend the winter roosting in flocks of hundreds of thousands of birds (Robertson et al. 1992; Winkler 2006).
We monitored a box-nesting population of tree swallows at the Queen’s University Biological Station in southeastern Ontario, Canada (44.5212°N, 76.3854°W) from 1975 to 2017. Nest boxes were arranged in grids over open areas or along roadsides, close to wetlands and lakes, which is the species’ preferred foraging habitat, as they rely heavily on insects with aquatic larvae, particularly for feeding their offspring. The spacing between boxes approximated their preferred distribution of natural nest sites (Robertson and Rendell 1990). Since the 1980s, first stovepipe and then cone-shaped skirt predator guards have protected nest boxes from predation, with limited success. Unfortunately, we did not record what type or when individual guards were installed. However, we would expect our ever-improving anti-predator defenses to only decrease predation rates, increasing overall population success.
Boxes were visited every 1−3 days during the breeding season, from late April or early May through July. Tree swallows lay one egg per day until incubation, so we recorded the date of laying of the first and last egg as well as the total number of eggs (clutch size). We also monitored hatch date and the number of nestlings that hatched. Nestlings either successfully fledged (survived to depart from the nest), died in the nest, or disappeared from the nest prior to the age of possible fledging (~ 21 days, recorded as a predation event). We recorded the date nesting attempts ended due to fledging or failure and the cause of failure (i.e., predation or death of nestlings in the nest, likely due to starvation and/or abandonment by parents).
During the breeding season, we banded adults and all nestlings that survived to 12 days old with uniquely numbered Canadian Wildlife Service leg bands. We identified returning adults by band number and banded new immigrants (63 ± 16% of females captured were unbanded new breeders). Immigration rates may account for local apparent population dynamics (i.e., whether box occupancy has gone up or down), but because this species is philopatric and immigrants are most likely from nests in nearby natural cavities (Winkler et al. 2004, 2005), immigration rates are likely influenced by the same factors affecting fledging success and overwinter survival in the nest box population and thus are unlikely to contribute distinctly to declines in AAIs. Adults were sexed according to the presence of a cloacal protuberance (male) or brood patch (female). Tree swallows display delayed plumage maturation in females, but not males; 1-year-old females have brown body feathers, while older females have iridescent blue-green plumage, allowing us to classify females as 1-year-old or older (Stutchbury and Robertson 1987).