A Review: Alternative Nesting Structures for Barn Swallows

Barn Swallows are closely associated with humans, and rely on human-made structures (barns, sheds, outbuildings) for nesting habitat. Natural nesting sites are very rare. In North America, there are accounts of Barn Swallows nesting in native dwellings in the early 1800s (Macoun and Macoun 1909). Accounts pre-dating this time are lacking. Recent research finds a close evolutionary relationship between humans and Barn Swallows (Smith et al. 2018).

Barn Swallow populations have declined by ~50% since the 1960s across North America (Sauer et al. 2017). Population declines were originally attributed to habitat loss—the decline of farming, and loss of nesting sites, loss of pastureland and associated livestock, agricultural intensification, and the regrowth of forests. Within the last 20 years however, population declines have become alarming, particularly in northeastern North America. Factors responsible for more recent declines are largely unknown, but are thought to be related to changes in insect prey abundance, pesticide use, climate change, loss of nesting structures and foraging habitat, and threats along migration routes and on non-breeding grounds. Of all of the factors thought to be contributing to declines, one that we can directly address is loss of nesting and foraging habitat.

Barn Swallows were assessed by the Committee on Status of Endangered Wildlife in Canada as Threatened in 2011. In 2017, they were added to the federal Species at Risk Public Registry as Threatened and thus were officially protected by the Species at Risk Act (SARA) in Canada. They had previously been list as Threatened in the province of Ontario in 2012, and Endangered in Nova Scotia in 2013. In Ontario, this has led to efforts to mitigate the loss of nesting habitat (e.g., barns) by creating alternative nesting structures. Guidelines were produced by the Ontario Ministry of Natural Resources and Forestry for those fulfilling requirements of an authorization or regulatory provision under the Endangered Species Act. In 2013, the Ontario Ministry of Transportation erected kiosk-like nesting structures complete with artificial nest cups for Barn Swallows along highways where work on bridges and culverts was disrupting nesting Barn Swallows. Two types of structures were tried; large, with 64 nest cups and small, with 12 nest cups. A second, similar project was tried along another highway a few years later. Bird Studies Canada has initiated similar projects, also in Ontario, at sites where older barns and other structures that hosted Barn Swallow colonies were being taken down or birds were being excluded from nesting habitat for other reasons. At one site in Townsend Ontario, where a barn that hosted approximately 25 pairs was torn down, an engineering firm experimented with two nesting structures designed by a biologist at the firm. The structures were 8 feet wide and 20 feet long. During the three years they were tested they hosted between them 5 nesting pairs in 2016, ~8 pairs in 2017, and ~5 in 2018.

In spite of these efforts, alternative nesting structures for Barn Swallows have had poor success, hosting at most a few pairs and often they are not used at all. They have not been shown to mitigate the loss of nesting habitat provided by larger structures, such as barns. In one study, when social cues (Barn Swallow vocalizations and decoys) were tested as a way to attract more pairs to the structures, they did not result in increased use.

Habitat selection in Barn Swallows is poorly understood. What appears to be “extra space” in large structures may in fact convey thermal protection by providing more stable temperatures; smaller structures generally have increased air flow and cooler temperatures. As strict insectivores, this has important implications for Barn Swallows—during colder weather they spend less time brooding young and more time foraging. Moreover, according to recent research, the “extra space” in large structures also may provide roosting areas at night (T. Imlay pers. comm.). Fledglings are seen perching at various locations inside barns for several days after leaving nests. At a large colony of 37 pairs nesting in a 22,000 square foot building, adults are seen flying throughout the entire structure, not just to and from their nests, during the nesting season. Other research has found that Barn Swallow colonies shift location within a larger structure over time. Additionally, for Barn Swallows nesting in colonies, the extra space may provide visual barriers between nests. Even when Barn Swallows nest in close proximity, nests almost always have different approaches (Snapp 1979). A study in Europe found that colony size increased with livestock farming and was larger on farms with traditional stables compared with farms with stables of other types (Ambrosini et.al. 2002). Another European study found that both the microhabitat of the nest site and the macrohabitat of the foraging grounds were associated with annual reproductive output (Grüebler et. al. 2010).

It appears that both large and small colonies are important to the integrity of Barn Swallow populations within a given area. Large colonies both produce young that colonize smaller colonies, and attract first-year breeders, while generally, more experienced breeders nest singly (Shields and Crook 1987) Therefore, it would appear that the best way to maintain an adequate supply of nesting habitat to support Barn Swallow conservation across broad geographical areas is to make efforts to save the larger structures that host large colonies, and utilize alternative structures at sites that provide suitable foraging habitat, but where no nesting structure is present.