Spatiotemporal Patterns in Nest Box Occupancy by Tree Swallows Across North America

Abstract

ABSTRACT. Data from the North American Breeding Bird Survey (BBS) suggest that populations of aerial insectivorous birds are declining, particularly in northeastern regions of the continent, and particularly since the mid-1980s. Species that use nest boxes, such as Tree Swallows (Tachycineta bicolor), may provide researchers with large data sets that better reveal finer-scale geographical patterns in population trends. We analyzed trends in occupancy rates for ca. 40,000 Tree Swallow nest-box-years from 16 sites across North America. The earliest site has been studied intensively since 1969 and the latest site since 2004. Nest box occupancy rates declined significantly at five of six (83%) sites east of -78° W longitude, whereas occupancy rates increased significantly at four of ten sites (40%) west of -78° W longitude. Decreasing box occupancy trends from the northeast were broadly consistent with aspects of a previous analysis of BBS data for Tree Swallows, but our finding of instances of increases in other parts of the continent are novel. Several questions remain, particularly with respect to causes of these broadscale geographic changes in population densities of Tree Swallows. The broad geographic patterns are consistent with a hypothesis of widespread changes in climate on wintering, migratory, or breeding areas that in turn may differentially affect populations of aerial insects, but other explanations are possible. It is also unclear whether these changes in occupancy rates reflect an increase or decrease in overall populations of Tree Swallows. Regardless, important conservation steps will be to unravel causes of changing populations of aerial insectivores in North America.RÉSUMÉ. Les données provenant du Relevé des oiseaux nicheurs (BBS) de l'Amérique du Nord semblent indiquer que les populations d'insectivores aériens sont en déclin, particulièrement dans les régions du nord-est du continent et depuis le milieu des années 1980. Grâce aux espèces qui utilisent les nichoirs, comme l'Hirondelle bicolore (Tachycineta bicolor), les chercheurs ont accès à une grande quantité de données qui révèlent davantage les tendances des populations à l'échelle fine. Nous avons analysé la tendance du taux d'occupation par l'Hirondelle bicolore à partir de 40 000 nichoirs-années provenant de 16 sites répartis en Amérique du Nord. Le site le plus ancien est suivi intensivement depuis 1969 et le plus récent, depuis 2004. Le taux d'occupation des nichoirs a diminué significativement dans 5 des 6 sites (83 %) situés à l'est du 78° de longitude ouest, tandis qu'il a augmenté significativement dans 4 des 10 sites (40 %) à l'ouest du 78° de longitude ouest. La tendance à la baisse de l'occupation observée dans le nord-est concorde dans les grandes lignes avec les résultats d'une analyse antérieure des données du BBS pour cette espèce, mais les hausses que nous avons observées dans d'autres régions de l'Amérique du Nord s'avèrent nouvelles. Plusieurs interrogations demeurent, notamment en ce qui a trait aux causes de ces chang...

Severe recent decrease of adult body mass in a declining insectivorous bird population

“…Tree swallow populations in northeastern North America are experiencing a welldocumented decline, similar to that noted in several aerial insectivores, but in sharp contrast with their stable or even increasing numbers in the western part of the continent [4,18]. Data from the Breeding Bird Survey (BBS) [19] for the province of Québec illustrate this regional decline (figure 1a).…”

No effect of insect abundance on nestling survival or mass for three aerial insectivores

“…The concept of a common driver for aerial insectivore declines, in this case, insect abundance, is appealing for conservation efforts because, presumably, if the driver was addressed it would benefit multiple species. However, despite sampling in an area where population declines for all three species are particularly steep (Nebel et al 2010, Shutler et al 2012, Michel et al 2016, insect abundance did not affect swallow breeding success. The effect of insect abundance on aerial insectivore breeding success may be constrained to areas experiencing land use changes, e.g., loss of livestock and agricultural intensification, which, in addition to weather conditions, may also reduce insect abundance (Møller 2001, Ambrosini et al 2002, Ghilain and Bélisle 2008.…”

“…Insect availability has been suggested as a possible explanation for declines in aerial insectivores, including swallows, across North America (Nebel et al 2010, Shutler et al 2012. The concept of a common driver for aerial insectivore declines, in this case, insect abundance, is appealing for conservation efforts because, presumably, if the driver was addressed it would benefit multiple species.…”

“…The most obvious common factor is that these species all rely on aerial insects for food. Therefore it has been hypothesized that reductions in insect abundance may be driving these trends (Nebel et al 2010, Shutler et al 2012. These reductions could be the result of a mistiming between peak insect abundance and offspring demands.…”

Trade-offs in the early development of tree swallows (Tachycineta bicolor): offspring characteristics, offspring quality, and parental care

“…It is also unclear if this means that the actual number of tree swallows in North America is declining overall, or if there are declines in other less-easily studied species of aerial insectivores, such as bats (Shutler et al 2012). Population sizes of tree swallows in my study areas have increased in the past decade, but populations in eastern North America have declined over the past several decades (Shutler et al 2012;Paquette et al 2014). It is therefore important to gather as much …”