Differences in spatial synchrony and interspecific concordance inform guild‐level population trends for aerial insectivorous birds

Abstract

Many animal species exhibit spatiotemporal synchrony in population fluctuations, which may provide crucial information about ecological processes driving population change. We examined spatial synchrony and concordance among population trajectories of five aerial insectivorous bird species: chimney swift Chaetura pelagica, purple martin Progne subis, barn swallow Hirundo rustica, tree swallow Tachycineta bicolor, and northern rough‐winged swallow Stelgidopteryx serripennis. Aerial insectivores have undergone severe guild‐wide declines that were considered more prevalent in northeastern North America. Here, we addressed four general questions including spatial synchrony within species, spatial concordance among species, frequency of declining trends among species, and geographic location of declining trends. We used dynamic factor analysis to identify large‐scale common trends underlying stratum‐specific annual indices for each species, representing population trajectories shared by spatially synchronous populations, from 46 yr of North American Breeding Bird Survey data. Indices were derived from Bayesian hierarchical models with continuous autoregressive spatial structures. Stratum‐level spatial concordance among species was assessed using cross‐correlation analysis. Probability of long‐term declining trends was compared among species using Bayesian generalized linear models. Chimney swifts exhibited declining trends throughout North America, with less severe declines through the industrialized Mid‐Atlantic and Great Lakes regions. Northern rough‐winged swallows exhibited declining trends throughout the west. Spatial concordance among species was limited, the proportion of declining trends varied among species, and contrary to previous reports, declining trends were not more prevalent in the northeast. Purple martins, barn swallows, and tree swallows exhibited synchrony across smaller spatial scales. The extensive within‐species synchrony and limited concordance suggest that population trajectories of these aerial insectivores are responding to large‐scale but complex and species‐ and region‐specific environmental conditions (e.g. climate, land use). A single driver of trends for aerial insectivores as a guild appears unlikely.

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.…”

“…Swallows, along with other aerial insectivores, i.e., birds that feed in the air during flight, are experiencing severe and widespread population declines across most of North America, particularly in the northeast (Nebel et al 2010, Smith et al 2015, Michel et al 2016. The temporal and spatial synchrony in population declines suggest that there is a common threat(s) affecting aerial insectivores (Nebel et al 2010, Smith et al 2015, but see Michel et al 2016).…”

“…The temporal and spatial synchrony in population declines suggest that there is a common threat(s) affecting aerial insectivores (Nebel et al 2010, Smith et al 2015, but see Michel et al 2016). The most obvious common factor is that these species all rely on aerial insects for food.…”

Stable Isotopes from Museum Specimens May Provide Evidence of Long-Term Change in the Trophic Ecology of a Migratory Aerial Insectivore

“…In North America, many of the bird species currently experiencing the steepest population declines are aerial insectivores that migrate long distances (Blancher et al, 2007;Nebel et al, 2010;North American Bird Conservation Initiative Canada, 2012;Smith et al, 2015). Since these species occupy a variety of breeding and wintering habitats, loss of any single habitat type cannot explain the similarities in population trajectories (Smith et al, 2015;Michel et al, 2016). In contrast, all of these species share a diet dominated by the adult life-stages of insects, and have foraging behaviors that are particularly energetically demanding and dependent on weather conditions conducive to insect flight.…”

“…Some of the sharpest population declines for avian aerial insectivores have occurred in northeastern North America (Nebel et al, 2010;Smith et al, 2015;Michel et al, 2016). Amongst these, Eastern Whip-poor-will are visual predators that rely on twilight or moonlight to detect prey, which are then captured during short bursts of flight from a perch on the ground or a branch.…”