Abstract
North American populations of aerial insectivorous birds are in steep decline. Aerial insectivores (AI) are a group of bird species that feed almost exclusively on insects in flight, and include swallows, swifts, nightjars, and flycatchers. The causes of the declines are not well understood. Indeed, it is not clear when the declines began, or whether the declines are shared across all species in the group (e.g., caused by changes in flying insect populations) or specific to each species (e.g., caused by changes in species’ breeding habitat). A recent study suggested that population trends of aerial insectivores changed for the worse in the 1980s. If there was such a change point in trends of the group, understanding its timing and geographic pattern could help identify potential causes of the decline. We used a hierarchical Bayesian, penalized regression spline, change point model to estimate group-level change points in the trends of 22 species of AI, across 153 geographic strata of North America. We found evidence for group-level change points in 85% of the strata. Change points for flycatchers (FC) were distinct from those for swallows, swifts and nightjars (SSN) across North America, except in the Northeast, where all AI shared the same group-level change points. During the 1980s, there was a negative change point across most of North America, in the trends of SSN. For FC, the group-level change points were more geographically variable, and in many regions there were two: a positive change point followed by a negative change point. This group-level synchrony in AI population trends is likely evidence of a response to a common environmental factor(s) with similar effects on many species across broad spatial extents. The timing and geographic patterns of the change points that we identify here should provide a spring-board for research into the causes behind aerial insectivore declines.
Highlights
Avian aerial insectivores are declining, and the causes of these declines are poorly understood
Further study into the mechanism behind these changes in Aerial insectivores (AI) trends should help researchers understand the causes of aerial insectivore population declines
Our results show that there have been numerous change points in the population trends of aerial insectivorous birds and that many change points were shared across most or all species in the group
Summary
Avian aerial insectivores (i.e., swallows, swifts, nightjars, and flycatchers; hereafter AI) are declining, and the causes of these declines are poorly understood. Synchronous Change Points in Aerial Insectivore Trends term population decline for AI are similar or greater than declines in any other group of birds in North America [1]. For two regions of eastern Canada, population trends for many AI species became may have become more negative during the 1980s, i.e., the declines began or intensified during the 1980s [2]. If the declines intensified or were initiated in the 1980s, it would imply a group-level change point in population trends: that is, a change in population trends synchronized across many or all species in the group. All, AI species shared a common change point in their population trends, it suggests some common ecological or environmental cause behind the group’s decline, and that the timing and geographic patterns of that change point could help to identify potential causal factors in the group’s declines
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