From declines to recovery: 52 years of changes in autumn migratory songbird abundance at an island stopover site in southern New England

Migratory birds are declining across North America, and many studies have
used long-term datasets to estimate trends in abundance for migratory
birds. To most efficiently prioritize at-risk populations, it is critical
to know the current trend status of migratory species, which involves
accurately assessing the most appropriate timescale for monitoring changes
in populations. In this study, we analyzed autumn hatch-year bird banding
data from the Block Island Banding Station on Block Island, Rhode Island
over a 52-year period (1970-2021) to examine long-term abundance trends
for 22 species of migratory songbirds and compared our results to trends
from Manomet Conservation Sciences and the USGS Breeding Bird Survey. We
ran four models (no change, linear, quadratic, and breakpoint model) for
each species and used model selection with AICc to determine the best
model. Eighteen of the 22 species were best represented by breakpoint
models, indicating that they experienced a sudden change in slope during
the study period. For our 18 breakpoint species, all experienced the
majority of their decline in the first two decades of the study (between
1976 and 1986), and 17 of 18 species were stable or recovering after the
breakpoint. Of the four species without a significant breakpoint, one
species had no change over the study period, two were best represented by
linear models, and one was best represented by a quadratic model. Trend
classifications varied across time, space, methods, and the geographic
regions of the sub-populations sampled. These results present evidence
that many species are currently stable or in recovery, despite initial
declines, and that modern conservation efforts need to account for abrupt
changes in trend direction within long-term time series analyses to most
accurately assess current abundance trends for the conservation of
vulnerable species.