Dramatic Declines of Evening Grosbeak Numbers at a Spring Migration Stop-Over Site

Authors: W. Douglas Robinson, Jessica Greer, Juliana Masseloux, Tyler A. Hallman, and Jenna R. Curtis

Year: 2022

Publication: Diversity

Publication Link: https://www.mdpi.com/1424-2818/14/6/496

Keywords: biodiversity benchmarks; Coccothraustes vespertinus; Evening Grosbeak; American elm; Dutch elm disease; migratory stopover; bird population decline; snapshot surveys; spruce budworm; Ulmus americana

Abstract: Evening Grosbeak (Coccothraustes vespertinus) populations have been hypothesized to
be in steep decline across North America. Data characterizing long-term changes are needed to
quantify the magnitude of the declines. We surveyed grosbeaks at a spring migratory stop-over
site in Corvallis, Oregon, USA, where birds gather annually during April and May to feast on elm
(Ulmus spp.) seeds before departing to breeding sites. An estimate produced by a statistics professor
in the 1970s indicated peak numbers were 150,000 to 250,000 birds. Our surveys in 2013–2015
found annually variable numbers, from a few hundred grosbeaks in the lowest year to less than five
thousand birds in the highest year. If the original estimate is approximately true, Evening Grosbeak
numbers have experienced dramatic declines, averaging ?2.6%/year, over the last four decades.
Our local observation of declines during spring aligns with declines documented in winter across
North America by bird feeder studies and in summer by the Breeding Bird Survey. We explore
potential explanations for the changes in population size, such as influences of spruce budworm
outbreaks, disease, and decreased structural diversity of forests owing to harvest practices. We also
consider the challenges of interpreting changes in abundance of species with exceptionally variable
populations, especially if population fluctuations or cycles may have long periodicities. Finally, we
call for additional planned surveys to track the numbers of this enigmatic and charismatic species.

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