The evening grosbeak, a loud and charismatic songbird, once arrived at Oregon State University in spring flocks so large that an OSU statistics professor estimated there were up to a quarter of a million birds on campus every day.
Gone are the days, however, when the birds were so plentiful that students, staff and faculty felt the need to seek shelter from grosbeak droppings.
A study from the state of Oregon published in the journal Diversity shows that the number of evening grosbeaks using the campus as a migratory stopover has declined an average of 2.6% per year over the past four decades. The bird has seen decades of decline throughout its range, which includes most of the United States, said Douglas Robinson of OSU’s College of Agricultural Sciences.
It’s not yet known, Robinson said, why there are fewer evening grosbeaks than before. It could be related to disease, climate change or changes in land use, or a combination of these, or a different factor that scientists have not yet discovered.
“Planned surveys designed to take advantage of opportunities presented by spring staging sites will improve our understanding of population fluctuations over time,” Robinson said. “Our observations suggest that more attention should be given to risk analyzes and hypothetical explanations for the decline of this charismatic bird species.”
Research by Robinson produced a single-day high count of 1,442 birds during 117 surveys that took place on the Oregon State campus from 2013 to 2015. In total, the researchers counted 8,407 grosbeaks.
“The numbers that appeared here every spring were in the hundreds of thousands,” said Robinson, who was chair of the Mace Watchable Wildlife Endowed Chair at OSU during the study. “Now we’re down to a few thousand, sometimes a few hundred. We’ve gone from newspaper stories of students in the 1970s carrying umbrellas on sunny days to keep bird droppings out of their head to people who barely notice the birds are around.”
The evening grosbeak, a black, white and bright yellow member of the finch family, is an irruptive species – if food is scarce in winter, it moves south. But it also migrates on predictable schedules, particularly in the spring, allowing populations to be monitored at stopover locations like OSU’s Corvallis campus.
“They eat elm seeds here before heading out to their forest breeding grounds in June,” Robinson said. “They are usually here for a few weeks in April and May.”
Impressed by their large numbers, Fred Ramsey, professor of statistics at OSU from 1966 to 2003, used a random sampling strategy to count the birds in selected elms. Ramsey, who during his career made major contributions to the field of wild animal population estimation, calculated that on a spring day in the mid-1970s there were 150,000 to 250,000 birds feeding on campus.
Ramsey has produced several papers on estimating wildlife abundance and has been involved in bird population surveys on many Pacific islands, although his estimation of evening grosbeak is not part of his a scientific article.
“But Ramsey was a professional statistician, and even in the unlikely event that he was off by an order of magnitude, it seems safe to conclude that the number of evening grosbeaks is significantly lower than it used to be. 45 years ago at our study site,” Robinson said. “When we compare the lower value of Ramsey’s estimate – 150,000 – with our maximum daily count of 1,442, this represents an average annual decline of 2.6%. Meanwhile, declines across the wholesale range – wandering beaks are quantified by the North American Breeding Bird Survey. data show a decline of 2.5%.”
The number of elm trees on campus has also declined since Ramsey’s time, although the link between the tree decline and the grosbeak population is uncertain.
Elms were planted on campus starting at 35 in 1913 and eventually became the dominant canopy tree on campus, numbering more than 330 mature trees, Robinson said.
“In 1978, Dutch elm disease was spreading across North America, and it was feared that the arrival of the disease would cause widespread death of elm trees, leaving the campus without shade trees,” said he declared. “This led to the culling of elm trees to ensure root connections between infected trees would not allow the disease to spread too quickly.”
Within 10 years, all at-risk elms were cut down and replaced with disease-resistant elm varieties or other tree species; today, the campus has 143 mature elms. Robinson knows of no evidence that disease-resistant elm trees produce less abundant or less desirable food for grosbeaks, but notes that it could be a possibility.
It’s unlikely, however, that the grosbeaks simply moved their feeding sites to other parts of Corvallis or the Willamette Valley, he said. eBird data from 2004 to 2021 showed some higher counts around Corvallis than what was seen on campus, but the highest single-day count was less than 2,000 birds, he said. declared.
Most high counts were between 90 and 500 birds, with similarly low counts reported throughout the Willamette Valley.
Four students from Oregon State University, all now at other research institutions after graduation, participated in the study: Jessica Greer (University of California Davis), Juliana Masseloux (Zoological Society of London) , Tyler Hallman (Swiss Ornithological Institute) and Jenna Curtis (Cornell Lab of Ornithology). Greer and Masseloux were undergraduates, and Curtis and Hallman were doctoral students.