New survey suggests charismatic songbird’s numbers have dramatically declined
The evening grosbeak, a noisy and charismatic songbird, once arrived at Oregon State University in spring flocks so large that an OSU statistician estimated that there were up to a quarter of a million birds on campus each day.
Gone are the days when the birds were so abundant that students, staff and teachers felt the need to take cover for grosbeak droppings.
An Oregon State study published in the journal diversity shows that the number of evening quails using the campus as a migration stop has decreased by an average of 2.6% per year over the past four decades. The bird has experienced decades of decline throughout its range, which includes most of the United States, said Douglas Robinson of the OSU College of Agricultural Sciences.
It’s not yet known, Robinson said, why there are fewer evening grosbeaks than there used to be. It could be disease, climate change or land use shifts, or a combination of these, or some other factor scientists have yet to discover.
“Scheduled surveys designed to take advantage of the opportunities presented by spring stopping points will improve our understanding of population fluctuations over time,” Robinson said. “Our observations suggest that more attention should be paid to risk assessments and putative explanations for the decline of this charismatic bird species.”
Research led by Robinson produced a one-day high of 1,442 birds during 117 surveys that took place on the Oregon State campus from 2013 to 2015. In all, the researchers counted 8,407 grosbeaks.
“The numbers that showed up here each spring were in the hundreds of thousands,” said Robinson, who served as the Mace Watchable Wildlife Endowed Chair at OSU during the study. “Now we are at most a few thousand, sometimes just a few hundred. We ranged from newspaper reports of students in the 1970s who wore umbrellas on sunny days to keep bird droppings off their heads, to people who barely noticed the birds in be near .”
The evening grosbeak, a black and white and bright yellow member of the finch family, is a disturbing species – when food is scarce in winter, it migrates south. But it also migrates on predictable schedules, especially during the spring, allowing populations to be tracked in layover locations like OSU’s Corvallis campus.
“They eat elm seeds here before going to their nesting areas in the forest in June,” Robinson said. “In April and May they are usually here for a few weeks.”
Impressed by their sheer numbers, Fred Ramsey, an OSU professor of statistics from 1966 to 2003, used a randomized sampling strategy to count birds in selected elms. Ramsey, who has been instrumental in estimating populations in the wild over the course of his career, calculated that 150,000 to 250,000 birds were foraging on campus on a spring day in the mid-1970s.
Ramsey produced multiple papers on wildlife abundance estimation and participated in surveys of bird populations on numerous Pacific islands, although his evening grosbeak estimate was not part of a scientific paper.
“But Ramsey was a professional statistician, and even in the unlikely event that he was wrong by an order of magnitude, it seems safe to conclude that the number of evening grosbeak is significantly lower than 45 years ago at our study site,” said Robinson. “If we compare the lower value of Ramsey’s estimate – 150,000 – with our maximum daily count of 1,442, that is an average annual decline of 2.6%. Meanwhile, declines are declining across the range of evening quail, as quantified by the data from the North American Breeding Bird Survey down 2.5%.”
The number of elms on campus has also declined since Ramsey’s time, although the link between the decline of the trees and the grosbeak population is uncertain.
Elms were planted on campus, starting with 35 in 1913, and eventually became the dominant canopy on campus, with more than 330 mature trees, Robinson said.
“In 1978, Dutch elm disease spread across North America, and there were concerns that the disease’s arrival would cause widespread death of the elm trees, leaving the campus without shade trees,” he said. “That led to the removal of elms to ensure root connections between infected trees wouldn’t spread the disease too quickly.”
In 10 years all risk elms have been cut down and replaced by disease resistant elm varieties or other tree species; today the campus has 143 mature elm trees. Robinson knows of no evidence that disease-resistant elms produce food that is less abundant or desirable for beaks, but notes that this could be a possibility.
However, it’s unlikely that grosbeaks simply moved their feeding grounds to other parts of Corvallis or the Willamette Valley, he said. Data from eBird from 2004 to 2021 showed a few higher counts from around Corvallis than what was observed on campus, but the highest count in a single day was less than 2,000 birds, he said.
Most high numbers were between 90 and 500 birds, with similarly small numbers reported all over the Willamette Valley.
Survey shows massive decline in bird populations in Europe and UK over the past 40 years
W. Douglas Robinson et al, Dramatic declines in evening grosbeak numbers at a spring migration stopping point, diversity (2022). DOI: 10.3390/d14060496
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