New London — A Connecticut College biology professor has written a report detailing changes in the makeup of bird populations living in the college's arboretum that he says demonstrate the effect of a complex combination of factors both internal and external to the wooded area west of the college's campus.

The bulletin, released last week, details eight decades of ornithological history in the arboretum, with a focus on the Bolleswood forested area donated to the college in 1911 that became a central part of the arboretum when it was founded in 1931.

It draws on research that the professor, Robert Askins, has collected since the 1980s, historical records collected by College birders and biologists and a paper Askins co-wrote in 2016 with 2014 college graduate Mary Buchanan and botany professor Chad Jones. It also builds on two previous bulletins detailing bird populations in the arboretum published in 1958 and 1990.

As trees in the arboretum regrew after the 1938 hurricane blew over all but three of 130 large hemlocks in the Bolleswood Nature Area, the numbers of forest birds increased relative to species more suited for meadows and shrubby landscape, Askins said. So, while shrub-dependent species like ruffed grouse, northern bobwhites, ring-necked pheasant and grasshopper sparrows are now gone from the area, forest birds like pileated woodpeckers now are common.

The types of species also have fluctuated along with changes in the makeup of the trees and vegetation growing or retreating in the Bolleswood area, and also are susceptible to changes in the environment surrounding the arboretum and in the tropical habitats where some migratory birds spend the winter.

"It is sobering to compare these bulletins and realize how much bird populations have changed in the past few decades," Askins wrote in the introduction to the new report. "These changes occurred despite the fact that the Arboretum properties are carefully protected and managed to protect natural diversity."

While naturalists, biologists and members of a college ornithology club frequented the forest in the 1930s and '40s, it wasn't until 1953 that a serious long-term study of birds in the arboretum began.

Before then, the only published account of bird activity was an observation by a College art professor and member of the ornithology club that a northern bobwhite had laid eggs in an eastern meadowlark nest in a field near the present location of the Williams School.

"People were obviously out there observing and keeping personal field notes, but none of that was saved, or it wasn't done in a systematic way," Askins said.

He said he was frustrated that he found no reports of bird populations before the 1938 hurricane.

"I just wish I had just a list of birds from the summer of 1932," he said.

By 1953, when Barbara Rice Kashanski, a junior at the college, pushed for a "remarkably detailed" record of population changes in the arboretum, the forest still was recovering from the hurricane, Askins said.

Since the 1950s, he said, due to changes in the arboretum's vegetation and damage to the tropical habitats of migratory birds, the number of forest migrants that spend the winters in the tropics has decreased while the numbers of birds suited to permanent residency in the forest has increased.

Migrants, especially blue-gray gnatcatchers and ovenbirds, may be making a comeback, though, as they have moved their homes northward in recent years.

Bird populations in the arboretum also may have been affected by woodlands being destroyed to make way for highways, shopping centers and apartment buildings in New London south of the arboretum, as well as an infestation of hemlock wooly adelgid, an aphid that killed off many of the hemlock trees that had grown back since the 1938 hurricane.

The bulletin, titled "Birds of the Connecticut College Arboretum," is available for purchase at the arboretum office in the Olin Science Center.

Askins said the information in the bulletin could be helpful to conservationists trying to manage forests around the species they want to protect.

Knowing how populations of caterpillar-eating birds respond to the abundance of certain trees "might change the way you manage a forest," he said. "If one of your priorities is to provide habitat for insect-feeding birds ... knowing that yellow birch is a critical species for insect-eating birds is important," he said.

m.shanahan@theday.com