Successful Nesting Brings Bobolink Back to Bald Hill

As the buzzards return to Hinckley, Ohio, and the swallows to Capistrano, the bobolinks are just starting to think about winging their way back to Bald Hill, N.Y., and meadows and hayfields all over North America.

Judging from the festivals and media hoopla, you might think a bird’s spring return is arranged by local Chambers of Commerce to further the tourist trade.

But not so, say two bird experts at Cornell University in Ithaca, N.Y., who have studied returning bobolinks.

In a recent article in the journal Ecology, Thomas Gavin and Eric Bollinger said the reason bobolinks--and probably many other species--return faithfully to their nesting sites each year is successful parenthood.

Breeding success is such an important determinant of return, in fact, that the Cornell scientists were able to use a bird’s reproductive history to make correct predictions about 70% of the time on whether or not it would return to nest in study fields in Upstate New York.

Gavin, an associate professor of wildlife in Cornell’s department of natural resources, and Bollinger, his doctoral student, banded, marked and observed 171 bobolinks that lived on Bald Hill in Tompkins County, N.Y., or at the Cornell Biological Field Station in Madison County.

For each adult bird, the pair recorded the number of mates the polygamous males had, the number of nests built, the number of eggs laid and the number of young that survived to fly from the nest.

Over the course of the four-year study, Gavin and Bollinger found that the male bobolinks that returned were birds that in the previous year had had more nests, eggs, hatched eggs and fledged young than the males that did not return.

For female birds, the main factor seemed to be simply whether or not they had raised at least one young bird to fledgling stage.

Oddly, “supermom” birds that managed to raise several young were not more likely to return: “Successful females that built more nests and laid more eggs tended not to return, suggesting that increased reproductive effort discouraged females from returning, even though they had fledged one or more young,” the scientists wrote.

“It seems, in general, that individuals have a rather simple decision rule, which is to return to the same site if successful and not to return if not successful” in raising young, they concluded.

In a telephone interview, Gavin said that, on average, the returning bobolinks nested within 45 to 55 yards of the spot where they had nested the year before. They homed in on the spots after a roughly 6,000-mile transequatorial journey to Argentina and back. All North American bobolinks make the trip, Gavin said, taking several weeks for the return flight in the spring that brings them back about May 1.

One of the main reasons bobolinks fail to raise young, and thus would not return to a site, is predators that eat the eggs and young.

Where bobolinks go in their first breeding season, when they have no track record to go on, Gavin said, “We just don’t know.”

Gavin and Bollinger listed 15 other species of birds that, like the bobolink, appear to use successful parenting as their guide on whether to return to a site. Buzzards and Capistrano’s species of swallows are not on the list.

But, Gavin said, that is probably because scientists haven’t studied the birds yet. “We now think it’s the rule rather than the exception that most individuals come back.”