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FeederWatch Data Confirm Decline of Evening Grosbeaks

November 05, 2008 - In today's column, I will recap some of the articles that have been published in major North American ornithology journals this year. The emphasis will be on birds that occur in Maine.

All of us are thrilled when evening grosbeaks come to visit at our bird feeders. For readers who have been birding for at least 20 years, you will remember times when evening grosbeaks descended in large flocks, quickly devastating the sunflower seeds in your feeder. Sadly, these large flocks seem a distant memory.

In a recent article in the Condor, David Bonter and Michael Harvey of the Cornell Laboratory of Ornithology used 18 years of Project FeederWatch data to quantify the changes in evening grosbeak abundance.

Their analysis confirms our impressions: evening grosbeaks seem to be in a population decline. Over the past 18 years, the number of sites reporting evening grosbeaks fell by 50 percent. Flock sizes decreased by 27 percent at feeders where the grosbeaks still visited.

The authors do not know why the population is decreasing, but argue that the reason for these declines needs urgent investigation.

The boreal chickadee has a more northerly distribution than the black-capped chickadee. Boreal chickadees also prefer coniferous forest. In Maine, these chickadees occur in the mountains, in the spruce-fir forest of northern Maine and in the spruce-fir coastal forests from Mount Desert Island eastward.

Adam Hadley and Andre Desrochers of Universite Laval recently described the effects of logging practices on boreal chickadee habitat use in Quebec.

They clearly showed that boreal chickadees prefer the taller (greater than 7 meters in height), commercially valuable stands of conifers (mostly balsam fir in their study area) during the winter. The birds typically move in stable flocks of four birds.

The average flock territory is around 50 acres. Boreal chickadees occurred less often in regenerating forest where the trees were between 4 and 7 meters tall. The chickadees avoided clear-cut areas and younger stands with trees less than 4 meters in height.

The authors expect that forestry practices in Quebec will result in substantial loss of prime boreal chickadee winter habitat over the next 20 to 30 years. Boreal chickadees will likely show apparent habitat declines, although the population may remain stable. The reason is that the winter flocks will need to expand their territory size in less preferred areas and therefore will be harder to detect.

On the positive side, wild turkey populations have risen rapidly over the past 20 years. In the early 1990s, the sighting of a wild turkey was an unusual event in Maine. Seeing flocks of wild turkeys now is commonplace, and many of us have them digging through our flower beds and vegetable gardens.

During the breeding season, male turkeys display and gobble to attract female mates. The toms have no parental role and, therefore, a tom seeks to have as many female partners as he can attract. We think that female turkeys may have multiple male partners as well, so eggs from a single clutch may be fathered by multiple males.

Alan Krakauer of the University of California has made a fine contribution to our understanding of turkey mating systems in an article published in the Condor. His work was done in central California; one expects that similar results would be seen in Eastern turkeys.

Krakauer used DNA fingerprinting techniques to examine the paternity and maternity of nestling turkeys. The DNA results do not lie. Krakauer took DNA samples from all the eggs of 32 nests.

He showed that wild turkeys at his study site did not have as many partners as one might expect. Nestlings in 15 of the nests had the same mother and father. The broods from 14 nests were often half siblings; they had the same mother but a different father.

In seven nests, the embryo DNA showed that the eggs were produced by more than one female. Clearly, a sneaky female dumped one or more of her eggs into the nest of an unsuspecting female.

In songbirds, the female usually incubates the eggs. She develops a brood patch, an unfeathered area with an extensive blood supply to allow the female to transfer heat to her eggs.

Margaret Voss and colleagues from Penn State described male incubation in barn swallows. Unlike female barn swallows, the males do not develop a brood patch. As a result, they are much less efficient in keeping the eggs warm than the female.

Nevertheless, male incubation is better than having no incubation at all, so male incubation gives the females the chance to feed for a longer period of time when she takes a break from her incubating duties.

Herb Wilson teaches ornithology and other biology courses at Colby College.

HERB WILSON, Portland Press Herald, October 30, 2008


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