Where have all the grosbeaks gone?

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This article was published 27/07/2008 (6619 days ago), so information in it may no longer be current.

Manitoba could soon lose one of its most distinctive and popular birds. The evening grosbeak is undergoing an inexplicable continent-wide population collapse, particularly in eastern and central Canada.

Scarcely more than a decade ago, it was one of the most common birds reported at bird feeders, but Project Feeder Watch confirms that it has fallen completely off the list of common avian feeder species range-wide.

A new sophisticated analysis of the evening grosbeak population status by the National Audubon Society indicates grosbeak numbers have fallen by almost 80 per cent since 1967, the second-most acute free fall of any land bird during that time period

According to the Cornell University Laboratory of Ornithology, when Project Feeder Watch began in 1987, the evening grosbeak was an abundant species. At present, it is completely absent in many areas where it was plentiful as recently as the early 1990s.

“The overall picture is one of massive decline over the past few decades,” said Rudolph Koes, an avian specialist with the Manitoba Naturalists Society. “In Winnipeg, where it used to be a fairly common feeder-bird in the 1970s and 1980s, it is rare now and the same goes for other towns on the southern Prairies.”

According to the Laboratory of Ornithology, evening grosbeaks used to be so common that people complained they muscled other birds away from winter bird feeders; they crowded at feeders in huge numbers, foraging ravenously on sunflower seeds.

North American Breeding Bird Survey information suggests losses are especially dramatic in many former population strongholds. In the 1970s, the continental population was 17 million; today, there are three million.

The evening grosbeak is an irruptive species and significant local population fluctuations often occur as a result of massive movements. But, the current population decline is so widespread that researchers do not think population redistribution is responsible.

Although banding studies show regular nomadic movements in this species, there is no evidence that current losses are due to population movements.

In many U.S. states, such as Nebraska and Washington, losses range between 78 per cent and 97 per cent. The National Audubon Society has placed the evening grosbeak on its watch list for species in serious trouble. Bird Banding magazine calls the rate of grosbeak decline “spectacular.”

The Canadian Wildlife Service’s action plan for evening grosbeaks claims that the greatest losses in the Canadian breeding range have been in southern British Columbia and the Atlantic region.

Evening grosbeaks are newcomers in eastern North America. They did not occur east of the Great Lakes until the early 1900s. A series of eastward incursions from western population hubs, took place in 1945, 1951, 1955-7 and 1960. They firmly established the species in the east. Flocks totaling 300 individuals, or more, were often reported at bird feeders.

Taxonomists think there are several evening grosbeak geographic variants. Research by Kendra Sewall and Rodd Kelsey at the University of California suggest each variant might have its own local vocalization dialect. Such variation is most evident in the flight call and it suggests that regional subpopulations, such as those that occur in Manitoba, are largely isolated from one another much of the time. The Manitoba subpopulation is part of the group that is experiencing the sharpest decline.

Scientists are not agreed on what factors might be responsible for the current decline.

According to the National Audubon Society, logging, mining, drilling, acid rain, pesticides and development are impacting on its breeding grounds.

But, a far more plausible explanation has recently surfaced. Scientists now speculate that evening grosbeak populations are vulnerable to fluctuations in abundance of spruce budworms. Budworm larvae are key elements in the diets of nestling grosbeaks, and adults eat them in substantial numbers as well. Since spruce budworms are largely considered to be forest pests, because they attack valuable trees, they are widely controlled by pesticides. Sometimes, pesticides used to control budworms are ingested by grosbeak, with fatal consequences. In addition, budworm population reductions impact on grosbeak food availability.

So far, the link between budworm abundance and grosbeak population losses is speculative, but accumulating evidence suggests that it is valid.

Alternatively some researchers have suggested that reduced seed production, generated by climate warming, can dramatically cut the grosbeak food supply. Data so far indicates that seed generation is dramatically influenced by warming climate regimes. Seeds are a major part of the grosbeak fall, winter and spring diet.

New data indicates that female grosbeaks tend to pioneer into new areas more than males. Over 35 years of data indicates males are more sedentary, and more likely to be impacted by negative local environmental pressures.

Evening grosbeaks are very secretive breeders. Consequently, breeding bird surveys do not accurately reflect their numbers. But, Audubon Christmas Bird Counts indicate the scope of population losses in Manitoba. Current winter numbers are 25 per cent to 85 per cent below figures from the 1970’s and 1980’s.

The Great Backyard Bird Count, a continental survey of birds people see most often around their homes, indicates the three best places to see evening grosbeaks in Manitoba are Pinawa, Belair and Onanole, each with more than 100 grosbeaks reported in the most recent count tabulation.

The National Audubon Society warns that the demise of the evening grosbeak could be the blueprint for major losses in other North American birds, urging study to determine the causes in order to plot remedial management strategies.

Robert Alison is a Victoria-based

wildlife biologist and writer

with a PhD in zoology.