Why are Rusty Blackbirds Declining?

New research and conservation efforts underway

By Russell Greenberg

"Don?t it always seem to go, that you don?t know what you?ve got till it?s gone?."

These well-known words from the pen of Joni Mitchell should be the watch words of everyone interested in bird conservation. The history of North American avifauna is dotted with once abundant birds that declined, sometimes to extinction. Therefore, we must be vigilant for common species showing consistent decreases in abundance, and alarmed when we see what might be population crashes.

Jack Bartholomai

The Rusty Blackbird is a case in point. Prior to 1920, more than half of the Rusty Blackbird accounts for eastern North America listed the bird as very common to abundant, compared with only a few accounts since 1950. A long-term decline like this is troubling, but the information from monitoring programs such as the Christmas Bird Count and the Breeding Bird Survey are downright scary. While the trend estimates are imprecise, the total decline since the 1960s is consistently estimated at 85 to 95 percent. Taken together, this portrays a species whose decline has accelerated to a free fall.

A number of factors have probably contributed to the decline. Rusty Blackbirds have an extensive breeding range in forested wetlands of the boreal zone from New England to Alaska and are strongly associated with forested wetlands in the winter as well. Rusty Blackbirds have lost their wintering habitats as woodlands have been converted to agriculture. At least 80 percent of bottomland hardwood habitat has been lost since European colonization.

Rusty Blackbirds sometimes forage in agricultural areas. Wintering ground problems could have been exacerbated by blackbird control programs in the 1960s and 1970s and more modest control programs today. Rusty Blackbirds are also more insectivorous than other blackbirds. Researcher Claudia Mettke-Hofmann found that they are more averse to novelty when feeding than other blackbirds, an observation that suggests they may be less adaptable to rapid environmental change.

Another cause of the decline could be loss and degradation of breeding habitats. Birds associated with boreal wetlands have shown consistent crossspecies declines. Global warming is suspected in causing major changes in the extent of boreal wetlands, the chemistry of the waters, and the structure of invertebrate communities. Peat production, logging, and reservoir formation have contributed to losses of boreal wetland and profound changes in hydrology, particularly in the eastern portion of the species range.

Acid rain and mercury accumulation may also be affecting boreal wetlands in the East, where Rusty Blackbirds are showing the greatest declines. Rusty Blackbirds may be at higher risk for accumulated mercury than other blackbirds because of their preference for eating aquatic invertebrates and small fish. Their preference for wetlands with acidic soils may also put them at greater risk from calcium loss in areas with acid rain.

An International Rusty Blackbird Technical Working Group is engaged in research and conservation of this troubled species, including studies on breeding and winter ecology, isotope and genetic studies to connect populations between seasons, and assays of methyl mercury accumulation in tissues.

You can join the effort to help Rusty Blackbirds by sending sightings to eBird at www.ebird.org. The Smithsonian Migratory Bird Center is also collecting sightings from birders and feather samples from bird banders. Visit http://nationalzoo.si.edu/ConservationAndScience/MigratoryBirds for more information.

Working together, we hope to determine the cause and come up with management solutions before the species joins the growing ranks of the threatened and endangered.

Russell Greenberg is director of the Smithsonian Migratory Bird Center.

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu