The rusty blackbird (Euphagus carolinus) was once a common species in North America. But both birders and field ornithologists have noticed a steady decline in the species, particularly over the past 40 years. Overall, the total decline of the rusty blackbird population is estimated to be between 88 and 93 percent over the past 4 decades. And no one knows why.
Even though the bird has been included on the IUCN Red List of Threatened Species, the U.S. Fish and Wildlife Service Birds of Conservation Concern, and Canada’s Committee on the Status of Endangered Wildlife Species of Special Concern, the decline continues. The fact of the matter is, not much is known about the rusty blackbird or its way of life.
Recognizing this severe lack of knowledge, scientists formed the International Rusty Blackbird Working Group (IRBWG) in 2005. The goal of the group is to gather as much information as possible about the rusty blackbird, its ecology, and resource requirements in order to understand and begin to reverse the population decline. The founding members initiated a series of collaborative research projects to further that end. The working group that initially began with only 10 members now has more than 60, and they have discovered a great deal about the rusty blackbird in 5 short years.
The rusty blackbird lives in wooded wetlands year-round and eats aquatic arthropods and fish as well as nuts from forest trees. It is a medium-sized bird with an extensive breeding range from New England to Alaska. It winters throughout the southern United States. Wetland habitat loss, climate change, and industrial pollution may be possible reasons for the decline. Since the rusty blackbird depends on wooded wetlands during both the breeding and winter seasons, it is easy to imagine why their numbers have been decreasing.
Man-made, or anthropogenic, habitat change is often at the root of migratory bird population declines. The case of the rusty blackbird is no exception. As much as 80 percent of the original wooded wetland habitat suitable for the rusty blackbird has been cleared for agriculture or urban areas, logged, or converted to pine forests. This is especially true in the wintering range of the blackbird in the Mississippi Alluvial Valley and the Southern Coastal Plain. In these areas, only small fragments of wooded wetlands and bottomland forests remain.
To the north, anthropogenic habitat change also affects the boreal wetlands. Oil and gas drilling, timber harvesting, and reservoirs all affect forests and wetlands. One IRBWG study found that birds nesting in small, native spruce trees in undisturbed wetlands in Alaska were highly successful during the breeding season, but birds nesting in replanted spruce and balsam firs in New England logging areas near wetlands were not so lucky. Nest survival was down 70 percent in New England.
The effects of global warming are becoming apparent. Not only has the breeding range of the blackbird been retracting in the south, but climate change appears to be drying the wetlands of the boreal forest. This affects water chemistry and reduces one of the primary food sources of the rusty blackbird, macroinvertebrates such as crayfish and aquatic worms.
Pollution also affects water chemistry by lowering the pH and elevating levels of mercury, specifically the highly toxic methylmercury, found in aquatic invertebrates. Since rusty blackbirds feed so heavily on aquatic invertebrates and fish, they are particularly at risk for methylmercury poisoning. Because of industrial pollution, methylmercury levels are higher in blackbirds breeding on the east coast in New England than those in Alaska.
Monitoring rusty blackbirds has proved to be quite a challenge. Winter populations fluctuate from year to year; the birds are very difficult to capture; and it is even more difficult to recapture a bird in a long-term study. They also move around quite a bit from year to year. Breeding rusty blackbirds usually are not concentrated in large flocks and are often difficult to find. Despite all this, figuring out where they go and why they choose a particular habitat is critical to understanding their needs and preserving good habitat for the birds in the future.
A study published by IRBWG members in 2010 found 2 distinct flight paths for the migrating blackbirds, separated by the Appalachian Mountains. Using stable hydrogen isotope ratios from feathers, the scientists discovered birds either migrate from Eastern Canada to the Southern Coastal Plain, located between the Gulf of Mexico and the Atlantic Ocean, or from central Canada and Alaska to the Mississippi Alluvial Valley, located along the Mississippi River. This discovery is important because it allows scientists to correlate research, bird monitoring data, and habitat management practices for the appropriate flight path.
Additionally, IRBWG members found the birds were widely spread across large home ranges which include multiple wetlands used for nesting and feeding. The birds in the Mississippi valley have been spotted in a variety of wooded wetlands as well as on nearby agricultural sites, where they were foraging for supplemental grains and seeds in the winter.
Rusty blackbird research has come a long way, but there is still a lot to learn about the rusty blackbird. And you can help. From January 29 through February 13, 2011, you can help scientists learn more about rusty blackbirds and where they spend their winters by participating in the Rusty Blackbird Blitz. Volunteers are asked to search for rusty blackbirds in any potentially suitable habitats and submit their observations via e-Bird.
This article summarizes the information in this publication:
Greenberg, R. and S. Matsuoka. 2010. Rusty Blackbird: Mysteries of a species in decline. Condor 112:770-777.
The Rusty Blackbird (Euphagus carolinus) breeds across the boreal forest zone of North America and winters throughout the eastern United States. Over the past four decades, the North American Breeding Bird Survey and the Christmas Bird Count have shown high rates of population decline ranging from approximately 5 to 12% per year. Regional surveys suggest declines and range retractions in the southern boreal zone. Analyses of historical accounts suggest that the Rusty Blackbird’s abundance has been dropping steadily for over a century. A number of hypotheses have been proposed to explain the decline. The species relies on wooded wetlands throughout the year, so loss and degradation of these habitats—particularly in the winter range—is a prime suspect. Blackbird-control programs may have contributed. In recent decades, habitat disturbance, global warming, and environmental contamination in the boreal zone may have taken their toll on breeding populations. In 2005, the International Rusty Blackbird Working Group was formed to develop research efforts toward understanding the mysterious decline. This special section presents the group’s research findings—the first on the species’ use of breeding and winter habitat, reproductive success, parasite prevalence, patterns of molt, and migratory connectivity. Data on the levels of methylmercury in tissues and the role of timber management on reproductive success are intriguing. We outline research needed for assessment of the roles of various factors in causing the decline of the Rusty Blackbird.
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