Most species of North American blackbirds (family Icteridae) are highly adaptable and have expanded, at least historically, in the face of human development. The success of the group as a whole may explain why it has taken a particularly long time to recognize both acute and long-term chronic declines in at least two species: the tricolored blackbird (Agelaius tricolor) in the west (Beedy and Hamilton 1999) and the rusty blackbird in the northern and western portions of the North American continent.
The Tricolored Blackbird is at present the focus of a concerted monitoring and conservation effort with participation from scientists at the Point Reyes Bird Observatory, United States Fish & Wildlife Service, United States Geological Survey, and the California Department of Fish & Game, among others. No such similar program exists for the rusty blackbird, despite the fact that it was recently featured in a report by the National Audubon Society as the North American species showing the sharpest decline.
Conservation efforts are most effective when they are initiated while the target species is still common. However, it is impossible to develop a meaningful conservation strategy for a species when the cause or causes of decline are undetermined. Despite the fact that the combined breeding and wintering ranges of the species cover much of North America north of the U.S.-Mexican border, the causes of the species' decline are still far from certain.
Clues to the cause of the precipitous drop in numbers may be gleaned from the species' unique natural history. The rusty blackbird is arguably the most ecologically specialized of the North American blackbirds, both in its feeding habits and habitat uses. Throughout the year this species feeds to a considerable extent on animal prey and is one of the few bird species restricted year-round to wooded wetlands.
The species breeds either in isolated or small clusters of pairs in boreal wetlands from northern New England and the Maritime Provinces of Canada, north and west to central Alaska (Figure 1, from Avery 1995). In fact, the rusty blackbird is the passerine species whose breeding is most closely tied to boreal forest wetlands for breeding, where it nests near open water and feeds primarily on the adults and aquatic larvae of wetland insects.
Rusty blackbirds winter primarily in wooded wetlands of the southeastern United States. An analysis of Christmas Bird Count (CBC) data suggests that the greatest winter concentrations are found in the Mississippi River Valley (Niven et al. 2004). The species seems to roost with many other blackbird species, but often is found foraging in single species flocks or together with common grackles (Quiscalus quiscula) in or near wooded wetlands and only occasionally in agricultural fields with other blackbirds.
An analysis of the literature on the distrubution of North America birds from the late-18th to the late-19th century shows a consistent long-term decline in the qualitative assessment of this species' abundance. More alarmingly is that both national indicators of songbird abundance, the Breeding Bird Survey (BBS) and the CBC, show sharp declines over the past three decades (Figure 2). BBS data indicate a population trend that averages a decline of more than ten percent per year for the last 30 years.
This precipitous decline equates to a loss of more than 95 percent of the population that existed when the Breeding Bird Survey was initiated. Therefore, it appears that rusty blackbirds have shown both long-term chronic and short-term acute patterns of decline. Despite the severity of the declines in this species, the bird research and conservation community has been slow to recognize and investigate the plight of this species.
The major features of its life history are known, but none are known thoroughly enough to support the various hypotheses that have been proposed to account for the species' decline. In the following paragraphs we list what is known about the decline followed by potential explanations.
Avery, M. L. 1995. Rusty Blackbird (Euphagus carolinus). In The Birds of North America, No. 200 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, and The American Ornithologists’ Union, Washington, D.C.
Evers, D. C., N. M. Burgess, L. Champoux, B. Hoskins, A. Major, W. M. Goodale, R. J. Taylor, R. Poppenga, and T. Daigle. 2005. Patterns and interpretation of mercury exposure in freshwater avian communities in northeastern North America. Ecotoxicology 14:193–221.
Greenberg, R. & S. Droege (1999). On the decline of the Rusty blackbird and the use of ornithological literature to document long-term population trends. Conservation Biology 13: 553-559.
Hefner, J. M. and J. P. Brown. 1984. Wetland trends in Southeastern U. S. Wetlands 4:1-11.
Hobson, K.A. 1999. Tracing origins and migration of wildlife using stable isotopes: A review. Oecologia 120:314-326.
Hobson, K.A., E.M. Bayne, and S.L. Van Wilgenburg. 2002. Large-scale conversion of forest to agriculture in the boreal plains of Saskatchewan. Conservation Biology 16:1530-1541.
Niven, D. K., J. R. Sauer, G. S. Butcher, and W. A, Link. 2004. Christmas bird count provides insights into population change in land birds that breed in the boreal forest. American Birds 58:10–20.
Orians, G. 1985, Blackbirds of the Americas: University of Washington Press, Seattle.
Sauer, J. R., J. E. Hines, and J. Fallon. 2004. The North American Breeding Bird Survey, Results and Analysis 1966–2003. Version 2004.1. USGS Patuxent Wildlife Research Center, Laurel, Maryland. Available at [http://www.mbr-pwrc.usgs.gov/bbs/bbs.html, April 2005].