Decline of the Rusty Blackbird
October 26, 2007 by Russ Greenberg
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.
Fig. 1. Rusty blackbird breeding and winter range (from Avery 1995)
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.
Fig. 2. Population trends over the last 30 years (line with dots is CBC index trend, line with diamonds is BBS index trend, from Niven et al. 2004)
The Decline and Possible Explanations
What is known
- The species seems to be rare to absent from a number of areas in the boreal forest where it had been known to be a common breeder. This includes northern Alberta and Saskatchewan, Northwest Territories, and northern Ontario. The only area we know where researchers report reliably large breeding populations is Alaska and Northern Yukon Territories.
- Although not well sampled on BBS, the species has shown a sharp global decline for the past 35 years. The rate of decline has been quite variable, with the highest rates in the central and eastern portion of the boreal forest
- Rusty blackbirds were common on the CBC until about 1970 when the species slipped into a decline from which it has not recovered. Although the decline occurred across the entire winter range of the species, interestingly, the onset of the decline varied between regions. The magnitude of the declines has also been quite variable, with the greatest drop in numbers found along the South Atlantic coastal plain
Possible Reasons for Decline
- Winter habitat loss due to conversion of wetlands to agriculture. The long-term and more recent conversion of wooded wetlands to agriculture has been well documented. At least 80 percent of the bottomland hardwood habitat has been converted since European colonization. Whether the rate of habitat conversion in the past 30 years is sufficient to account for the recent declines in rusty blackbirds is unclear.
- Large losses during blackbird control programs in the 1960s and 1970s
- Joining large roosts of blackbirds in winter exposes this species to a disease for which it has low immunity.
- Increased competition with other blackbird species. Habitat loss may have caused the rusty blackbird to feed in more open habitats where it is more exposed to competition with common grackles (Quiscalus quiscula) and red-winged blackbirds (Agelaius phoeniceus). Particularly females are at risk of suffering from such competition.
- Breeding habitat loss and degradation, including boreal wetland drying and changes in water chemistry due, directly or indirectly, to global warming. Birds associated with boreal wetlands have shown consistent cross-species declines. In additions to rusty blackbirds, lesser yellowlegs (Tringa flavipes), solitary sandpipers (T. solitaria), lesser scaup (Aythya affinis), and boreal-nesting scoters (Melanitta fusca and M. perspicillata) have experienced among the highest rates of decline of North American species. In addition to rusty blackbirds, severe long-term declines have been documented across all or major portions of the breeding ranges of co-occurring lesser scaup, white-winged and surf scoters, horned grebe, lesser yellowlegs, and solitary sandpiper. Global warming is suspected to be 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 both to direct loss of boreal wetland and profound changes in hydrology, particularly in the eastern portion of the species range. The eastern portion of the range is where, historically, the species may have achieved the highest breeding densities.
- Acid rain is effecting boreal wetlands, particularly for the eastern portion of the breeding population, affecting trophic resources for a bird restricted to already low-Ph environments.
- Mercury accumulation in tissue, known to affect other blackbirds including common grackles, may decrease reproductive success. The rusty blackbird may have a higher risk of accumulating mercury than other blackbird species because of its preference for feeding on aquatic invertebrates and small fish.
- Loss or change of habitats along the migratory route.
- The rusty blackbird may be less able to adapt to environmental changes due to its ecological specialization. Even slight changes in the environment may render a habitat unsuitable for this species.
- As yet unknown causes of mortality or reproductive failure that will not be known until research is undertaken.
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