Today these reports seem unbelievable since Rusty Blackbirds populations have suffered one of most staggering population declines of any bird in North America. An understanding of the Rusty Blackbird's habitat requirements is urgently needed to conserve its remaining populations. This is especially true during spring migration when Rusty Blackbirds congregate in large flocks which may be particularly vulnerable to habitat losses, blackbird control programs, or other disturbances.
Russell Greenberg, director, Migratory Bird Center on the Great Backyard Bird Count (coming Feb. 17-20, 2010!) webpage:
It is particularly disturbing to monitor a decline and not have a specific, definitive underlying cause. But considering the distribution and ecology of this elusive species, the search for the culprit becomes almost like the Agatha Christie novel where all the suspects were guilty.
So this is what we know. The Rusty Blackbird has a geographically extensive breeding range from New England to Alaska throughout the boreal zone in forested wetlands. The species is strongly associated with wooded wetlands in the winter as well, although foraging birds can be found in agricultural settings, particularly in association with livestock. The species is more insectivorous than other blackbirds, often foraging in single species flocks, not associated with blackbirds, and (based on experiments by Claudia Mettke-Hofmann) is more averse to novelty when feeding than other blackbirds. The last observation suggests the bird may be less adaptable in the face of rapid environmental changes.
From this we can suggest a number of factors leading to the decline: Winter habitat loss due to conversion of wooded wetlands to agriculture. At least 80 percent of the bottomland hardwood habitat has been converted since European colonization. Habitat loss may have caused the Rusty Blackbird to feed in more open habitats where it is more exposed to competition with birds such as Common Grackles and Red-winged Blackbirds. Wintering ground problems could have been exacerbated by large losses during blackbird control programs in the 1960s and 1970s. It should be noted that the species is still listed as a pest species in a number of states and thus still subject to more modest control programs aimed at blackbirds.
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. 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 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 and is also the region where it has shown the greatest decline. Acid rain and mercury accumulation (an only recently detected problem in songbirds) may be differentially affecting boreal wetlands in the East. The Rusty Blackbird may be at higher risk for accumulated mercury than other blackbird species because of its preference for feeding on aquatic invertebrates and small fish. Its preference for wetlands with acidic soils may also make the effect of acid rain on calcium loss particularly great in this species. (see our blog for mercury-related entries)