Comments: On wintering grounds, potential threats and/or causes for observed population decline include destruction of wooded wetlands and blackbird control programs; on breeding grounds, acid precipitation and conversion of boreal forest wetlands have been implied (Greenberg and Droege 1999).
Habitat degradation: Land-use practices that degrade or reduce wooded wetlands are detrimental to this species' habitat needs (Avery 1995). Greatest loss of wooded wetlands is on the wintering grounds. Between the mid-1950s and mid-1980s, about 25% of remaining wooded wetlands in the southeastern U.S, an area that encompasses most of the species' winter range, were drained and converted (Hefner and Brown 1984, Greenberg and Droege 1999). However, modern rates of wooded wetland conversion may not be sufficient to explain the severity of recent declines; changes on the breeding grounds may also be limiting this species (Greenberg and Droege 1999). Several other species that utilize high-latitude wetland habitats for breeding, such as the Horned Grebe (Podiceps auritius) and Lesser Yellowlegs (Tringa flavipes), are also experiencing survey-wide (BBS) declines (Sauer et al. 2005).
Clearcut logging on breeding grounds removes habitat and may also encourage establishment of competitors Common Grackles and Red-winged Blackbirds (Agelaius phoeniceus) (Ellison 1990), or encourage invasion by Brown-headed Cowbirds (Molothrus ater), a common and potentially problematic nest parasite (Avery 1995). Conversely, recently logged habitat, when saturated with water, may provide breeding habitat for this species; Ellison (1990) found several Rusty Blackbird nests and fledglings in recent clearcuts in Vermont. Although recent clearcuts may satisfy habitat requirements for this species, no data exists on the relative quality of these sites (Hannah 2000, 2004).
Blackbird control programs: Rusty Blackbirds may form mixed-species flocks in winter with other blackbirds and starlings, regularly exceeding 1 million birds. As a result, species has been subjected to lethal control to reduce nuisance, health, and crop damage problems (Avery 1995). Winter roost control programs in the eastern U.S. coincided with declines in Common Grackle populations (Avery 1995, Greenberg and Droege 1999). The overall effect on Rusty Blackbird populations is unknown but suspected localized and nominal, as this species typically constitutes <1% of winter roosts (Avery 1995).
Wetland acidification: Acidification of boreal wetlands due to industrial emissions is also of concern, particularly in eastern North America, but overall effects are unknown (Greenberg and Droege 1999). Since Rusty Blackbirds inhabit areas with naturally high soil acidity, it is difficult to determine the real impact of acidification (Darveau et al. 1989, Savignac 2004). Declines in snail abundance in acidified soils in the Netherlands have been linked to declines in passerine production (Graveland et al. 1994); given the high proportion of snails and mollusks in Rusty Blackbird diets, the impacts of acidification on food resources could be of concern (Greenberg and Droege 1999).