Rusty blackbird (Euphagus carolinus) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Habitat loss

Wintering habitat

The conversion of the wetland forests in the south of the United States, on the wintering grounds of the Rusty Blackbird, is thought to be one of the most significant factors in the decline of this species (Greenberg and Droege 1999). Over the last 150 years approximately 80% of the forest massif has disappeared and this area now consists of detached forest tracts (Hefner and Brown 1984). Loss of other important habitats has also been reported for the upper Mississippi (http://www.birds.cornell.edu/ ivory/story11.htm, en anglais seulement).

Breeding habitat

Wetland conversion for agriculture and urban development has also resulted in habitat loss in the southern part of the species’ breeding range in Canada (Hobson et al. 2002). Approximately 5% of Rusty Blackbird habitat in Canada has been lost and another 4% loss is anticipated in the next 50 years (Global Forest Watch Canada 2000; Hayeur 2001; Griffiths and Woynillowicz 2003; Kling et al. 2003; Wildlife Habitat Canada 2003)

Bird control programs

Rusty Blackbird populations may also be affected by bird control programs in the U.S. that are designed to reduce populations of nuisance birds that damage crops (Avery 1995). Between 1974 and 1992, Red-winged Blackbird and European Starling control programs were responsible for the extermination of 100,000 Rusty Blackbirds, which was 1% of the total number of birds killed in roosts (Dolbeer et al. 1997). Although blackbird control is currently carried out in several American states there are no studies showing the number of Rusty Blackbirds exterminated by these programs.

Migratory populations of Rusty Blackbirds could also be affected by bird control programs currently being planned in central and southeastern U.S. states where sunflower and rice crops are subject to significant damage from Red-winged Blackbirds (Linz et al. 2002; Blackwell et al. 2003; Custer et al. 2003).

Habitat degradation on breeding grounds

Although no studies have been conducted, Rusty Blackbird populations could also be affected by: 1) the degradation of boreal forest wetlands due to climate change (Sillett et al. 2000; Jones et al. 2003; Both et al. 2004); 2) the acidification of wetlands by industrial waste (Schindler 1988; Graveland et al. 1994, but see DesGranges et al. 1998); 3) mercury contamination of boreal wetlands following forestry activities and the creation of hydroelectric reservoirs (DesGranges et al. 1998; Gracia and Carignan 2000; Gerrard and St. Louis 2001).

Other limiting factors

Rusty Blackbirds might also be affected by the invasion of dominant species such as the Red-winged Blackbird into boreal wetlands (Ellison 1990; Erskine 1992). 

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