Golden-winged warbler (Vermivora chrysoptera) COSEWIC assessment and status report: chapter 8
Limiting factors and threats
Golden-winged Warbler declines are likely due to three main pressures they face while on the breeding grounds: a decrease in early successional scrub environments required for breeding, hybridization with the Blue-winged Warbler and perhaps Brown-headed Cowbird parasitism.
Of these three threats, loss of breeding habitat and hybridization with Blue-winged Warblers are the most significant contributors to Golden-winged Warbler declines. However, further research is needed to elucidate the impacts of these threats in concert, or to determine which one is having a more pronounced impact on Golden-winged Warbler populations in Canada.
a) Habitat loss
Due to their reliance on anthropogenic disturbances (e.g. logging, hydro right-of-ways, and abandoned farmland) the Golden-winged Warbler is vulnerable to local extirpation throughout the breeding range. Declines occur with advancing succession and reforestation (Confer 1992) and a general loss of early successional scrub habitat in eastern North America (Confer and Pascoe 2003; Dettmers 2003). As such, it is likely that habitat loss is playing a major role in Golden-winged Warbler declines throughout the breeding range. This is further supported from across-species data showing that the majority of avian species undergoing population declines in North America depend on disturbance-generated ecosystems (Confer and Pascoe 2003; Sauer et al. 2005).
However, habitat limitations cannot be solely responsible for Golden-winged Warbler declines as they sometimes are extirpated from areas where suitable habitat remains, but only when sympatric with Blue-winged Warblers, suggesting that interactions between these species are also playing a role in declines (J. Confer, pers. comm.)
b) Hybridization with Blue-winged Warblers
As detailed above (see Interspecific Interactions), hybridization between Blue-winged and Golden-winged Warblers is ongoing and widespread, and replacement of Golden-winged Warblers after 50 years of Blue-winged Warbler arrival is the norm (Gill 1997). Only one region of sympatry has persisted for more than 100 years and reasons for this coexistence are unknown (J. Confer, pers. comm.; Confer 2000).
At present there is no clear genetic reason why Blue-winged Warblers are causing the regional extirpation of Golden-winged Warblers whenever the two species come into contact. However, with the movement of Blue-winged Warblers into currently allopatric populations of Golden-winged Warblers further declines are expected.
c) Parasitism by Brown-headed Cowbirds
Brown-headed Cowbird parasitism is playing a role in population declines of Golden-winged Warblers in some areas of the United States. Up to 30% of all nests in some regions of New York State are parasitized, decreasing the number of fledged Golden-winged Warblers by 17%; Confer et al. 2003). The rate of parasitism varies greatly across the Golden-winged Warbler’s geographic range. While territories located in areas of high herb cover are able to support larger clutch sizes, these areas are also more susceptible to cowbird parasitism (Confer et al. 2003).
Parasitism does not appear to be a major factor in at least one population in Ontario (3.8% of nests parasitized; R. Fraser, unpub. data) but data from other areas in Canada are lacking and the true impact of Brown-headed Cowbird parasitism in Canada is unknown.
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