Olive-sided flycatcher (Contopus cooperi) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

The causes for the declines in Olive-sided Flycatcher populations are unclear but are almost surely related to habitat loss and change. Several authors have suggested habitat alteration on the wintering grounds may be a significant factor (Petit et al. 1993, Altman and Sallabanks 2000). Diamond (1991) predicted that the Olive-sided Flycatcher would lose 39% of its non-breeding habitat between 1980 and 2000, in addition to habitat lost prior to 1980. However, precise estimates for changes to non-breeding habitat are not available, and there are no data linking declines in particular breeding locations with specific wintering populations.

In eastern North America, Olive-sided Flycatcher habitat has changed with alterations to forest structure, urbanization, loss of wetlands and their associated edge habitats and the reforestation of abandoned farms. This may explain declines in the Atlantic provinces, as well as parts of southern Ontario and southern Quebec. However, Gauthier and Aubry (1996) have suggested that the large-scale clearcutting of older forests in eastern Canada may have changed forest structure to favour the Olive-sided Flycatcher and may explain the peak in their abundance in Quebec in the 1980s.

Hutto and Young (1999) have speculated that Olive-sided Flycatchers are an early post-fire dependent species that is attracted to managed (harvested) forests that have similar structural conditions to early post-fire habitat, but that these habitats may function as ecological sinks. In a Montana study, Robertson and Hutto (2007) found that Olive-sided Flycatchers preferred to nest in selectively logged habitats, but that breeding success in that habitat was only half of that in natural burned openings. Their data suggested that increased nest predation in the logged habitats was the reason for reduced success. These findings are supported by data fromAltman and Sallabanks (2000), who report that nest success for Olive-sided Flycatchers was highest in early post-fire habitats (62%, n=16) in the Cascade Mountains of west-central Oregon compared to semi-open forest (49%, n=33), to harvest units that retained trees (39%, n=89) or at forest edge (33%, n=31). Conversely, in northwest California,Meehan and George (2003) found that the probability of nest loss was lower in unburned habitat (early seral forest) than in burned habitat (formerly predominantly clearcut). These differences may be explained by different amounts of standing trees following fire in a clearcut versus fire in a mature stand, although this has not been tested. Burned habitat in the Californian study also had reduced arthropod biomass and lower foraging rates than unburned forest.

The continued decline of Olive-sided Flycatchers across their breeding range despite the continued addition of early seral habitat (through harvest) to the landscape suggests that forest management practices may be a significant factor in population decline. Regional differences in population trends, though difficult to assess because of low sample sizes, may result from differing forest harvest practices that could impact nest predator and insect prey populations in different ways.

Another possible, but undocumented, cause of continued population declines could be a general reduction in insect prey, either on the breeding or wintering grounds. Pesticides have been suggested as the cause of such a decline, but there is no specific evidence for this in the case of Olive-sided Flycatchers (Altman and Sallabanks 2000). Similar population declines have occurred in a wide range of aerial insectivore bird species (e.g. Chimney Swift, Common Nighthawk), most of which winter in South America as well.

The low reproductive rate of this species suggests that adult survivorship would have to be high to maintain populations.

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