Kirtland's warbler (Dendroica kirtlandii) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

The Kirtland’s Warbler has always been rare in Canada and the factors limiting population size in Canada have not been identified. Several factors have, however, been associated with declines in the Kirtland’s Warbler population in the U.S. and could apply to birds breeding in Canada.

Reduced habitat quality

Habitat quality for Kirtland’s Warbler has been reduced because of fire suppression. Historically (pre-European settlement), the supply of extensive patches of young jack pine (4-20 years of age) was renewed on an ongoing basis by lightening-caused fires in the Michigan pine barrens. For example, in 1871 a fire of 400,000 ha burned in the heart of the species’ breeding range in Michigan (Mayfield 1992) and three fires, each more than 6,000 ha in extent, occurred between 1939 and 1946. These stand-replacing fires produced jack pine habitat of the appropriate successional stage (6-23 years), tree height (1.7-5.0 m tall), tree density (> 2000 stems/ha), and ground cover for breeding (Probst and Donnerwright 2003). However, fire suppression has drastically reduced the extent and frequency of these wildfires in the breeding areas. Indeed, the small but stable Michigan population expanded after extensive fires during the 1970s and 1980s (Mayfield 1977; Harwood 1981; Probst and Weinrich 1993; Kepler et al. 1996). In the absence of fire, management (timber harvest, prescribed burns, elliptical leave patches, 25% unplanted) has been geared to maximizing habitat suitability for Kirtland’s Warblers. The increase in warbler population in Michigan has largely been attributed to management of jack pine plantations (Probst and Weinrich 1993).

Available wintering habitat could also be reduced by hurricanes in pine forest habitat in the non-breeding areas in the Bahama Islands.

Habitat loss and fragmentation

In the past, habitat loss through permanent conversion of jack pine barrens to agriculture or forestry has reduced the amount of suitable habitat for this species. Fragmentation and isolation of jack pine stands may also have contributed to decreases in Kirtland’s Warbler populations. The species prefers and has higher breeding success in larger patches of jack pine, thus activities that fragment forest patches could result in population declines.

Cowbird parasitism

Kirtland’s Warblers appear to be particularly vulnerable to cowbird parasitism. In the 1960s and 1970s, about 70% of all Kirtland’s Warbler nests in Michigan were parasitized by cowbirds and produced an average of less than one young per nest (Ryel 1981). After cowbird control was implemented in 1972, the number of parasitized nests fell to 5% and the average number of young per nest increased to almost three (Kelly and DeCapita 1982; Walkinshaw 1983). Cowbird control appeared to stabilize the population, but increases in Kirtland’s Warbler numbers occurred only when cowbird control wasaccompanied by increases in habitat that followed from management and the two large wildfires that occurred in 1975 and in 1980.

Brown-headed Cowbirds are common throughout southern Ontario, but less common in the parts of Ontario where Kirtland’s Warblers have been recorded. In addition, recent Breeding Bird Survey trends show statistically significant long-term population declines for cowbirds in Canada (Downes and Collins 2007), so this factor may not be a significant threat to Kirtland’s Warblers in Canada.

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