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Black-throated Green Warbler

Limiting Factors

Black Throated Green WarblerThere is considerable debate in the scientific literature as to the relative significance of events occurring on breeding, wintering, and migratory stopover habitats in terms of their effects on songbird populations. This section will mainly deal with events occurring in Alberta, within the breeding range of the Black-throated Green Warbler, including Habitat Loss and Fragmentation, Agriculture, Forest Management, Energy Sector Activities, Nest Predation and Parasitism as well as Winter and Migration Stopover Habitat.

Habitat Loss and Fragmentation:  The loss and fragmentation of forest habitat are closely allied processes. "Habitat loss" refers to the conversion of suitable habitat into unsuitable habitat, while "fragmentation" is the increasing isolation and division of remaining habitat. Habitat fragmentation has been implicated in the declines of neotropical migratory songbird populations across North America. As the area of patches of suitable habitat declines, and the distances between those patches increases, the likelihood of individual patches supporting a subpopulation of birds declines. There is little data available on individual species, but it is reasonable to assume that Black-throated Green Warblers will respond to habitat fragmentation in a similar fashion as other neotropical migrants. 

These birds may also actively avoid edge habitats associated with forest harvesting and be reluctant to cross habitat openings as has been shown for other species of songbirds. In eastern North America, local extirpations of the Black-throated Green Warbler have been documented in heavily fragmented forests. Much of our understanding of the effects of forest fragmentation on birds developed from studies in agricultural landscapes in eastern North America, but recent research in Alberta is shows similar but less severe results. In an area of northern Alberta where much of the landscape remains forested, Black-throated Green Warblers have declined by roughly 50% in forest fragments only five years after harvesting.

These factors together (habitat loss, fragmentation, and edge avoidance) may lower bird reproductive success in fragmented forests by influencing pairing success or other factors. However, no specific studies have been conducted on Black-throated Green Warblers in relation to these factors. Habitat corridors may facilitate bird dispersal in a fragmented landscape, but there is likely to be a critical threshold in the degree of landscape fragmentation beyond which populations may decline more rapidly. Overall, it is thought that the effects of habitat loss outweigh the effects of habitat fragmentation. Thus, although the two processes are clearly linked, conservation efforts are probably best directed at slowing the rate of direct habitat loss.

Agriculture:  Agricultural, as well as urban, expansion may beHayfield implicated in the possible northward contraction of the Black-throated Green Warbler's breeding range. In the Alberta breeding range of the Black-throated Green Warbler, agriculture is largely limited to part of the Peace River drainage, and a smaller part of the Athabasca River drainage. In the Peace Country,
45 000 kilometres, or 12%, of the land is in agricultural production. However, agricultural expansion in this area is nearing its limits as all economically viable land is already in use. 

Forest Management:  Timber harvesting has increased significantly in Alberta in recent years. Until relatively recently, most harvesting by the forestry sector was focused on coniferous Pulpwood Forest Clearcutting stands, but as of the mid-1980s, deciduous and mixedwood forests have also come under pressure. Large forested areas have been allocated to forest companies under Forest Management Agreements (FMAs). As of December 1995, there were 11 FMA holders in Alberta covering more than 13.6 million hectares of the province's forested area; by November 1998 this had increased to 17 FMA holders covering roughly 19.6 million hectares. These figures represent 60% and 87% allocation of the province's landbase, respectively. The proportion of the Annual Allowable Cut (AAC) allocated has also steadily increased. As of January 1995, roughly 85% of the province's AAC of timber had been allocated (94% coniferous, 73% deciduous), and the provincial government anticipates further increases in allocation and harvesting.  Black-throated Green Warbler habitat will, likely, be reduced in quantity and quality under the current forest harvesting strategies. Current operating ground rules for forestry in the province dictate a two- or three-pass clear cutting system, with the oldest stands prioritized for harvest. Furthermore, with rotation lengths of 60-80 years, stands won't be allowed to reach an age where they will support Black-throated Green Warblers. Overall, current forest harvesting strategies will lead to a reduction in the proportion of old stands in the landscape, and will fragment previously contiguous forest. It is estimated that habitat availability for the Black-throated Green Warbler could be reduced by 75-95% over a 300 year time horizon, depending on the harvest rate and other factors, with reductions of 50-70% forecast over the next 100 years. 

Energy Sector Activities:  Oil and gas development in the forested region of Alberta impacts the landscape through the clearing of forest for seismic exploration lines, pipelines, and wellheads. Currently, roughly 14 000 kilometres of new seismic lines are cut each year, and an additional 20 000 kilometres of existing lines are recleared annually. At an average width of 6 inches, this translates to approximately 8400 hectares of new forest cutting annually, and 12 000 hectares of forest being reopened. Thus, the total amount of forest harvested annually by the oil and gas sector is substantially less than by the forest industry. However, due to the linear nature of these disturbances, the total area of forest that is affected may be significantly higher. This adds to the impacts of forestry in causing habitat loss, and further reduces the availability of forest undisturbed by human activities as well as creating semi-permanent open corridors into forested landscapes.

Nest Predation and Parasitism:  In the heavily fragmented landscapes of eastern North America, where agriculture is the dominant land-use, predation and parasitism of nests is thought to be a significant limiting factor of songbird populations. The Brown headed Cowbird, regularly parasites nests of neotropical migrant songbirds, and predation of eggs by corvids can be significant. Black-throated Green Warblers are known to be regular hosts for cowbirds in the east. Studies in boreal Alberta suggest that, over the short-term, neither predation nor parasitism rates increase following forest fragmentation from logging, but whether the predator community or cowbirds simply had not yet responded to recent harvesting in these studies is unknown. Cowbirds and corvids are currently present at low densities in this region. However, species such as the Brown-headed Cowbird may gain access to forested landscapes via linear corridors, such as resource extraction roads and seismic lines. Edge habitats, which may facilitate predation or parasitism, are short lived adjacent to cut blocks, but are longer-term features associated with linear disturbances. Most nest predation events were attributed to small mammals, whose response to habitat may be complex, and may be species- and habitat-specific.

Winter and Migration Stopover Habitat:   Winter habitat degradation is likely a significant factor affecting songbird populations and may, in fact, be more significant than factors on the breeding grounds. Forest habitats in the wintering range of most North American songbirds are being depleted at an alarming rate: forest loss in Central America has been estimated at 2% annually. Little is known about the habitat requirements of the Black-throated Green Warbler during migration. In fact, there is no documentation available on its migratory routes or frequently used stopover sites. However, the importance of migratory stopover sites is becoming recognized. 

Reprinted from Alberta Wildlife Status Report No. 23 (1999), with permission from Alberta Sustainable Resource Development.

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