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Black-throated Green Warbler

Black Throated Green WarblerThe Black-throated Green Warbler (Dendroica virens) is a neotropical migrant. The species breeds across the boreal forests of Canada and the north eastern United States, and winters in Mexico and Central America. Although common in some areas of suitable habitat in Alberta, little is known about its ecology in the province. Recent concerns over the degradation and loss of its breeding habitat through forest harvesting have led to the Black-throated Green Warbler being included on the Blue List of species that may be at risk of extirpation in Alberta.  

In general, descriptions of the habitat of the Black-throated Green Warbler emphasize an association with coniferous tree species in mature forests. However, across its breeding range the Black-throated Green Warbler exhibits fairly broad patterns of habitat use. In Canada, it has been described as characteristic of the most fertile needle-leafed forest community present in a region. In Alberta, records for the species exist from the Boreal Mixedwood, Boreal Foothills, and Boreal Uplands Ecoregions. Recent work has revealed two consistent features of the Black-throated Green Warbler's habitat in Alberta: deciduous or coniferous dominated mixedwood, with a dependence on the presence of some mature coniferous trees in the forest canopy older stands; perhaps 80-130 years old or older. General habitat descriptions for the Black-throated Green Warbler have been published from across its range, including Ontario, Quebec, Maine, New Hampshire, Minnesota, and British Columbia. The species is fairly plastic in its habitat requirements at a continental scale, and has been described in association with a variety of coniferous stands, and specifically with Balsam Fir and Hemlock stands. Mixed coniferous-deciduous stands have been frequently described including pine-aspen, spruce-aspen and spruce-poplar. 

In almost all cases, the Black-throated Green Warbler is classified as a forest specialist: it inhabits larger tracts of forest and tends to avoid disturbed or edge habitats and small forest patches. Recent work in Alberta reveals a consistent association of the Black-throated Green Warbler with deciduous or coniferous dominated mixedwood, with a dependence on the presence of some coniferous canopy trees.

Black-throated Green Warblers in Alberta are typically found in Black Throated Green Warbler mixed stands of Trembling Aspen and White Spruce. Both aspen-dominated and spruce-dominated stands are used. The age of forest stands also appears to influence their use by Black-throated Green Warblers. Near Lac La Biche, the Black-throated Green Warbler was found in significantly higher numbers in old (greater than 120 years) aspen stands than in mature (51-63 years) or young (23-26 years) stands.  The Black- throated Green Warbler's use of individual spruce trees for singing and foraging increases significantly with increasing tree diameter, which is correlated with stand age.

Other evidence corroborates the Black-throated Green Warbler's reliance on older stands. Both White Spruce and Paper Birch occur in greater abundance in the canopy of older stands, and, in fact, these tree species are two of the vegetative descriptors that most clearly distinguish mature from old stands. 

The Black-throated Green Warbler is a small, foliage gleaning wood warbler. Males and females are of similar size (11-12 centimetres long and weighing 8-11 grams), but males are more brightly coloured. Birds are reproductively mature in their first year, and first year males are commonly known to breed. Almost no data exists on longevity, but, based on a band return, the oldest bird on record was a minimum of 5 years and 11 months of age.

The Black-throated Green Warbler is almost entirely insectivorous. Breeding birds tend to take a high volume of lepidopteran (butterfly and moth) larvae, but will forage opportunistically on other abundant prey such as midges along a lakeshore. This warbler is also known to consume beetles, true bugs, wasps, ants, gnats, moths, flies, spiders, and mites and, together, these other prey types may outnumber lepidopterans. Unlike several other related wood warbler species, the Black-throated Green Warbler is not known as a major predator of Spruce Budworms (a destructive pest of coniferous trees), except when the insect is at epidemic levels. Most foraging by the Black-throated Green Warbler is diurnal, and involves gleaning insects off leaves or small branches, but birds will also hover at branch tips and "hawk" insects in midair. During wet weather individuals will forage more along the bark and trunks of trees, and during migration they will also eat berries and other vegetative material.  These particular Warblers arrive in Alberta from mid to late May. Very few spring migrants are ever observed in southern Alberta, suggesting that most birds enter the province from the east. Males arrive several days earlier than females, and immediately begin establishing territories by singing and exhibiting aggression towards other males. 

Fall migration in Alberta occurs between mid-August and mid September. Many more Black-throated Green Warblers are observed in the fall migration than in spring in Alberta, and virtually all banding records for the province are from late August and early September, suggesting the species may follow different fall and spring migration routes. Black-throated Green Warblers, especially young birds, are often observed in mixed species flocks of warblers. Birds turn up regularly, but in very low numbers, at Calgary. 

Red squirrels and Blue Jays have been identified as major predators of egg, and young.  In northern Alberta, other corvids may also be important nest predators, although small mammals are probably responsible for more egg predation events in nests at lower heights. 

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

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