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Black Throated Green Warbler

The Black-throated Green Warbler (Dendroic 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 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.

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.

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.

Black-throated Warbler

Black-throated Warbler

Black-throated Warbler

Black-throated Warbler