Birds in boreal Canada : communities, densities and adaptations

All boreal conifer forests share the same community of birds, with some additions or subtractions, whereas much greater differences are found in comparison with conifer habitats of other, nearby regions to the south, where such communities do not form the vegetational climax. The regular and abundant members of the conifer forest avifauna include Spruce or Ruffed Grouse (Tetraonidae), Canada* or Blue Jays (Corvidae), Boreal Chickadee (Paridae), Swainson's Thrush (Turdidae), Goldenand Rubv-crowned Kinglets (Svlviidae), Magnolia and Yellow-rumped Warblers (Parulidac), Northern Junco*, * Note on bird names: English names follow those of the AOU Cheek-list (1957) and its Supplements (1973, 1976), exrept for Canada Jay (in preference to Gray Jay), and Northern Juneo (in preference to Dark-eyed Juneo as in ABA Cheeklist, [1975]). Separate names'for those forms reduced to subspecific rank in the AOU Supplement (1973) have been used where appropriate. and Chipping or White-throated Sparrows (Fringillidae), plus the carduelinc finches. Densities of breeding birds rise through the succession, from 100 to 200 pairs per km in open jack pine stands or low bog forests through 200 to 100 pairs per km in spruce to 300 to 600 pairs per km in balsam fir, the richest conifer habitat in the boreal region. Still higher densities, up to over 800 pairs per km, occur in the presence of spruce budworm outbreaks, owing to the response of a few warbler species to this food source, and in mixed coniferousdeciduous stands. Broad-leafed forests in the boreal region are relatively limited both in duration on a site and in site variety, and they lack a unique bird community. Birds of such sites arc drawn partly from the eastern deciduous fauna and partly from that of successional conifer habitats. The regular and abundant members of this group include Ruffed Grouse (Tetraonidae), Yellowbellied Sapsueker (Picidae), Least Flycatcher (Tyrannidae), Black-capped Chickadee (Paridae), American Robin and either Swainson's Thrush or Veery (Turdidae), Red-eved Virco (Vireonidac), Chestnutsided and Mourning Warblers, Ovenhird, and American Redstart (Parulidac), and Rose-breasted Grosbeak (Fringillidae). West of the Rockv Mountains, Dusky Flycatcher and Western Wood Pewee (Tyrannidae), Warbling Vireo (Vireonidae), Orange-crowned and MacGillivray's Warblers (Parulidac) at least partially replace other members of the same families. Densities of breeding birds fall to the north and west, from between 400 and 600 pairs per km in New Brunswick to Ontario, down to 200 to 300 pairs per km in northern British Columbia, but rise again west of the Rockies. All birds of open habitats in the boreal region also occur in other biomes, for example: Sharp-tailed Grouse (Tetraonidae), Killdecr (Charadriidae), Common Nighthawk (Caprimulgidae), and Savannah Sparrow (Fringillidae). Many widespread species also occur in the low shrubby stages of early forest succession, including Alder Fly catcher (Tyrannidae), House Wren (Troglodytidae), Robin and Hermit Thrush (Turdidae), Mourning Warbler and Common Yellow throat (Parulidac), American Goldfinch, Northern Junco, White-throated and Song Sparrows (Fringillidae). Many others are associated with edges, both those between forests and open areas within the region, and with adjoining open biomes. Bird densities in open habitats are typically lower than in forest, usually in the range from 150 to 450 pairs per km. Wetland birds are less evenly distributed than those in terrestrial communities, and quantitative data are less satisfactory. Recognizable groupings include birds of deep marsh, of open water, and of shallow marshes, fens, and assorted hogs, the last probably the wetland types most typical of the boreal region. Most birds are restricted to one or a few of these wetland groups, but the interdigitation of the types means that many wetland birds are widespread and abundant, for example: Mallard, Green-winged Teal, Lesser Scaup (Anatidae), Common Snipe (Scolopacidae), Alder Flycatcher (Tyrannidae), Yellow Warbler, Northern Waterthrush, and Common Yellowthroat (Parulidae), Red-winged and Rusty Blackbirds (Icteridae), Savannah, Lincoln's, Swamp, and Song Sparrows (Fringillidae). Density data are lacking except for some types of bogs, in which densities seldom exceed 100 pairs per km; more fertile types of wetlands probable support much higher densities of birds. Artificial environments include those cleared for agriculture or right-of-way, and artificial structures such as buildings and dams. Birds of the open habitats are associated largely with remnants of shrubbery, or with the edges between disturbed and more natural areas, but a few such as Killdeer (Charadriidae) and Savannah Sparrow (Fringillidae) are almost ubiquitous. Urban settlements feature especially three introduced species, plus four sw allows (Hirundinidae), American Robin (Turdidae), Chipping and Song Sparrows (Fringillidae). Densities in towns vary directly with time