Northwestern Crow Corvus caurinus This information was scanned from The Birds of British Columbia (Campbell et al.), Volume III, pages 228-235. Volumes I, II and III of The Birds of British Columbia, can be ordered electronically at: orders@ubcpress.ubc.ca from UBC Press in Vancouver, British Columbia..

 

Range Northwestern Crow is resident along the Pacific coast from south-coastal and southeastern Alaska south through British Columbia west of the Coast Mountains and Cascade Mountains, to northwestern Washington, including the Queen Charlotte Islands, Vancouver Island, and all other coastal islands.

Status Fairly common to common summer resident along the coast. Throughout the rest of the year a common to very common resident in the Georgia Depression Ecoprovince, including the Sunshine Coast, Gulf Islands, and southeastern Vancouver Island; fairly common to common resident on Western Vancouver Island, the Southern and Northern Mainland Coast, and the Queen Charlotte Islands, in the Coast and Mountains Ecoprovince. Locally abundant to very abundant from late summer through early spring in the vicinity of nocturnal roosts, particularly in the Georgia Depression.

Status Change No change.

Nonbreeding The Northwestern Crow occurs along the length of the British Columbia coastline, including Vancouver Island and the Queen Charlotte Islands. Numbers dwindle rapidly with increasing elevation and distance from the sea. The absence of records from the northern Boundary Ranges of British Columbia confirms evidence from the Alaskan southeast (Swarth 1922; Gabrielson and Lincoln 1959) that this crow, at the northern end of its range in regions with intact natural habitats, does not normally occur more than a few kilometres from tidewater. A notable exception to this coastwise concentration occurs when spawning runs of fish enter the large rivers and the Northwestern Crow follows the food source as much as 120 km upriver (W. Prescott pers. comm.). This is especially notable on the Fraser, Skeena and Nass rivers. Although the Northwestern Crow is widespread along the coast, its centre of abundance is in the Georgia Depression, including the southeast coast of Vancouver Island and the lower Fraser River valley. The Northwestern Crow occurs at elevations from sea level to 1,700 m, but it is most numerous at lower elevations. Although it frequents a wide range of natural and human-influenced habitats, it is closely linked to marine shorelines, where it forages much of the time. Thus, shallow marine shores with interspersed rocky headlands, boulder beaches, and sand, gravel or mudflats are the primary feeding grounds of this crow. It occurs infrequently or is absent from the sheer rock margins of the many fiords that characterize the mainland coast. There the steep, rocky shorelines plunge directly into deep water, providing few foraging opportunities for this bird. Moreover, the surface water of these inlets is low in salinity, and the shore fauna of invertebrates, on which the crow feeds, lacks variety. Tidal estuaries at the heads of the fiords normally support a small population of crows. The availability of exposed intertidal areas to crows varies with the season. During the winter months, the lowest tides, and therefore the richest feeding habitats, occur at night; during the spring and summer, low tides occur during daylight hours. This crow responds by changing foraging practices with the seasons and concentrating more on human-altered habitats when the beaches are less available. At such times, the tameness of the Northwestern Crow becomes notable as it forages on lawns, roadsides, parking lots, ferry wharfs, marinas, refuse piles, garbage landfills and similar areas. It adapts easily to the environmental changes imposed by people and, during the non-breeding parts of the year, is now more abundant around towns and villages than in its natural environment. Deforestation has helped increase crow populations in some areas. Habitats now include farm fields, pastures, playgrounds, residential neighbourhoods, golf courses, cemeteries and bush camps. Farm fields sown with corn and grains are favoured after the harvest. Garbage landfills have been particularly important in providing a winter food source. This close association with humans and the attendant ease of acquiring food, particularly through the winter months, has likely contributed to the increase in the populations of the Northwestern Crow in urban areas. An analysis of Christmas Bird Counts for the period 1975 to 1994 in select areas of southwestern coastal British Columbia indicates that the larger cities attract more crows in winter than do smaller cities and rural sites, and that agricultural areas are also important foraging sites, where this crow uses seasonally available food sources such as fruit and grain. The Northwestern Crow is not normally considered migratory in British Columbia, but local movements appear to be widespread. Birds move from exposed coasts to more protected areas in autumn. With the coming of spring, winter flocks disperse, and areas of coastline that were almost without crows during the winter receive their courting pairs. Total numbers of crows reported from the Georgia Depression illustrate this point. They average about 30,000 birds from September through March, then drop to an average of about 7,000 birds from April through August. This difference can be only partly attributed to flocking behaviour during the non-breeding season. Pearse (in Bent 1946) reports evidence of movements to farmlands on eastern Vancouver Island from the lightly inhabited mainland coast. Behaviour important to the Northwestern Crow during the non-breeding season is its congregation into large flocks that move between feeding areas and roosting sites. Of 608 records consisting of 100 or more birds (excluding Christmas Bird Counts or other such surveys), 87% were reported from September through March. In most areas, local movements occur twice daily: once at dawn, when the entire population of the roost leaves for foraging areas, and again in late afternoon, as the crows muster in noisy flocks to assemble at the roost sites for the night. On the south coast, some of these movements are spectacular, with birds streaming by in flocks of hundreds or, occasionally, thousands. For example, in March 1988, 2,071 crows were counted arriving at a roost on Sidney Island between 1810 and 1859 hours. Twenty-one flocks were counted. They ranged in size from 6 to 460 birds, with an average of 99 birds per flock; average time between flocks was 2.4 minutes (R.W. Butler pers. comm.). Some of the largest known roosts are on Chatham Island, James Island, Grouse Mountain, Douglas Island (Port Coquitlam), Burns Bog and Burnaby, but there are many local roosts occupied by smaller numbers of crows. The roosts are usually situated in undisturbed areas of large, old conifers, sometimes interspersed with arbutus, Garry oak, and bigleaf maple. Frequently they are on islands away from human activity. Little is known about the characteristics of these traditional roosts sites, or of their role in the biology of the Northwestern Crow. Northern populations have not been reported flocking in such numbers. The Northwestern Crow is present throughout the year in British Columbia.

Breeding Nests Eggs Nest Success The Northwestern Crow has a widespread breeding distribution along the British Columbia coast from southern Vancouver Island north to at least the Queen Charlotte Islands and Portland Inlet. Its eastward breeding distribution is imperfectly known but reaches at least to Chilliwack in the Georgia Depression and to Kitimat on the Northern Mainland Coast. Most birds nest within a few kilometres of the sea. In summer, as in winter, its highest numbers are in the Georgia Basin. An analysis of Breeding Bird Surveys for the Northwestern Crow in British Columbia shows that the mean number of birds on coastal routes increased at an average rate of 2% over the period 1968 through 1993. This species has been reported breeding from near sea level to 510 m elevation. Much of the coastline of British Columbia is margined by coniferous forests with a varied understorey of berry-producing shrubs. Where the coastal strip of forest has not been removed by clearcut logging, it provides nesting and roosting sites for this crow close to the foraging area of the beach. Nesting sites are usually near a forest edge that supports many smaller trees and tall shrubs. Other nesting habitats include estuaries, river valleys, lakeshores, offshore islands, marshes and sloughs. The population of crows nesting within cities is increasing, but often goes unnoticed except perhaps for the early morning, prelaying, courtship-begging of females. As nesting proceeds, however, some crows begin to harass passers-by, occasionally making contact with them, usually about the head. Once the young are off the nest and begin their raucous begging for food throughout the day, they seldom go unnoticed. Autumn leaf-fall exposes the crow nests, which can be seen even in boulevard trees no more than 3 m high. In selecting its nesting territory, the Northwestern Crow tends to avoid extensive areas of dense forest far removed from the ocean. Nesting habitat includes the hundreds of small rocky islets scattered along the coast. Many of these support nesting colonies of marine birds and feature a lush vegetation of fortes, bulbous plants, grasses, stunted crab apple, elderberry, salmonberry, snowberry, spirea, Pacific ninebark and even a few wind-wracked conifers. On these islands and islets, the Northwestern Crow frequently nests in low shrubs or on the ground. The Northwestern Crow has been recorded breeding in British Columbia from 23 March (calculated) to 24 July. Colonies: Colonial nesting has been reported for this species (Bent 1946). In British Columbia, groups of 2 to 19 nesting pairs occurring on small islands and along hedgerows and boulevard trees have been referred to as colonies. However, we do not consider this species to be a colonial nester in British Columbia. On Mitlenatch and Mandarte islands, each pair defends a territory of about 0.5 ha in which the nest is situated (R.W. Butler pers. comm.); 22% of the breeding pairs are assisted by yearlings in defending the territory (Verbeek and Butler 1981).

Nests:  Most nests (64%; n = 391) were found in trees, both coniferous (31%) and deciduous (18%). The most often used conifers were Douglas-fir (11%), Sitka spruce (10%), and lodgepole pine (4%), followed by Western red-cedar, hemlock, and grand fir. Pacific crab apple (5%) and bitter cherry (4%) were the most often used deciduous trees, followed by willow, red alder, black cottonwood, arbutus, maple, Pacific dogwood and Garry oak. On rocky island sites, where trees were scarce or absent, nesting in shrubs (16%) or on the ground (14%;) was commonplace, a behaviour first reported by Darcus (1930). Shrubs included saskatoon (4%), Pacific ninebark (2%), willow, Nootka rose, thimbleberry and elderberry. In addition, virtually every other local species of native tree and shrub was reported to have harboured a nest. Where the nest situation was described, most nests (63%; n = 269) were found among the branches of trees or shrubs. Nests in conifers were often saddled on the branches, sometimes close to the trunk. In deciduous trees and shrubs the nests were almost always placed among the branches well away from the trunk. Ground nests were found under or among ground cover (10%), beneath a log or rock, on rocky ledges, in a rock crevice, or among the lower branches of shrubs. The bulky nests were composed primarily of twigs, sticks, and small branches. Nests were lined mainly with bark strips, followed by grasses, moss, feathers, fine rootless, plant fibres and sometimes hair or human-made items such as paper, rope, fiberglass insulation and mattress filling. On Mitlenatch Island, all nests were lined with cedar bark strips even though no cedars grew on the island (Butler 1980). The heights of 372 nests ranged from ground level to 45 m, with 59% between 1.2 and 8 m. Eggs: Dates for 270 clutches ranged from 27 March to 8 July, with 52% recorded between 2 and 26 May. Calculated dates indicate that eggs can occur as early as 23 March. Sizes of 241 clutches ranged from 1 to 6 eggs (1E-10, 2E-19, 3E-46, 4E-98, 5E-63, 6E-5), with 67% having 4 or 5 eggs. In a sample of 187 clutches from Mandarte and Mitlenatch islands, 54% contained 4 eggs (Richardson et al. 1985). On Mitlenatch Island, the mean incubation period from the day the last egg was laid until all were hatched was 18.3 + 0.85 days (n = 19) (Butler et al. 1984). Nest Success: Of 28 nests found with eggs and followed to a known fate, 16 produced at least 1 fledgling, for a success rate of 57%. On Mandarte and Mitlenatch islands, 79% of nests fledged at least 1 young (Butler et al. 1984). On the same islands, Richardson et al. (1985) found that the proportion of young that survived from hatching to fledging was 67% and 37%, respectively. Clutches of 3,4, and 5 eggs fledged an average of 0.98, 1.22, and 1.20 young, respectively; 51% of the nestlings died within 7 days of hatching, and starvation appeared to be the predominant cause of death. Young: Dates for 174 broods ranged from 10 April to 24 July, with 53% recorded between 25 May and 20 June. Sizes of 148 broods ranged from 1 to 6 young (1Y-29, 2Y-49,3Y-43,4Y-23,5Y-3,6Y-1), with 62% having 2 or 3 young. The nestling period was reported as 32 + 2.5 days (n = 20) on Mandarte Island and 26 + 3.4 days (n = 55) on Mitlenatch Island; the earlier departure in the latter case may have been induced by observer disturbance (Butler et al. 1984).

Remarks Opposing views have been expressed regarding the systematic status of the two crows in British Columbia. The American Ornithologists' Union (1983) notes that the Northwestern Crow and American Crow are closely related and are considered nonspecific by some authors (Johnston 1961; American Ornithologists' Union 1983; Sibley and Monroe 1990). Difficulties in field identification of the two species make it impossible to determine, under field conditions, the precise areas of overlap in their distribution (see American Crow). On the basis of specimens collected and possible differences in call notes, the Northwestern Crow is thought to occur exclusively west of the coastal mountains, and the American Crow east of the mountains. However, numerous potential zones of contact are available along major river systems such as the Fraser, Skeena, Nass and Stikine, or along highway corridors. We have deliberately excluded observational reports from these areas, because of the uncertainty of identification. All coastal records of American Crow and all interior records of Northwestern Crow that are not supported by specimens have been excluded.Any interior reports of Northwestern Crow should be treated cautiously, pending further research. The Northwestern Crow may be highly philopatric. With the exception of one report of a crow banded near Comox on 11 June 1979 and recovered north of Victoria on 26 November 1980, all recoveries (n = 38) of the Northwestern Crow were from the banding location, some after a number of years had passed. For example, a bird banded near Campbell River on 22 June 1969 was recovered there in January 1981, nearly 12 years later; another, banded in the same area on 3 June 1977 was recovered there over 7 years later, on 15 November 1984. The Northwestern Crow is a major predator and scavenger on coastal seabird colonies. On Mandarte Island, for example, it is a major predator upon the eggs of cormorants, the Glaucous-winged Gull and the Pigeon Guillemot. There it consumes 22% of all eggs laid in the first clutches of Double-crested and Pelagic cormorants (Verbeek 1982), and also preys on the newly hatched young of the cormorants (Butler et al. 1985). The disturbance caused by people landing among the cormorant nesting colonies results in the parent birds leaving the nests unprotected. The nests are then raided by the crows. Thus the number of fledglings produced by a colony is strongly influenced by the frequency of human disturbance at the colonies. Even a single landing party a week can result in many cormorant nests failing to produce fledglings. On islands with burrow-nesting seabirds, crows not only scavenge food droppings but also prey on the nestlings as they emerge from their burrows (Butler et al. 1985). The Northwestern Crow frequently preys upon the eggs and nestlings of many species of small birds. Old nests of the Northwestern Crow are used as nest sites by several species of birds. In British Columbia, the Great Horned Owl, Merlin and Long-eared Owl have been documented nesting in them. British Columbia probably supports most of the world's population of Northwestern Crows. Although considered wildlife under the British Columbia Wildlife Act, Northwestern Crows are afforded no protection from shooting for much the same reasons discussed under American Crow. For additional information on the biology and behaviour of the Northwestern Crow, see Butler (1974, 1980), Verbeek and Butler (1981), Butler et al. (1984), and Richardson et al. (1985).

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