The Northwestern Crow (Corvus caurinus) is a species of crow that lives on seashore and lowlands within about 100 km of the Pacific Ocean coast between southern Alaska and the northmost corner of Washington.
Corvus caurinus can be told apart from the American crow (Corvus brachyrhynchos) by its body size, which is about 10% smaller, and its smaller feet. The northwestern crow is a seashore predator of marine invertebrates, birds’ eggs, and chicks. It is found mostly around human habitation but also occurs along beaches and on seabird islands.
Studies have shown that crows have complex cultures and social interactions. There are stories from credible sources of crows ‘holding court’, executions, and funerals for fallen comrades (Marzluff and Angell 2005). It is difficult to make sense of these observations and it indicates how little we understand of these common birds. Crows also are among the cleverest of birds possibly nearing the intelligence levels of great apes.
Diet And Population
There have been no systematic censuses of northwestern crows but Verbeek and Butler (1999) indicated that the North American Breeding Bird Survey did not show an increase between 1980 and 1995. They thought that sudden increases in local areas are probably a result of shifts in distribution rather than population changes but further study is needed.
Northwestern Crow diet includes a beach smorgasbord of clams, whelks, crustaceans, sea urchins, and small fish. On land it eats, insects, eggs, and nestlings of cormorants, gulls, songbirds, oystercatchers, auklets, and herons, as well as fruits, and carrion. Some surplus food is cached for later consumption.
Northwestern Crow Behavior
Feeding, Hunting & Foraging
Northwestern crows are well known to scientists for their ability to crack clams on rocks to open the shells. Crows find clams buried on beaches that they transport in their bills to rocks, wharves, paved walkways and roads where the clams are dropped until they break.
In a series of clever experiments, Howie Richardson and Nico Verbeek (1986) showed that crows were adept at knowing the right size of clam that was worthwhile to crack open. They showed that crows often abandoned clams that were too small to carry to the rocks in favor of the large and more profitable clams.
The crows decide on whether a clam is worth eating or not by its weight (O’Brien et al. 2005).
Flight Patterns
Flocks of several thousand crows form in late summer in Victoria and Vancouver, British Columbia, and Seattle, Washington to fly to evening roosts. The number of crows entering roosts numbers from a few birds to many thousands of individuals.
The large communal roosts are used all year round but the greatest number is present from about October to March. In the early morning, crows return from the roost to feed in parks, suburban lots, farmlands, and beaches.
Territory-holding crows in Vancouver defend territories through the winter and non-territorial crows forage as a group on the fringes. Near sunset, flocks begin to form to fly to the roosts.
The flights to the roost can take some crows up to 45-60 minutes to complete. The flights often involve tens or hundreds of crows that call to one another along the way. Their arrival at the roost is even more boisterous than their commuting flights. Calling, swooping, bowing, and tail flicking displays are signs of their heightened anxiety.
Why crows fly to roosts is not clear but it might have to do with predator avoidance. Red-tailed hawks and especially great-horned owls are predators of crows.
Unlike most songbirds that discretely enter roosts near where they feed, crows fly great distances to join in large numbers making a commotion. Possibly the raucous behavior discourages predatory owls and hawks from waiting hidden to pounce on unsuspecting sleeping crows in the roost. Crows might use roosts to find a new mate, for warmth in winter, or to share information on the whereabouts of food.
Aggressive, Defensive & Territorial Displays
Crows vigorously defend nesting territories and bully other crows around food items. A threat display begins with the erection of feathers on the head held rigid over the shoulders. The bill is pointed slightly downward and the feathers on the belly and flanks are splayed.
As aggression mounts, the wings droop ready to strike blows, and the tail is raised about 45o off the horizontal. Fights are vicious affairs that begin with lunges of the feet, and jabs with the bill.
The strategy is to pin the opponent to the ground and direct several hard jabs of the bill into the body. There is much slapping with the wings, snapping of the bill, and squawking at opponents.
Northwestern Crow Vocalizations
Crows have a very wide repertoire of sounds whose meanings are largely unknown to us. They use some calls in a variety of situations that has left researchers confused to the meaning.
There is still much that is unknown about crow society. People often ask if crows can speak. Although they utter a host of sounds, they do not appear to mimic noises around them unlike well-known mimics such as the mynahs and starlings.
Courtship & Breeding Behaviour
Crows are very secretive about their breeding behavior. Pairs will be seen together preening each others feathers and around the eyes in a very tender fashion.
During mating, the female crouches in front of the male who mounts onto her back. They push the tails aside and contact their cloacas. It is a quick affair after which the birds fly to a perch where they often preen.
Nesting Habits
The northwestern crow begins to breed after its first birthday but nothing is known about when they establish breeding pairs. Nests are built beginning in late February and through March in trees and shrubs and on the ground on small islands.
Both parents build the nest. Twigs provide shape to the nest and finely shredded cedar bark, moss, wool, and grass stems form the lining. An average of 4 eggs are laid per nest in April, May, and June and they hatch in May and June.
Only the female incubates the eggs and her mate provisions her with food. This behavior allows constant vigilance of the eggs and young chicks from other crows intent on cannibalizing them.
Chicks hatch naked with a few bits of down. They are fed by both parents and attain 80% of the adult mass of 310 g at about one month of age when they leave the nest. The adults continue to feed them as the young scramble about the branches near the nest for another week or two gradually following the parents on foraging forays.
One brood of chicks is raised each year by both parents. Some pairs are assisted in raising the young by the previous year’s offspring. By July, families of crows make a racket when the young beg for food, and the parents ward off cats, raccoons, dogs, and humans that venture too close.
People are sometimes the recipient of the wrath of adult crows that swoop down from trees to peck or slap at heads. Crows are not villains – they are defending their nestlings. A careful search will likely reveal a recently fledged crow hiding in the shrubbery. Alfred Hitchcock’s horror movie The Birds has permanently etched into some minds that crows are villainous. Hollywood films like this have inflicted much undue harm to the public view of wild animals.
References:
- Butler, R. W. 1974. The feeding ecology of the northwestern crow on Mitlenatch Island, British Columbia. Canadian Field-Naturalist 88: 313-316.
- Butler, R.W., N. A. M. Verbeek and H. Richardson. 1984. The breeding biology of the northwestern crow. Wilson Bulletin 96: 408-418.
- Emery, N. J. and N. S. Clayton. 2004. The mentality of crows: convergent evolution of intelligence in corvids and apes. Science 306: 1903-1907.
- James, P. C. and N. A. M. Verbeek. 1983. The food storage behaviour of the northwestern crow. Behaviour 85: 276-291.
- Marzluff, J. M. and T. Angell. 2005. In the company of crows and ravens. Yale University Press, New Haven Connecticut.
- O’Brien, E. L., A. E. Burger and R. D. Dawson. 2005. Foraging decision rules and prey species preferences of northwestern crows (Corvus caurinus). Ethology 111: 77-87.
- Richardson, H. and N. A. M. Verbeek. 1986. Diet selection and optimization by northwestern crows Corvus caurinus on little neck clams Venerupis japonica. Auk 104: 263-269.
- Verbeek, N. A. M. 192. Egg predation by northwestern crows: its association with human and bald eagle activity. Auk 99: 347-352.
- Verbeek, N. A. M. and R. W. Butler. 1981. Cooperative breeding of the northwestern crow Corvus caurinus. Ibis 123: 183-189.
- Verbeek, N. A. M. and R. W. Butler 1999. Northwestern Crow Corvus caurinus. Birds of North America, No. 407. The Birds of North America, Inc., Philadelphia, PA.