American Crow (Corvus brachyrhynchos) Julie A. Craves
Oakland Co., MI. March, 2009. © Robert Epstein
As Verbeek and Caffrey (2002) declared, everybody knows crows. The smart and (Click to view a comparison of Atlas I to II) conspicuous American Crow is found nearly across the continent in almost every type of were in human-influenced environments such as habitat, being absent only from far northern old fields, row crops, and residential areas Canada and Alaska and arid southwestern areas (Wenger 1991). (Verbeek and Caffrey 2002). Crows winter mostly within their breeding range, withdrawing Breeding Biology from the northern portions, generally where the American Crows build platform nests of sticks mean minimum January temperature is 0°F (- on horizontal branches or in the crotches of 18°C) (Root 1988). deciduous or coniferous trees, usually at heights of 10 to 20 meters (33 to 66 feet) (Verbeek and Distribution Caffrey 2002). In southern Michigan, nest Up until the mid-1800s, the Common Raven building may begin in early April, but usually in was the large corvid found throughout Michigan May (Kelley 1978); it commences slightly later (Gibbs 1879, Barrows 1912). American Crows further north (Wenger 1991). American Crows first appeared in the state some time around often breed cooperatively, with helpers 1850, eventually displacing ravens over most of consisting of the yearling offspring of a the state (Hubbard 1887, Wood 1951). Soon breeding pair, although these auxiliaries will enough, crows were one of the most abundant, sometimes continue as non-breeding helpers for widespread, and recognizable species in four to six years (Verbeek and Caffrey 2002). Michigan, thriving on the mixed land use and open spaces created by human settlement Abundance and Population Trends (McPeek 1994). (Click to view a trends from the BBS) Statewide, the number of townships where In both Atlases, crows were found in every county crows were found was nearly identical in both and the vast majority of townships. They were a bit Atlases, with the species being recorded in just sparser in the UP, especially the extreme western over 89% of all townships. Crows were part of the peninsula, and more concentrated in the confirmed in a smaller percentage of townships SLP. This pattern is not surprising, as settled areas in which they were found in MBBA II than are favored by crows. More than half of the crow MBBA I, but there was a small increase in the sightings in MBBA I number of probable plus confirmed records in
© 2011 Kalamazoo Nature Center American Crow (Corvus brachyrhynchos) Julie A. Craves each region of the state, with the biggest bump roosts that can confound analyses (Bonter and occurring in the UP. Hochachka 2003, Caffrey and Peterson 2003, Hochachka et al. 2004). Two such data sets The Atlas map and the apparent increase in from southeast Michigan provide examples of records do not really tell the story of what has the decimation of local populations. been, and is happening to American Crow populations in Michigan. In fewer than ten Winter bird surveys have been conducted twice- years, a disease has altered crow numbers not weekly for 18 years at the Rouge River Bird only in the state, but across the country. Observatory on the University of Michigan- Dearborn campus, Wayne County. The average The mosquito-borne West Nile virus (WNV) number of crows per survey dropped 96% for was first identified in North America in New the post-WNV period compared to the ten York in 1999 and spread throughout most of the previous years (Craves unpubl. data). Likewise, continent within six years (McLean 2006). the average number of crows per party hour on American Crows are especially sensitive to the the Detroit River Michigan-Ontario Christmas North American strain of the virus, which is Bird Count dropped 97% for the post-WNV typically fatal to crows inside of a week (Komar period versus the prior 25 years (NAS 2002). et al. 2003, Brault et al. 2004). Risk of These results are similar to the declines noted mortality does not typically differ with age, sex, for marked breeding populations, where single- or breeding status, but substantial social year mortality was 68% in central Illinois and disruption is expected in the long run in this 72% in Oklahoma (Yaremych et al. 2003, family-oriented, often cooperatively breeding, Caffrey et al. 2005). species (Clark et al. 2006). Conservation Needs The impact of WNV on crow populations can be Prevention or management of WNV in crow profound. WNV was first detected in Michigan populations will prove extremely difficult due to in late summer 2001 (SOM 2007), and the BBS the ecology of the virus. Eradication of WNV in trends covering the period of MBBA II show a North America is highly unlikely. We have yet disturbing, highly significant annual decline of to discover how far crow populations can 7.2%. However, outbreaks of WNV tend to be decline without local collapse, or how the steep more common in urban areas where container- reduction of this common and widespread breeding Culex mosquitoes, the primary vector, species will impact the functioning of are prevalent, and habitat fragmentation brings ecosystems. mosquitoes and birds closer together (McLean 2006, LaDeau et al. 2008). Since BBS coverage Six decades ago, Alfred Otto Gross stated, “In in urban areas tends to be poor, it may not be the spite of incessant persecution the crow has been best data set to use to examine population trends able to outwit his human adversaries by its for crows (Hochachka et al. 2004, Ladeau et al. unusual intelligence and instinct of self- 2006). preservation” (Bent 1947). It appears that the American Crow has finally met its match. About half the wintering crows in the SLP are resident birds, with the remainder from northern breeding areas (Black 1941). Even so, winter surveys have been used to assess declines in American Crows in northern latitudes, especially from sites which do not have winter
© 2011 Kalamazoo Nature Center American Crow (Corvus brachyrhynchos) Julie A. Craves
Literature Cited New York 1999 strain of West Nile virus. Emerging Infectious Diseases 9: 311–322. Bent, A.C. 1947. Life histories of North LaDeau, S.L., A.M. Kirkpatrick, and P.P. American Jays, Crows, and Titmice. U.S. Marra. 2006. West Nile emergence and National Museum Bulletin No. 191, large-scale declines of North American bird Washington, D.C., USA. populations. Nature 447:710-713. Bonter, D.N., and W.M. Hochachka. 2003. LaBeau, S.L., P.P. Marra, A.M. Kilpatrick, and Declines of chickadees and corvids: possible C.A. Calder. 2008. West Nile virus impacts of West Nile virus. American Birds revisited: consequences for North American rd 103 Count: 22-25. ecology. BioScience 58:937-946. Black, C.T. 1941. Ecological and economic McLean, R.G. 2006. West Nile virus in North relations of the crow, with special reference American birds. Ornithological Monographs to Illinois. Ph.D. diss., University of Illinois, 60:44-64. Urbana. McPeek, G.A. 1994. American Crow. In Brault, A.C., S.A. Langevin , R.A. Bowen, N.A. McPeek, G.A., and R.J. Adams, Jr. (eds.). Panella, B.J. Biggerstaff, B.R. Miller, and N. 1994. The Birds of Michigan. Indiana Komar. 2004. Differential virulence of West University Press, Indianapolis, Indiana, Nile strains for American crows. Emerging USA. Infectious Diseases 10:2161–2168. National Audubon Society [NAS]. 2002. The Caffrey, C., and C.C. Peterson. 2003. West Nile Christmas Bird Count Historical Results virus may not be a conservation issue in [Online]. northeastern United States. American Birds
© 2011 Kalamazoo Nature Center American Crow (Corvus brachyrhynchos) Julie A. Craves
Yaremych, S.A., R.E. Warner, P.C. Mankin, J.D. Brawn, A. Raim, and R. Novak. 2004. West Nile virus and high death rate in American Crows. Emerging and Infectious Diseases 10:709–711.
Suggested Citation
Craves, J.A. 2011. American Crow (Corvus brachyrhynchos). In Chartier, A.T., J.J. Baldy, and J.M. Brenneman (eds.). 2010- 2011. The Second Michigan Breeding Bird Atlas. Kalamazoo Nature Center. Kalamazoo, MI. Accessed online at:
© 2011 Kalamazoo Nature Center