PART I: COOPERS HAWK

In North America, raptors, or birds of prey, are thought to be one of the most exciting species of wildlife to see. The Bald Eagle should basically be on the American flag, falcons have been the prized possession of kings, noblemen and a total of three NFL football fans, and owls are portrayed as wise and mysterious, or on Drake’s clothing line. Today, for many people, raptors symbolize the majesty and wildness of nature (BC Resource Inventory Committee, 2001) and not much else suites that title better than a lively little dare devil known as Cooper’s Hawk. You may be vaguely familiar with the Cooper’s Hawk (Accipiter cooperii), potentially catching a glimpse of this species swooping into your backyard bird feeder to catch an unsuspecting songbird or perching on top of a light post along your street, scanning for their next meal. However, you may not know the details of this excitable accipiter’s life, so take a load off and let this blog bring you up to speed.  

ONE BIG FEISTY FAMILY

Cooper’s Hawk is a medium sized raptor belonging to the Genus Accipiter, within the family Accipitridae and Order Accipitriformes, the largest and most diverse genus of the birds of prey. Species belonging to this group range from the little sparrowhawk (A. minullus), which is approximately the size of a thrush at 7 inches (it’s as adorable as you think it would be), to the northern goshawk (A. gentilis) which stands around 25 inches tall (Rodriguez, 2019). Cooper’s Hawks are fairly variable in size, however females are generally larger than males with an average wingspan of 32 inches, length of 17 inches, and weight of 18 ounces. Males have a wingspan averaging 28 inches, average length of 15 inches, and average 11 ounces in weight (All About Birds).

KNOW YOUR HAWKS NEXT TIME

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Cooper’s Hawk sports the classic accipiter shape, a long tail rounded at the tip (National Geographic), and broad, rounded wings (All About Birds) which allows for mind boggling maneuverability when chasing its prey. A good identification feature of Cooper’s Hawk is their flat, dark cap combined with a sightly paler nape (back of the neck; Sibley, 2016) as well as distinct red eyes and a short sharply curved beak. Plumage along the back is blueish-gray which contrasts with a pale white underside (upper-breast area) infused with dense reddish barring (All About Birds). The tail of Coopers Hawk is dark with thick, black horizontal striping. Juvenile Cooper’s Hawks aren’t quite as eye catching, with dark brownish plumage on the back and a white underside combined with vertical brown streaking.

MISIDENTIFICATION WOOPS

Cooper’s hawk can be puzzling to identify at times because they are often confused with the smaller but similar Sharp-shinned hawk (Accipiter striatus). However, there are subtle but key differences between these two species.

BREEDING BEHAVIOUR

Cooper’s Hawk breed in a wide array of habitat, from urban locations adjacent to densely populated neighborhoods or busy roads to remote woodlands far from development (Bielefeldt et al, 1998).  Cooper’s Hawk establish territories of approximately 0.8 km² in size and begin seeking mates when two years of age or older (Norman). In British Columbia, courtship displays begin in mid-April to early May, with egg laying occurring anywhere from late April to late July. Young are at the nest from early June to late August (BC Resource Inventory Committee, 2001) and Cooper’s Hawks typically support a brood size between 3 to 5 (Millsap et al, 2013; Pericoli, 2004).

As with most birds of prey, Cooper’s Hawk exhibit reverse sexual dimorphism, with females nearly a third larger than males. Due to their larger size, the male is generally submissive and cautious when interacting with his mate, building the majority of the nest and feeding the female for up to a month before she begins laying eggs.   

HABITAT AND DISTRIBUTION

Cooper’s Hawk is broadly distributed throughout North America, and is situated in a wide range of habitat, however, the species is usually found in mixed deciduous/coniferous forests (BC Resource Inventory Committee, 2001), gravitating to areas with good canopy cover adjacent to an clearing or break in vegetation. In a study observing the breeding behavior of Cooper’s Hawk, (Millsap et al, 2013) found nests to be in areas close to water drainages or small streams and usually within 100 metres of a large opening such as a field or road. Being relatively tolerant of human disturbance Cooper’s Hawk can also live developed areas such as a city parks or golf courses.

Cooper’s Hawk can be found year-round in a large portion of their range, predominantly in the continental US, with some residents in northern Mexico and Southern Canada. Many cooper’s hawks do not migrate south during the winter, particularly those in the middle and southern united states, with the exception of those that breed further north, particularly in the Canadian prairies (Audubon). This northernmost population migrates as far as central America to regions where prey is abundant in winter.

HUNTING AND DIET

Cooper’s hawks tend to hunt in areas with high visibility such as open forest understory, fields, and estuaries, taking advantage of these habitat with its stellar eyesight to locate unsuspecting prey species. Cooper’s hawk often utilizes the element of surprise, concealing itself in foliage close to its prey and then exploding through in a high speed chase that would put James Bond to shame, pulling off mind blowing moves between thick foliage and squeezing through gaps not much larger than itself. However, hunting birds is not an easy pastime, and accidents do happen. One study conducted by Northeastern University of over 300 Cooper’s hawk skeletons observed healed over fractures in the bones of the pelvic girdle, especially the wishbone, in approximately 20 percent of the birds (Adirondack Almanack, 2014).

Cooper’s hawks are not picky eaters, and although passerines compose a large part of their diet (BC Resource Inventory Committee, 2001), they have been observed going after a diverse variety of prey species ranging from waterfowl, to small mammals and reptiles (Millsap et al, 2013). Though due to their longer lifespan (10-12 years) and high position on the food chain, Cooper’s Hawk are more susceptible to poisoning from pollution and pesticides as some chemicals accumulate in organisms over time and become concentrated as they move up the food chain (BC Resource Inventory Committee, 2001; Brogan, 2016).

CONSERVATION STATUS

Numbers of Cooper’s Hawk declined in the mid-20^th century as a result of human persecution (American Bird Conservancy) and the widespread use of DDT and other chemicals (Audubon). However, today the species has made a significant recovery (252% increase) over the past 40 years (data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven, 2007). This population boost is most likely attributed to the protection of the species, the ban of DDT in North America, and Cooper’s Hawks ability to exploit a variety of niches, especially urban habitat. Today the population is listed as a species of least concern by the International Union for Conservation of Nature (IUCN) (Birdlife International, 2016).

PART II: ASSESSMENT OF EXPOSURE AND EFFECTS OF PERSISTANT ORGANIC POLLUTANTS IN COOPERS HAWK

Urban areas are often developed over top of highly productive areas such as coastal floodplains and riverine corridors, leading to local declines in biodiversity. Few species generally thrive in these built-up areas, however, Cooper’s Hawk has done just that and has made a name for itself in the big city. However, this lifestyle is not all glamour like they show it to be in the movies, as urban raptors are subject to hazards such as collisions with vehicles and buildings as well as exposure to increased bioaccumulation of toxic substances and pesticides produced from human activity.

Urban environments are generally contaminated by a variety of persistent organic pollutants (POPs). This can include industrial chemicals such as polychlorinated biphenyls (PCBs), which have been used for decades in many commercial and manufacturing products, and polybrominated diphenyl ethers (PBDEs) which are used as flame retardants in building materials, electronics, and furnishings (Brogan et al, 2016). Organochlorine (OC) insecticides such as DDT are also detected in urban areas previously used for agriculture (Harris et al, 2000). One study conducted in Vancouver, Canada by Brogen et al (2016) sought to assess the factors influencing the exposure of Cooper’s Hawks to legacy pollutants by examining the effects of land use and population density on containment levels, diet, and reproductive success in the study population.

Sites within the western Metro Vancouver region were identified and monitored through the use of call-back surveys and citizen science (collected information from eBird.com, BC breeding Atlas, and local naturalists). Land use and average population density was quantified within 1.23km (size of area equal to average home range) of each sampled Cooper’s Hawk nest (Brogan et al, 2016). Land use was classified into several categories: rural/agriculture, recreation, lakes and water, industrial/utility, undeveloped, and residential commercial. Average population density within each home range was calculated by multiplying the proportion of residential and commercial land within the home range by the associated population density (Brogan et al, 2016).

Both adults and nestlings were captured from nests, with a blood sample no more than three millilitres being extracted from each bird. From these extracted samples, plasma (colourless fluid of blood which excludes red blood cells) samples were prepared and analyzed, providing information on diet and trophic level (Brogan et al, 2016), along with the concentrations of a number of organic contaminants including PBDEs, PCBs, and several organochlorine (OC) pesticides. Reproductive success was measured by observing the number of young that fledged from each nest, with fledging success being defined as the number of young present at each nest after 21 days or longer (Brogan et al, 2016).   

The study found blood plasma sampled within the urban area of Metro Vancouver to contain a variety of legacy contaminants, specifically PCBs, OC pesticides, and of more recent origin, the flame retardant PBDEs (Brogan et al, 2016). Concentrations of PCBs and PBDEs in the blood plasma of the studied Cooper’s Hawks were found to be high, increasing with level of development, but not at critical levels which substantially affects population health. Concentrations of these contaminants were found to be similar to other populations of birds of prey across Canada, suggesting that these contaminants have essentially been in environmental equilibrium for some time (Brogan et al, 2016). Concentration of OC insecticides, which are likely derived from past agricultural use of the land, were found to be higher in Cooper’s Hawks than recent reports covering other birds or prey, with three individuals surpassing a critical threshold associated with reduced eggshell thickness. However, overall reproductive success did not appear to be impacted by these concentrations. Mean fledging success of the studied population was 71% with 1.95 fledging per active nest, similar to breeding populations elsewhere in North America (mean success=72%, fledging per nest=2.27) (Brogan et al, 2016). Dietary analysis revealed there to be little variation in the diet of Cooper’s Hawks studied as most birds only pursue medium-sized birds in this region (Cava et al, 2012). 

Consistent with previous studies of persistent organic pollutants in birds of prey, Brogan et al (2016) found there to be ongoing contamination in the resident population of Cooper’s Hawks in Vancouver, thus revealing that exposure is local. However, concentrations of these contaminants have not appeared to reach lethal concentrations at this point in time as this population has been able to colonize the built-up areas surrounding Vancouver.

Literature Cited

Adirondack Almanack. 2014. Adirondack Birds of Prey: Accipiters. Retrieved October 14, 2020, from: https://www.adirondackalmanack.com/2014/03/adirondack-birds-prey-accipiters.html  

American Bird Conservancy. 2017. Cooper’s Hawk: Bird of the week. Retrieved October 18, 2020, from: https://abcbirds.org/bird/coopers-hawk/

Audubon Field Guide. (n.d.) Guide to North American Birds: Cooper’s Hawk. Retrieved October 13, 2020, from: https://www.audubon.org/field-guide/bird/coopers-hawk  

Bielefeldt, J., R.N. Rosenfield, W.E. Stout and S.M. Vos. 1998. The Cooper’s Hawk in Wisconsin: A Review of its Breeding Biology and Status. The Passenger Pigeon 60(2): 111-122. Retrieved October 15, 2020, from: http://images.library.wisc.edu/EcoNatRes/EFacs/PassPigeon/ppv60no02/reference/econatres.pp60n02.jbielefeldt.pdf

Birdlife International. 2016. Accipiter cooperii. (n.d.) The IUCN Red List of Threatened Species. https://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T22695656A93521264.en

Brogan, J.M., D.J. Green, F. Maisonneuve and J.E. Elliott. 2016. An assessment of exposure and effects of persistent organic pollutants in an urban Cooper’s Hawk (Accipiter cooperii) population. Ecotoxicology 26: 32-45. http://doi.org/10.1007/s10646-016-1738-3

Butcher, G.S. and D.K. Niven. 2007. Combining data from the Christmas bird count and the breeding bird survey to determine the continental status and trends of North American birds. National Audubon Society. Retrieved October 14, 2020, from: http://www.audubon.org/sites/default/files/documents/report_1.pdf

Cava, J.A., A.C. Stewart and R.N. Rosenfield. 2012. Introduced species dominate the diet of breeding urban Cooper’s Hawks in British Columbia. The Wilson Journal of Ornithology 124(4): 775-782. https://doi.org/10.1676/1559-4491-124.4.775

Harris, M.L., L.K. Wilson, J.E. Elliott, C.A. Bishop, A.D. Tomlin and K.V. Henning. 2000. Transfer of DDT and metabolites from fruit orchard soils to American Robins (Turdus migratorius) twenty years after agricultural use of DDT in Canada. Archives of Environmental Contamination and Toxicology 39: 205-220. https://doi.org/10.1007/s002440010098

Norman, B. (n.d.) Cooper’s Hawks: Accipiter cooperii. Retrieved October 13, 2020, from: http://www.coopershawks.com/about.html  

Millsap, B.A., F.B. Breen and L.M. Phillips. 2013. Ecology of Cooper’s Hawk in North Florida. North American Fauna 78(78): 1-58. https://doi.org/10.3996/nafa.78.0001

National Geographic. 2006. Cooper’s Hawk. Retrieved October 20, 2020, from: https://www.nationalgeographic.com/animals/birds/c/coopers-hawk/

Pericoli, R.V. and A.M. Fish. 2004. Golden Gate Raptor Observatory’s East Bay Cooper’s Hawk Intensive Nesting Survey. Golden Gate National Parks Conservancy. Retrieved October 20, 2020, from: https://www.berkeleyside.com/wp-content/uploads/2013/04/CHINSforWeb.pdf

Rodriguez, E. 2019. Accipiter. Encyclopedia Britannica. Retrieved October 15, 2020, from: https://www.britannica.com/animal/accipiter  

Resource Inventory Committee. 2001. Cooper’s Hawk: Accipiter cooperii. In Inventory Methods for Raptors. British Columbia Ministry of Sustainable Resource Management, pp. 62-65

Sibley, D.A. 2016. Sibley Birds West: Field Guide to Birds of Western North America. Alfred A. Knopf, New York, New York. 478 p.

The Cornell Lab of Ornithology. (n.d.) All About Birds: Cooper’s Hawk. Retrieved October 14, 2020, from: https://www.allaboutbirds.org/guide/coopers_hawk/id