The evolutionary mystery of the hawk that defies the rules of size
In short: Most animals grow larger in colder climates, but the Cooper's hawk breaks the mold. While its size varies wildly by longitude rather than latitude, this agile predator hides a complex history of genetic shifts, shifting genera, and a remarkable recovery from 20th-century chemical threats.
The Cooper's hawk (Astur cooperii) is a master of woodland pursuit, known for a 'flap-flap-glide' flight pattern that allows it to navigate dense canopies. While often confused with the smaller sharp-shinned hawk, genetic testing reveals a deeper story: the Cooper's hawk is closely related to the American goshawk, whereas the sharp-shinned hawk's similarities are likely the result of convergent evolution. This species is also a biological outlier; unlike many species that follow Bergmann's rule—growing larger in colder, northern regions—the Cooper's hawk's size is more closely tied to longitude, with birds found east of the Mississippi River tending to be larger on average than those in the west.
The species' taxonomic history is equally dynamic. Originally placed in the genus Accipiter, a 202lar molecular phylogenetic study confirmed that the genus was polyphyletic. To resolve this, the genus Astur was resurrected to accommodate the Cooper's hawk along with eight other species. This evolutionary lineage is ancient; the Cooper's hawk may be the earliest Astur to have colonized North America, with a fossil record potentially dating back 0.5 to 1 million years.
Despite a period of decline during the 20th century due to anthropogenic causes like industrial pesticides and chemical pollution, the population was increasing as of 2024. This recovery is accompanied by striking sexual dimorphism. Females are significantly larger than males, sometimes weighing up to 125% more. This disparity may allow the sexes to utilize different food sources, reducing competition while the smaller, more agile males excel at gathering food for the family group.
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Sources
- Summary by Intellect · source: Cooper's hawk
- Also: www.allaboutbirds.org — Cooper
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