Friday, November 20, 2009
As someone who has been hawk watching for over 35 years, I’ve been fascinated by the changing fortunes of the Cooper’s Hawk. Following HMANA’s Hawk Migration Studies, Hawk Count, and RPI (Raptor Population Index), it is clear we’ve had a dramatic increase in Cooper’s Hawk over much of the continent during the past several decades.
When I started hawk watching in Massachusetts in the 1970s, one rarely saw a Cooper’s Hawk. The best time was in late September and early October, when you might hope to see one a day, maybe two, at selected watch sites. Most New England sites covered by experienced observers reported something like 35 to 40 Sharp-shinned Hawks for every Cooper’s. There was much discussion about identification difficulties, in part because people saw so few Cooper’s; they are so similar to Sharp-shinned Hawks; and there were no good field guides on hawk ID. There was considerable skepticism about some Cooper’s reports.
Looking on a broader geographic basis now, sharpie vs Cooper’s ratios remain very intriguing. For example, Lighthouse Point in Connecticut reported 9,080 sharpies in 398 hours in 1980, and only 84 Cooper’s, a ratio of 108:1. In 2009, in 544 hours of coverage so far, Lighthouse has reported 5,308 sharpies vs 1,221 Cooper’s: 4.3:1, a dramatic change.
Looking somewhat farther south down the coast, in 1980 Cape May in New Jersey reported 52,282 sharpies and 1,615 Coopers, 32.4:1. So far in 2009, in 784 hours of coverage Cape May reported 13,710 sharpies and 5,497 Cooper’s; 2.5:1 significantly lower than Lighthouse. (Both of these coastal sites report primarily birds of the year.)
Looking inland, Hawk Mountain in Pennsylvania reported 8,319 sharpshins, and 374 Cooper’s in 846 hours of coverage in 1980, or 22.2:1. In 2009, in 875 hours of coverage to date, Hawk Mountain has had 4,291 sharpies to 601 Cooper’s, or 7.1:1.
Going farther inland, and north, Holiday Beach in Ontario reported 12,460 sharpshins in 1980, vs 316 Cooper’s, a ratio of 39.4:1. In 2009 to date, in 582 hours of coverage Holiday Beach has had 9,699 sharpies and 938 Cooper’s; 10.3:1.
Intriguingly, out west where the data set does not go back as far, in 1991 in 707 hours of coverage the Goshute Mountain site in Nevada had 3,674 sharpshins and 2,726 Cooper’s: 1.3:1, far lower than any eastern sites. In 2008, the most recent data available, the Goshutes had 4,697 sharpies and 1,957 Cooper’s; 2:5 to 1, suggesting accipiter trends in the west might be quite different from those in the east.
Several things are abundantly clear. All four eastern sites are seeing far fewer sharpshins this year than they did 29 years ago. Second, they are seeing many more Cooper’s Hawks now. However, the magnitude of the changes in ratios varies, often somewhat dramatically, from site to site. What is going on? This comparing snapshots of accipiter migration in two different years (four, including the Goshutes) has clear limitations. Looking at moving averages can provide a better picture of what is going on. To look at RPI data for sharpies and Cooper’s from 17 sites across the continent, visit RPI.
Photos by Joseph Kennedy. Used with permission.