Friday, November 20, 2009

Sharpie-Cooper's Ratios

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.


  1. Very useful information. I think I would say the hawks were "identified as..." rather than "seen..." in the last paragraph,

  2. I think also that decreased coverage throughout
    many inland sites in the northeast,especially during peak accipiter migration,could also give somewhat of a incomplete picture.
    Like the broadwinged hawk,it could be that the migration flight paths have shifted (west?)and many hawks are migrating past areas that just are not covered.I think before good I.D. skills developed many watches would see the majority of Male Sharpies and if the less abundant female came by close then it may have gone down as a Coopers.I still think flight behavior/atitude is one of the best field marks for Coopers.They seem almost reluctant to flap those wings.Very stiff.arthritic...

  3. Bill's point is a valid one. There were undoubtedly some mistaken IDs (as there still are today) but I'm inclined to believe that most of the sharpies reported 29 years ago were indeed sharpies. With many more Cooper's now, the challenge is both easier because you see many more Cooper's and more difficult -- because you see many more Cooper's now.

    To Thom's point, all the sites I mentioned were covered virtually full-time until mid or late November, so their data does not have a significant "cover time" factor. (That coverage factor bias would be significant for those sites that focus primarily on broadwing season.) There was the old joke about Christmas Counts that the first accipiter was usually a sharpie, but almost invariably the second would be identified as a Cooper's.

    I was surprised to look at recent data for Mt. Desert, Maine, one of my favorite spots, and see that in 282 hours of coverage this year, they reported 1569 sharpies and only 20 Coopers! That's 78.4:1! I need to look at breeding bird surveys and CBCs for points north of east of Mt. Desert to better understand what might be going on with Cooper's Hawk there.

  4. I believe also,we must look at the prefered prey base and breeding, as well as wintering habitats for these species.Sinse I have moved here in Maine(I know it's only been 8 years)2009 had the lowest number of accipiters to date.
    Virtually no sharpies or coopers all summer.
    These fields and woods use to yield many a chase by these two in a way ,the songbirds got a least around here.
    I'm wondering what kind of toxins are ending up in the environment these days? Especially with hyped up fears of Asian bird flu,west nile virus,and Equine enceph...I worry that mankind still has the "spray first,ask questions later" mentality.

  5. Thom and others,
    Here is a great source for recent contaminant reports if you are interested..especially for the ME area.
    Biodiversity Research Institute:
    It's astounding the amount of contaminants including banned chemicals, making their way into wetlands, lakes etc. We may be past the DDT era but we have new host of contaminants to deal with.