Tuesday, February 07, 2012

Coops and Sharpies


Cooper's Hawk profile

Lately I've been thinking about Cooper's and Sharp-shinned Hawks. As bird photography has become more popular and accessible to birders, I've noticed Cooper's Hawk photographs far outnumber Sharp-shinned Hawk ones posted to Wisconsin's birding forums. This is what I expect based on my own observations at Pheasant Branch Conservancy, as I encounter far more coops than sharpies. In most cases, photographs of coops on our forums are incorrectly identified as sharpies. It's almost as if there is a predilection or bias to call any small accipiter a Sharp-shinned Hawk. This isn't exclusive to photography because I've noticed the same thing during my field trips. To be sure, these can be challenging bird species to separate, but I wondered if such a bias would be reflected in eBird data. Since I've only been recording my sightings in eBird since 2007, the graphs below were created using data from that year to the present:


Pheasant Branch Conservancy


Dane County

Looking at spring migration peak, the ratio of 9:1 coops to sharpies for Dane County is lower than 20:1 for Pheasant Branch. My comparatively small sample size could make this differential an anomaly, but I think it's true I see fewer sharpies at Pheasant Branch Conservancy than what Dane County birders are reporting. What I really found surprising was the comparison of coops to sharpies for the entire state of Wisconsin during fall migration:


Wisconsin

Though both coops and sharpies concentrate along shorelines during fall migration, perhaps this last graph is explained by a more rapid migration for sharpies along Lake Michigan and tallied at places like Concordia University. If true, I'm curious about why there would be such an extreme difference in-land versus shoreline sightings by count totals in the fall for such two similar species. Are lakeshore observers guilty of gross bias for sharpies during hawkwatch events? Or do the number of sharpies vastly outnumber coops at those particular locations during fall migration? Looking at the two maps below, Cooper's Hawks are seen in more places than Sharp-shinned Hawks. According to the Wisconsin graph, sharpies outnumber coops from September to November, but where?


Sharp-shinned Hawk - Fall Migration


Cooper's Hawk - Fall Migration

Cooper's Hawk image © 2012 Mike McDowell

4 comments:

  1. Interesting post. If you check out the hawk count data it looks like Sharp-shinneds outnumber Coopers Hawk by quite a bit during fall migration in Wisconsin. This is similar to what other fall watches see like Hawk Mountain in PA.

    6:1 - http://hawkcount.org/month_summary.php?rsite=516&ryear=2011&rmonth=12&sec=key

    ~8:1 - http://hawkcount.org/month_summary.php?rsite=694

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  2. What's the experience level of hawkwatchers that participate on these counts? I suppose it varies. My experience tells me not to trust even intermediate birders in making correct accipiter identification. Don't hawkwatches rely on volunteers who aren't necessarily experts at raptor identification? Something just seems dubious to me about the fall counts. It seems like there should be more coops being seen.

    Mike M.

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  3. I heard a program on NPR the other day and they were talking about the difference. The 'expert' said that the sharp shinned hawk is not much bigger than a blue jay. If that is true, then for sure what I thought was a Sharp Shinned hawk in my yard was more likely a Cooper's hawk. Would you agree?

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  4. Hi Wanda,

    Here's the problem with going by size:

    Sharp-shinned Hawk length:

    Male: 24 to 27cm
    Female: 29 to 34cm

    Cooper's Hawk length:

    Male: 37 to 39cm
    Female: 42 to 45cm

    Given variability in nature, it's possible that a smallish male Cooper's Hawk might actually be physically smaller than a large female Sharp-shinned Hawk.

    It's far better to be more familiar with shape and plumage characteristics than size with these two species.

    Mike

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