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:


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