Wednesday, October 13, 2010

Hawk Watch Year-to-Date Numbers & What They Mean

[Ospreys (like this juvenile over Cape May last week) are up 1847 birds over this date last year at the CMBO hawk watch. That's a 77% increase, but what does it mean? Click to enlarge - we know this is a juvenile because of the uniform good condition of the flight feathers and the lingering buffy-brown tones underneath, which rapidly fade to white. Ospreys are much easier to age when viewed from above, thanks to the juvenile's obvious pale edges to the upperwing coverts and pale spots on the back.]

People often ask about the hawk count year-to-date numbers compared to last year, especially this year since it seems like this year's count is substantially higher. And it is, 6354 raptors higher, or about 28%.  Break out the champagne, raptors are on the upswing, right? Well, not exactly.  First, here are the numbers:

Cape May
Cape May Point, New Jersey, USA
                       LAST YEAR           THIS YEAR
Species            2009 thru Oct. 12    2010 thru Oct 12
------------------ -----------------    ----------------
Black Vulture              90                 89
Turkey Vulture            368                438
Osprey                   2412               4259
Bald Eagle                336                315
Northern Harrier          349                979
Sharp-shinned Hawk       8145              11764
Cooper's Hawk            3963               2523
Northern Goshawk            0                  0
Red-shouldered Hawk         8                 25
Broad-winged Hawk         599               1104
Red-tailed Hawk           303                225
Rough-legged Hawk           0                  0
Golden Eagle                0                  0
American Kestrel         3341               4268
Merlin                   1455               1627
Peregrine Falcon          967               1075
Unknown Accipiter           0                  0
Unknown Buteo               0                  0
Unknown Falcon              0                  1
Unknown Eagle               0                  0
Unknown Raptor              0                  0
                        -----              -----
Total:                  22338              28692
Looks pretty good, right? Everything but Cooper's Hawk and Red-tailed Hawk is up.  We've been joking that maybe we've got a better hawk counter than last year, but the flat out truth is that there's a LOT of noise in migration count data, and one season does not a trend make. I was talking with Dr. David Mizrahi, NJ Audubon VP of Research, about this very thing yesterday.  The only reason we can make statements about population trends using migration count data is because we have many years of it, 34 to be exact, to smooth out the noise.

This year, in my opinion, one of the biggest noise makers is the fairly steady stream of cold fronts and following northwest winds. These conditions increase Cape May hawk counts - and butterfly counts, for that matter. Layer onto that variable the annually changeable timing of fronts and strong west winds; frequency of high flight days, when light winds and strong thermals allow some (or most) birds to fly so high they are literally out of sight and go uncounted; annual breeding success, which varies substantially year-to-year and yet may not amount to a trend; and yes, observer variability. Trend analysis does reveal statistically significant long term patterns, e.g. up in Bald Eagles and down in American Kestrels, but the only way we can keep track of these changes is to. . .keep track, every year!

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