Sunday, April 28, 2013

Raptor migration at Derby Hill

Broad-winged Hawk
Last week I visited New York, and part of my trip was an excursion to the Derby Hill hawk watch, where I was a raptor counter in 2006 and 2007. I consider those spring seasons at Derby Hill formative in some ways, for these were my first seasonal field gigs in North America, and the first of several hawk watch engagements I've been on. I've counted hawk migration in Michigan, Texas, Florida, New Jersey, Veracruz Mexico, and even in Honduras, where I now live, but it all began at Derby.

Golden Eagle

It was good to see familiar faces, meet new people also, and of course the birding wasn't bad either. Birding in Central America means that I don't get to see Golden Eagle, Rough-legged Hawk or Northern Goshawk on a regular basis anymore, so those were my target species for the trip to Derby. I was not disappointed. I was also lucky to catch a spectacular (500+ both days) Black-capped Chickadee flight along the bluff.

Northern Goshawk

Derby Hill, located in the southeastern corner of Lake Ontario, does best on southeast winds, and as luck would have it, the two days I was there the winds were from that most favorable direction. Such winds push the stream of migrant raptors to the lakeshore, which they then follow, trying to stay close to land where thermals form. Especially the second day started out beautifully with a parade of low-flying Broad-winged Hawks and other raptors. However, with plentiful sunshine and relatively light winds, the inevitable happened: the hawk flight got higher and higher, and birds were seen further and further out over the lake. This is a classic scenario that Derby regulars have observed many times. Gradually, the flight becomes a 'scope flight', in which observers scan just over the tree line in the direction of the lake, occasionally picking up distant kettles that shimmy in and out of vision, and sometimes are barely visible even in the scope. 

Whenever this happens, the locals speak of birds 'cutting the corner of the lake' or 'jumping off at Nine Mile (Point)' Nine Mile Point being the name of the nuclear power plant 13 km (8 miles) west of Derby Hill. If the thermals, which the SE wind gently blows out over the lake, take the hawks high enough, the distance to the east shore of lake Ontario doesn't seem so threatening anymore, and hawks can bypass Derby.

Back in 2006 and 2007, I remember looking at NEXRAD radar from weather sites and being fascinated to see a hawk flight out over the lake – on radar. We always wondered if this would be visible by an observer on the ground also.

To field test this, my travel companion and I went over to Selkirk Shores State Park on Tuesday afternoon. For about 35 minutes, we scanned the skies toward Nine Mile Point and observed small groups of raptors coming in off the lake. Between 3:00 and 3:35 PM, we observed 15 Turkey Vultures, 1 Osprey, 1 Golden Eagle, 9 Sharp-shinned Hawks, 2 Bald Eagles, 135 Broad-winged Hawks, and 14 Red-tailed Hawks making landfall at Selkirk Shores. Not bad for an afternoon half hour of hawk watching!

But if the observers at Derby Hill cannot see all the birds 'cutting the corner', we in turn probably didn't see the entire flight either.

Here are some radar images from today (28 April 2013), with a very similar weather situation (light SE winds):

 photo DerbyHill28APR2013_zps26835144.gif

The clusters of green dots are groups of raptors flying along the lake shore. We can see them fly over Oswego, but when they get to Nine Mile Point, where the lakeshore topography changes direction, they seem to simply continue in more or less the same direction, passing north of the corner where Derby Hill is located. The birds appear to make landfall between Selkirk Shores and Sandy Pond, after which the stream of green dots changes course and follows a more northerly route, parallel to Highway 81.

At Derby Hill, the official count got to 843 raptors today, and counter Steve Kolbe noted a high flight in blue skies. Here's some more radar imagery, from almost an hour later. From 1:22 EDT to 2:10 EDT, several flight lines are shown: one over the lake, one over Derby Hill, and even some flight south of Derby Hill. The change of direction west along Highway 81 remains visible.

 photo DerbyHill28APR20132_zps88b4a29f.gif

Isn't it incredible that hawk flights can be seen on radar this way? And what about other hawk watches? The NEXRAD radar network covers the entire United States, so in theory it ought to be possible to see these flights for other major hawk migration sites. The thing is: it all depends on how far the hawk watch location is from the nearest radar station.

Think about it: the earth is round, while the beam of the radar is straight. Thus, the radar samples relatively close to the ground in its immediate vicinity, but at higher air strata further away.

Here's a look at a slice of today's radar for Braddock Bay, like Derby Hill a Great Lakes raptor count site with a good spring hawk flight. At the time of writing, they hadn't posted their day totals yet. Around midday, there was only a modest flight visible on radar:

 photo BraddockBay28APR2013_zpsfc3ae35e.gif

Birds can be seen following the lakeshore east, but the gray and green blocks zip off the screen, presumably as the flocks gain altitude and fly above the radar.

What about Whitefish Point, Michigan? Sadly, that site is at the edge of the range of the nearest radar (in Marquette) and thus radar images for Whitefish show nothing. There may well have been a flight there today, but due to its location relative to the nearest NEXRAD radar, it will never be visible on that radar.

No comments: