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):

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.

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:



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.

Warbler song in migration

Black-throated Green Warbler

According to the Peterson Field Guide to the Warblers of North America, "male warblers sing almost incessantly early in the breeding season, and often in spring migration as well (especially as the breeding grounds are approached); many species begin singing before departing the wintering grounds." (Dunn & Garrett 1997).

This week I heard some migrant warbler song here and there among the rich chorus of resident species now singing down here in Honduras. A couple of days ago I obtained a poor recording of a singing Black-throated Green Warbler, while today I got a rather better recording of a singing Wilson's Warbler. Although the phenomenon is evidently not unknown, it remains poorly represented in bio-acoustic libraries such as Xeno Canto or Cornell's Macaulay Library. 

Unfortunately, the Black-throated Green Warbler is barely audible as its weak song is drowned out by a chorus of cicadas. Listen carefully for what I believe is a so-called 'unaccented song'. This Black-throated Green Warbler was in the company of at least three conspecifics, as well as two Blue-headed Vireos, a Black-and-White Warbler and a Magnolia Warbler. They were foraging in encino (thin-leaved) oaks on a sunny, south-facing slope at an elevation of about 1300 m.

This Wilson's Warbler was foraging in similar habitat, but did not appear to be associated with other insectivorous birds. The weak ending of his song suggests the nominate eastern subspecies. At the time of writing, Xeno Canto does not have any other Wilson's Warbler song from Central America, but the Macaulay Library has an excellent cut from Costa Rica.

Cited literature:

Dunn, J & K. Garrett. 1997. A field guide to the warblers of North America. Peterson Field Guide Series 49. Houghton Mifflin, New York.

New species for El Salvador: Clapper Rail

Clapper Rail has not been reported from El Salvador, but after recent documentation of the species in the Honduran and Nicaraguan parts of the Gulf of Fonseca, its occurrence on the Salvadoran side of that area was to be expected. Saturday afternoon, Oliver, Roselvy and I set out to find the species in a salt ponds complex called Salinera San Ramón in La Unión, not far from the Honduran border. We were there before in February and even did some unsuccessful playback of Clapper Rail back then.

This time we birded later in the day, and as it got dark, Clappers began to vocalize. We had been playing tapes along the southern side of the salt ponds earlier, when suddenly we heard Clapper Rails giving grunt calls from the mangroves on the northern side. Quickly we walked over a dike to the other side and started recording what sounded like at least three pairs - one close, two further in. Oliver recorded the following vocalizations, documenting the presence of Clapper Rails in El Salvador for the first time:

The next morning, we heard and saw a Clapper Rail in another salinera, La Ostia across the border in Valle, Honduras. We've had this species here several times before, since I first found two adults here last August tending chicks. Sunday I managed a poor quality shot of a bird that briefly came out of the vegetation to inspect my playback.

"documentation shot" of Clapper Rail (in Honduras, not El Salvador)

The first Honduran record dates from 2010, when Robert Gallardo and Mayron Mejía observed and photographed a pair in Choluteca, and the first Nicaraguan record dates from 2012 (van Dort 2013). Although recently discovered, the population in the Gulf of Fonseca may number in the hundreds, if not thousands, given the availability of suitable habitat. Why it has gone undetected for so long is anyone's guess, although I suspect that the relative lack of researcher attention to the Central American mangroves has something to do with it. Or perhaps this population is simply bouncing back from historical depressions after the ban of DDT.

Literature:
Van Dort, J. 2013. Clapper Rail breeding in Honduras. El Esmeralda, Vol II (1): 23–26. Asociación Hondureña de Ornitología (ASHO).

Raptors in La Muralla

Swallow-tailed Kite

La Muralla National Park in Olancho, Honduras is one of the lesser-known birding destinations in Honduras, at least if gauged by eBird checklists. Maybe that's because it's far away from the north-south axis of San Pedro Sula / Tegucigalpa, or because there are no mayan ruins nearby, or because there is little tourist infrastructure in the park. The birding itself, however, is as good as any in Honduras, and this place has quickly become one of my favorite birding destinations.

King Vulture

Roselvy and I went there this weekend, and stayed at the rustic visitor center inside the park. We went to bed with a calling Mottled Owl outside, and woke up with the songs of Black-faced Antthrush and Pheasant Cuckoo. Highlight species for us included Black-throated and Unicolored Jays, Scaled Antpitta, Dusky Antbird, Stub-tailed Spadebill, Resplendent Quetzal, Blue-crowned Chlorophonia, King Vulture (seen both days), Great Tinamou and Nightingale Wren - to name a few birds that we don't usually see or hear closer to home.

Strong-billed Woodcreeper

When we got there Saturday afternoon, sunny weather made for pleasant birding, and for decent lighting for photos. Sunday however was overcast. We saw some great birds, but practically all raptors I photographed against an off-white sky, not the best lighting conditions for raptor photography. Here goes anyway:

Ornate Hawk-Eagle

Ornate Hawk-Eagle was high on our wish list for the trip, so we were pleased to see this spectacular raptor make an appearance over the open space around the visitor center, right after we had birded the El Pizote trail for nearly six hours. 

Double-toothed Kite

It was soon followed by a very high Double-toothed Kite, and a Short-tailed Hawk.

A little later, our guide Naim Torres spotted this Plumbeous Kite perched on a snag at the edge of a coffee plantation. The bird allowed close approach and would occasionally fly out to catch a dragonfly, and then return to its perch to consume it.

Plumbeous Kite

Elsewhere in the park, we saw two more Plumbeous Kite hunting insects.

Plumbeous Kite

For good measure, a White-breasted Hawk made a brief appearance. Still not its own species, this tropical subspecies of Sharp-shinned Hawk is nonetheless distinctive. 

White-breasted Hawk

Although a solid five and a half hour drive from where we live, we will probably come back soon get a little more of that sweet Honduran Atlantic Slope birding in. (The drive ain't so bad though when you have to stop twice to look at little groups of Swallow-tailed Kites...)

Swallow-tailed Kite