Varied Thrush in Central Park, Nov 30th

It's just a number, but as of last weekend I'd been sitting at 194 species seen in Central Park for quite some time. Now that I'm no longer in Manhattan it's become a little harder to track down one-day wonders although over the years I've not done badly in that department. I did, however, take time to visit the park on Tuesday morning to see the Varied Thrush that had originally been spotted by Mike Bryant et al on Sunday.

This Varied Thrush is an immature, and so sex is indeterminate, and there's some indication that it's had a hard life - the feathers at the throat on one side show a cleft that is suggestive of some injury. And it was mentioned to me that a toe might be dislocated. I didn't notice this myself and the bird seemed quite mobile and robust. In fact the only other Varied Thrush I'd ever seen was in MA in the dead of winter at Quabbin Reservoir feeding in the snow (Feb 13th 2004). I'm grateful that this bird stayed around long enough to be park bird #195.

Now all we need is another 5 vagrants.....

The Cackling Goose quandary

Cackling Geese are somewhat sought-after in these parts, being relatively rare and a very small proportion of the often massive Canada Goose flocks that dominate inland ponds and lakes. There are big wintering flocks around the Princeton-ish NJ area. This is rather different from e.g. the Katy Prairie area of TX where Canada/Cackling are in the distinct minority amongst other geese.

On Thanksgiving morning I headed out to Etra Lake, south-east of Hightstown in search of both Greater White-fronted and Cackling Geese, and found one of the latter but no GWF.

Picking out Cackling from Canada requires a certain comfort with the variation within Canada, which can be quite considerable within a flock. In particular the cheek patch and breast color are rather variable. Cackling is sometimes hyped as being a grayer-frostier colored bird, especially on the back, but there's quite a range of variation just within Canada. When I finally found the Cackling - identified by square head structure, bill size and small size - it was a similar color as a Canada next to it and well within the range of color variation of the flock. In fact I had one bird pegged as a Cackling by color until it raised its head and proved me obviously wrong.

In this mix there's also the big question of Lesser Canada Goose, which shares many of the attributes of Richardson's Cackling Goose and the separation of which is a potential nightmare. At the moment I'm (mostly) sweeping that under the rug but there have been some "Cackling" Geese that have certainly got me to wondering, in particular last winter's Rahway River Park birds.

A recent post by Sibley on Cackling vs Canada goose bill structure is pertinent here. The whole ID morass is still being worked out. While that effort is ongoing I rather suspect that Cackling Goose in eastern NY and NJ is really a bit bucket for all sorts of small outlier White-cheeked Geese. Which is why I still seek out reliable locations to study them further.

Stamford, Bronx, Long Island, Jamaica Bay

One of my period dark-dark trips with long road miles, I started off at the Fork-tailed Flycatcher spot in Stamford, CT just after dawn and while the bird took a little while to turn up it did pose very nicely indeed for photographs. There were also a few roosting Wild Turkeys and a couple of Monk Parakeets. The Fork-tailed (FTFL) was at this site Nov 17th-Dec 4th.

I had chosen to look at the FTFL because it was fairly close to my main interest for the day - a banded Barnacle Goose of known wild origin in Pelham Bay Park at Orchard Beach. So after heading back into NYC from Stamford I spent two hours combing the local area for it, including some grassy areas slightly further afield. I was sadly unsuccessful. I later learned that some photographers had got far too close the previous day and scared it off. That's a lot of people hours wasted as a result of selfishness. Update: this Barnacle Goose turned up again on Dec 4th in Stratford CT, roughly 45 miles to the west, again associated with a Canada Goose flock.

After some frustration at Pelham Bay I went south to Jones Beach West End and spent a little time looking at the somewhat distant Northern Loggerhead Shrike and chatting with other birders - the shrike in particular is a little small and a little dark but otherwise seems a decent match for Northern over the rather similar Loggerhead (see discussion of this both above and below). Shai Mitra pulled out a distant Kittiwake over the ocean with his scope, but the Northern Gannets were rather more obvious. A Merlin was hunting the swallows (presumed Tree, but Cave is possible) and later on the pigeons at the Coast Guard Station. At the Coast Guard Station there was a big flock of Brant and shorebirds (Dunlin, American Oystercatcher, Black-bellied Plover and a few Red Knot) and a dispersed flock of Bonaparte's Gulls in the bay. Since there wasn't that much of interest at Jones I went around to Point Lookout which used to be a big spot for Bonaparte's several years ago, and there were some off the park. However they were not especially concentrated, being spread out all over the inlet, bay (and probably the beach). This flock holds multiple Little Gulls and at least one Black-headed Gull but unless they concentrate in one place finding them will be a challenge. A small flock of Common Eider flew in and out of the inlet while I was there.

Finally I spent the late afternoon at Jamaica Bay Wildlife Refuge, photographing ducks flying in and out of the West Pond. All the usual suspects were there, with the exception of Canvasback which seemed to be curiously absent. The Snow Goose count was a grand total of 5 birds seen in flight as I was exiting the refuge. The list from Jamaica Bay is:

Snow Goose
Brant
Gadwall
American Wigeon
American Black Duck
Mallard
Northern Shoveler
Greater Scaup
Lesser Scaup
Bufflehead
Common Goldeneye
Hooded Merganser
Red-breasted Merganser
Ruddy Duck
Northern Harrier
Cooper's Hawk
Dunlin

Barnegat, Brigantine

Saturday was sunny but breezy, and I headed out for a fairly traditional coastal route.

First, Barnegat Inlet (Barnegat Light) were at 7:44 the breakwater held a lot of people - mainly fishermen - out of all proportion to birding opportunities. The wind was already fresh, it was relatively cold, and there was a dearth of birds in the inlet itself so I went into quick scouting mode. Perhaps this was not totally unrelated to the extensive small boat traffic. Of interest was a fly-by immature Great Cormorant, and several small groups of Bonaparte's Gulls. Last winter there weren't many Bonaparte's Gulls around in general, so these were actually my year list ones. This winter there's a bonanza of them.

In search of more productive areas I went south to Brigantine (or if you want to be formal, the Brigantine Division of Forsythe National Wildlife Refuge). The wind had risen and was quite strong here, so there was little chance of finding a Cave Swallow. I did two tours around the impoundment and found decent numbers of ducks, a good flock of Tundra Swans (also a year bird) and Green-winged Teal and some shorebirds, a few of which were Western Sandpipers making for an implausible three year-bird day in late November. Northern Harriers were around in numbers, terrorizing the Teal again, but the Peregrines did not seem to be present or at least not hunting. A fuller list follows (Brigantine unless noted):


Snow Goose
Brant
Mute Swan
Tundra Swan (50-ish)
American Black Duck
Mallard
Northern Shoveler
Northern Pintail
Green-winged Teal
Long-tailed Duck (Barnegat)
Bufflehead
Hooded Merganser
Red-breasted Merganser
Red-throated Loon (Barnegat)
Pied-billed Grebe
Double-crested Cormorant
Great Cormorant (Barnegat)
Great Blue Heron
Northern Harrier
Red-tailed Hawk
American Coot
Black-bellied Plover
Western Sandpiper
Dunlin
Savannah Sparrow
Eastern Meadowlark

Fork-tailed Flycatcher, Loggerhead Shrike, Common Ground-Dove

My only previous Fork-tailed Flycatcher was one in Morrisville PA in early June 2006, seen right after my "flycatcher special" trip of May-June 2006. I got no photos of that bird, so I was interested in seeing and photographing the one that was up in Stamford CT. Usually a one or two day wonder (as with the PA bird) the FTFL in CT was found Weds and stayed through at least Sunday at Cove Island Park in Stamford (edit: found on Nov 17th, it stayed all the way through Dec 4th - remarkable). I saw it on early Sat morning where it was already drawing a crowd at dawn. The volunteer gathering names said that birders had come from as far away as SC and MI for it. Other birds at this place included a small flock of Cedar Waxwings and a similar-sized flock of Monk Parakeets.

Stamford is not that far beyond NY, so after a couple of hours with the FTFL I went down to Jones Beach. The wind was quite strong at that point, so I didn't see much of note from the Coast Guard basin (Common Loon, Long-tailed Duck, Red-breasted Merganser) but the flock of Snow Buntings was feeding on the grass growing up in the cracks of the West End 2 parking lot. I went on to Theodore Roosevelt Nature Center and found my target bird - the
Northern
Loggerhead Shrike - hunting from perches on and around the boardwalk and not particularly phased by the conditions. Considering that I've also seen Northern Shrike in Duluth MN in late December the conditions were almost tropical by comparison. The bird was a first winter one with faint barring on the breast. Two Northern Harriers were the only other predators I saw there, and there were some American Black Ducks and Gadwall in the pond between West End 2 and the Nature Center but otherwise things appeared pretty quiet. Inevitably there were Brant in the West End median strip and I also saw a fly-by flock of Dunlin.

Subsequent queries about the ID of the shrike species led to a discussion on NYSbirds-L and ultimately BIRDWG01 where it emerged that the consensus was strongly in favor of Loggerhead over Northern Shrike. While this is atypical of a Loggerhead it has too many features at variance with Northern Shrike to be consistent with it. Loggerhead is if anything even less expected than Northern in that location at this time of year.

And then since I was so close I went a little further east to Captree State Park. I wasn't really that psyched about the Common Ground-Dove but I'd been told it was a first NY State record so I went for a quick look for it. I found it skulking on the northern edge of the southern parking lot, mostly behind the concrete blocks in the parking lot, but it showed chestnut wing undersides as it flew. Very skittish that bird, and for that reason I decided not to hang out for photos and stress it more. Instead I headed for home.

Statistical Models of Bird Identification

A recent post on BIRDWG01 particularly piqued my interest because it ran counter to my viewpoints on the statistical nature of bird identification. I think likelihood is both implicitly used and also formally overlooked. In this case, an Aythya (diving) duck of unusual appearance was seen in the Eastern USA and proposed to be a Pochard X Scaup as opposed to what the final ID appeared to settle on, which was Canvasback X Greater Scaup. (Scaup in Great Britain is the Greater Scaup in the U.S.A.).

All duck hybrids are rare to very rare, but many of them certainly occur (an even more recent WG01 thread shows a nice selection of Goldeneye hybrids). Well I guess that the Mallard hybrids with American Black Duck, Mottled Duck and Mexican Duck are more frequent than "very rare".

For any bird ID the likelihood that it's correct is basically the ratio of the probability that the species displays those characteristics vs the probability that some/all other species displays those characteristics, weighted by the chance that you'd see the species in that spot. Much of bird ID stops before the comma in that last sentence, but that's naive.

So for this duck let's arbitrarily say that the chance of seeing a hybrid is 1/10,000. If you see a Canvasback then there's a 1/10,000 chance of it really being a hybrid, ignoring for a moment the evidence for hybrid traits (i.e. an "unusual Canvasback" based on appearance). Let's say this 1/10,000 holds true for the similar Aythyas on the UK side of the Atlantic. The fundamental issue is, the chances of seeing a Pochard or Tufted Duck or Scaup on the East Coast of the USA is very low. Pochard is essentially unknown here although some stray to the west coast from Asia, Tufted Duck is very rare, and Greater Scaup vs nominate Scaup from Europe is impossible to distinguish in the field. It's safe to say that very, very few diving ducks make that transatlantic voyage. But let's say that there's a 1/20,000 chance of the Aythya you come across in some pond in NJ being one of Pochard/Tufted/"European" Scaup. This means that the chance that you find a European PochardXScaup hybrid is (1/20,000)X(1/10,000) or 1/200,000,000.

One in 200 million. The odds of winning the Powerball lottery grand prize from one ticket is pretty much the same. I do not recommend this as a retirement strategy.

We can haggle over any of the numbers - actually I think that the real numbers are more remote odds than these ones. Simply plug your own estimates in and follow the bouncing ball.

Now, the chance that you'd stumble into a hybrid Aythya from the USA side of the Atlantic is only 1/10,000 so the relative chance that any hybrid is European is less long odds: a mere 1/20,000. This means that for any bird that you're sure is one of those rare Aythya hybrids there's a 99.995% chance it's going to be USA-origin.

For most of us, ID is done with far less certainty - we see an American Robin fly by and call it that because it's shape best fits American Robin and the color we can see best fits American Robin so the overwhelming probability is that it's an American Robin and not, say, an atypical-looking Varied Thrush. Or a Redwing (Turdus iliacus). When you're working with common vs rare species, or even rare vs very rare species, that occurrence ratio of 1/20,000 is quite a strong selector between different IDs. You would have to be very, very certain indeed that the fly-by bird is NOT an American Robin before Varied Thrush even becomes a reasonable probability.

So it is with the hybrid Aythya. For any possible hybrid, there's a range of appearance that is "typical" of the hybrids. Hybrids are notoriously variable so the range of appearance is often broader than for a pure species (think Golden-winged Warbler and Blue-winged Warbler vs the common hybrids of Lawrence's and Brewster's Warbler, both of which show quite a range of variability). Even if a hybrid were judged to be 80% likely to be Pochard X Scaup and 20% Canvasback X Gtr Scaup by appearance, when you add the population weighting in, it becomes a 99.925% Canvasback X Gtr Scaup by likelihood. Or put another way: you would have to be absolutely certain it could only be Pochard X Scaup based on the bird appearance before it became anything close to even odds on the overall population-weighted chances you'd got that ID right.

I.E. You should use population to weight your idea of what the bird is. Failure to do that is a failure to take proper account of birds of a common species showing an atypical appearance, which might be a far more likely explanation than a much rarer bird.

This subject is not new to WG01 - a more formal discussion of this using Bayes theorem was bounced around in October 2007 regarding the identity of a Catharus thrush, although the post I linked to uses the dangerous territory of race rather than thrush ID points.

However I think this aspect of bird ID is still under-appreciated, and quite an important one when attempting to put labels on strange-looking birds.

Pole Farm Sunday 11/14

Let's just say that I don't think it's going to be a big Short-eared Owl year based on the early winter going with a late afternoon visit to Pole Farm (Mercer Co. Park Northwest) in Lawrenceville:

American Kestrel: 1
Northern Harrier: 1
Short-eared Owl: 0

In recent years the number of Northern Harriers milling around have been a pretty good predictor of Short-eared Owl occupancy. Unlike the big year, I also haven't heard rodents milling around in the grass. Few rodents = few/no owls.

Other birds: Eastern Bluebirds, Hairy Woodpecker, large overhead flock of Grackles, some American Robin movement at sunset, Dark-eyed Junco, Song Sparrow. Really fairly quiet. Not even a Red-tailed Hawk perched along the fields, which might also correlate with the lack of mice.

White-tailed Kite and Brigantine

I made three visits to various parts of the Edwin B. Forsythe National Wildlife Refuge on Sunday. First and last were a visit to the Barnegat impoundment (Barnegat division of Forsythe NWR) where there was a persistent White-tailed Kite. The first time around, shortly after dawn, netted various ducks and herons but no kite. The second time around on the way back home the kite had settled on a dead tree in the far distance half-way across the bay, but was visible in scope views and flew briefly while I was there.

In the intervening time I went down to the Brigantine division, the more traditional area for birders visiting Forsythe NWR. En route I stopped at Barnegat Inlet, found the tide very high, a lot of chop in the inlet and very few birds milling around. A roving flock of Juncos and other fly-by passerines suggested migration movement off the north wind.

The tide was also very high at Brigantine, with much of the saltmarsh at least somewhat inundated. A lot of the ducks were coming in at middle altitude and settling into the north-east impoundment. Shorebirds were limited to several flocks of agitated Dunlin and a few Black-bellied Plovers. One of the reasons that they had good reason to be agitated were two Peregrines and several Northern Harriers, although those raptors were harassing a large flock of Green-winged Teal when I saw them. Blue-winged Teal was the best duck, and there was also Northern Pintail, Mallard, many American Black Duck, Bufflehead, Hooded Merganser and Ruddy Duck. A small flock of Brant and a moderate-sized flock of Snow Geese were in the impoundments but windy conditions made for few photo ops. The most active part was on the south-east sluice where the influx of water and small fish made for a Double-crested Cormorant and Gull feeding frenzy. Given the wind the passerines kept their head down but I did see a few fly-by Savannah Sparrows.

I returned north via Barnegat impoundment for the White-tailed Kite and a brief visit to Well's Mills County Park where the feeders held only titmice, chickadees and Juncos. At Well's Mills they've also removed the trees to the east of the visitor center which provided some protection around the periodic drainage pond for small birds drinking there. Since it's not a spring-fed pond that patch was also dry - a shame since last winter it did attract some birds in that small area.