Saturday, July 22, 2017

Long Island Cory's Shearwater variation

Cory's Shearwater (Calonectris diomedea) is a common summer-fall species off the southern New England and mid-Atlantic coasts. Currently two subspecies are recognized by the AOS, and both occur in these waters.

Calonectris diomedea borealis breeds in the northeast Atlantic Ocean and is the much more common subspecies in this region. It is characterized by dark undersides of the primaries and averages heftier in structure.

Calonectris diomedea diomedea (AKA 'Scopoli's' Shearwater) breeds in the Mediterranean and is usually greatly outnumbered by borealis in these parts. It shows quite a bit of white bleeding onto the underside of the primaries, including p10.

It is important to know that field identification of these forms (considered separate species by some authorities) is still being worked out. I certainly struggle with some individuals, while others fit pretty neatly into one form or the other (based on current knowledge, anyway). The bulky birds with little/no white on the underside of the primaries we pretty comfortably call borealis. On the flip side, whenever I see a slender, narrow-winged Cory's winging by, it pretty much always reveals quite a bit of white on the primaries when it banks. Those we call diomedea. But there are some birds on which I'm not comfortable putting a label; they tend to be birds that appear large or intermediate in size with some white bleeding onto those primaries. Are these borealis showing variation? Are they intergrades? Is there another explanation?

Last weekend I did some fishing south of Montauk, NY and came across quite a few Cory's Shearwaters. Though I was multi-tasking at the time thus not able to scour the Cory's as I would normally, I was seeing more birds with white on the primaries than without - the opposite of what I'm used to seeing around here. Over several hours, with a chum slick out, I was able to snap off photos of the occasional Cory's that came through. In this particular area, about 20 miles from Montauk, Scopoli's-types out numbered borealis. There were a couple intermediates. Here is a selection of record photos from that day illustrating the variation in underwing pattern of COSH, from least white to most white. I did not get any shots of birds completely lacking white, which is surprising.








this and the bird above are the same individual

You wonder why there was so much white on the underwing in this particular group of birds. Was it just by chance that we fished around birds that had traveled from the Mediterranean region that day? Is this a particularly 'good' year for Scopoli's Shearwater in our waters? Are Mediterranean birds more likely to investigate the scent of a chum slick (as pointed out to me by Kate Sutherland, Scopoli's do appear to be more likely to linger in the chum slick and closer to the boat than borealis)?

We are early in the local pelagic birding season, so I will be interested to see the ratios that are reported from our waters as the year progresses.

While I'm at it, here are some other photos from the same day. Seabirds were not terribly numerous, but there were always birds in sight. A few common shorebirds were seen and heard migrating offshore. The most surprising sighting of the day came in the form of an adult male Brown-headed Cowbird that circled the boat a few times - not something you expect to see in mid-July some 20 miles offshore.

Great Shearwater

Wilson's Storm-Petrel

Wilson's Storm-Petrel

Cory's (left) and Great Shewarwaters

Brown-headed Cowbird

Brown-headed Cowbird

 - Nick

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