Re: Cook's Petrels off Sonora?

[Michael Force]

Hi Birders,

To me, the photo reveals what appears to be a feeding flock of Black Terns, which can be incredibly abundant in the Gulf of California in the fall; I've seen flocks of 12,000-80,000 in early September as they migrate down the Mexican coast to wintering areas in the Panama bight. The dorsal surface of several birds in the photo is grey, and of the wrong colour for Cook's. The overall shape also points to Black Tern and not Cook's Petrel. When in a mixed seabird feeding flock, Cook's alight on the surface to feed, and do not pick from the surface as do Black Terns. The storm-driven provenance theory has some merit. After a hurricane one fall, I saw a Kermadec Petrel in the Gulf of California. Furthermore, a couple of Cook's Petrels I saw off Isla Santa Margarita (west side of Baja California Sur) earlier this week may have been a result of Hurricane Rick that had passed to the south of Cabo Falso a few days earlier. Until then we hadn't seen any Cook's on this leg of the cruise.

happy birding,

Michael Force

currently aboard NOAA ship McArthur II

30 nmi southwest of Punta Abreojos, BCS

pagodroma@...

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[Brian Sullivan-5]

Hello All

This is a really interesting discussion. I'm not sure we'll ever know for sure what these birds are based on the photos, as there just isn't quite enough detail there. Kimball is right that the bird on the lower left is suggestive of Cook's based on shape, but others in the photo look wrong to me. The wing shape of these birds looks a bit too triangular and short when compared with the body. On the birds that are banking, especially the middle bird about fifth from left, the tail shape looks wrong for Pterodroma, being fanned and square-tipped, more like a Black Tern. I couldn't find a Cook's in my photos that shows a similar shaped tail in any pose.

I'm not sure it helps, but I've posted a gallery of Cook's Petrels taken off California this summer here:

http://briansullivanphotography.com/Galleries/COPE/index.html

Granted these birds are closer, but there are a variety of shape shots here that might help. Most of these summer birds are finishing primary molt and their tails are in heavy molt. I would expect by late September that most have finished molting--not that we can see that on the MX birds, but I would expect them to look cleaner overall by late Sep.

In any case, for what it's worth....

RE feeding Pterodromas, I've only seen Black-cappeds actively feeding on chum, and as Chris said, they seem to be able to do a lot more than simply shoot around in steep arcs. The birds I saw were hovering and dip-feeding in strong 30 knot winds, looking more like giant Wilson's Storm-Petrels than the sleek fliers we typically think of for Pterodromas. Even with bouts of hovering and dipping, they interspersed steep arcs on crooked wings. It was an interesting sight. I have photos of these birds feeding with Wilson's Storm-Petrels if anyone is interested.

Brian

On Fri, Oct 30, 2009 at 5:59 AM, Michael Force <pagodroma@...> wrote:

Hi Birders,

To me, the photo reveals what appears to be a feeding flock of Black Terns, which can be incredibly abundant in the Gulf of California in the fall; I've seen flocks of 12,000-80,000 in early September as they migrate down the Mexican coast to wintering areas in the Panama bight. The dorsal surface of several birds in the photo is grey, and of the wrong colour for Cook's. The overall shape also points to Black Tern and not Cook's Petrel. When in a mixed seabird feeding flock, Cook's alight on the surface to feed, and do not pick from the surface as do Black Terns. The storm-driven provenance theory has some merit. After a hurricane one fall, I saw a Kermadec Petrel in the Gulf of California. Furthermore, a couple of Cook's Petrels I saw off Isla Santa Margarita (west side of Baja California Sur) earlier this week may have been a result of Hurricane Rick that had passed to the south of Cabo Falso a few days earlier. Until then we hadn't seen any Cook's on this leg of the cruise.

happy birding,

Michael Force

currently aboard NOAA ship McArthur II

30 nmi southwest of Punta Abreojos, BCS

pagodroma@...

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Brian L. Sullivan
Pacific Grove, CA

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