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Saturday, October 27, 2012

A lesson in light

One of the highlights from a boat trip to Bodega Canyon and Cordell Bank on 26 October 2012 was a Manx Shearwater (Puffinus puffinus).

The Birds of Sonoma County California lists two records for Manx Shearwater: the first at Bodega Canyon on 7 April 2008 and the second on the way to Cordell Bank on 16 November 2008.  I don't know if there have been other records since then, but this is currently a rare species in county waters.  We encountered it fairly close to Bodega Head.

There are several important field marks to look for:  the most critical is the color of the undertail coverts (feathers under the tail).  In the photo above, note that they're white almost to the tip of the tail.

In addition, notice the following:

- overall small size (wingspan of 75-84 cm, or 30-33 inches)
- strong black and white coloration (see black upperparts in next photo)
- an almost hooded appearance due to a white "hook" or "notch" extending upward behind the face
- a mostly pale unmarked underwing, with a narrow black border 
- gray underside of primaries (at wing tips)
- relatively short tail

The next photo shows the white "saddles" just in front of the black "thigh patch."

The image below is the same individual, but in different light and at a slightly different angle.  Note that with a quick view, it appears that the undertail coverts could be dark.  This is a lesson in light.  It was important to continue watching the bird until there was a clear view of the undertail coverts in good light.  Otherwise, this bird might have been mistaken for a Black-vented Shearwater.  [Note that Black-vented Shearwaters have dark undertail coverts, appear brown above rather than black, have a diffuse and dusky face pattern, and lack the white saddles.]

Until the last 20 years or so, Manx Shearwaters were considered rare throughout the Pacific Ocean.  They were first detected in California in the mid-1970s, and the first formally accepted record was from Monterey Bay in 1993.  There are now annual records in California (most between the Farallon Islands and San Luis Obispo County).  Late summer and fall (especially September and October) is the most likely time to see them.

Although primarily an Atlantic species (the common name means "from the Isle of Mann," located in the northeast Atlantic), Manx Shearwaters are now potentially breeding on Triangle Island off Vancouver Island and on Middleton Island in Alaska.  We may see more of them in the future!

(Facts from Rare Birds of California and Petrels, Albatrosses, and Storm-Petrels of North America by Steve Howell.)

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