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Wedge-rumped Storm-Petrel

Oceanodroma tethys


eBird Checklist S38807281
San Diego Pelagic Offshore Hour 5 of 10
San Diego, California, United States
Search this location, Illustrated Checklist
Latitude and Longitude 32.741, -117.721 Map

Not specified
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A really crisp plumage look in this image showing the strong flight and pointier wings. Squared off tail can be seen, maybe just a hint of a notch.

Observation details

Identified from photos by Gary Nunn, who will add a description and photos shortly. If accepted by the CBRC, this would be the first record for San Diego County. Exact location of observation: 32.822066, -117.806303 A smaller sized bicolored storm-petrel with prominent white rump and dusky blackish plumage. This bird had clearly swift looking longish pointier wings which I think is apparent in some of the images. The bird was observed for only a minute or so as it flew left-to-right across the bow at some distance away. It paused around about our 1 o’clock position, from the bow of the boat, making a few repeated quick towering flip back movements and then diving very quickly down to the water surface before moving away from our position with fluid smooth quite strong looking pumping flight including some wing locked glides. I took a total of 86 images spanning 18 seconds based on accurate time stamped camera data. The most prominent feature is the long tapered lozenge shaped bright white rump quite strongly bulbous at the sides sometimes presenting a groove or shadow down the center. This appears to be the two thicker white feather tracts of the rump sides meeting in the mid-point as in some images the rump appears entirely white in color. In one image the ends of the white upper tail coverts can be seen separating pretty cleanly into two halves at the center point over the darker tail base. The bird appears to be in nice fresh plumage with all dark remiges and rectrices without any signs of wear visible. This would agree with observations in Howell (2012), p.399, from S Baja California where mid-late Aug “60% birds with p10 growing or wings fresh”. The bird can be told from nominate Leach’s Storm-Petrel by the rump-tail coloration length ratio beyond the wings, about 1:2 on average in Leach’s Storm-Petrel but clearly a larger ratio on this bird more like the opposite in fact 2:1 in some open views of the upperparts. The white rump has a quite thickly feathered or bulbous looking appearance too unlike the more in-line tapered rump of Leach’s Storm-Petrel. In addition the squared off tail end is notable and not forked unlike the typical deeper forked tail of Leach’s Storm-Petrel. There is only maybe a hint of a small notch at the center of the tail so far as I can see. The tail shape of this bird can be clearly seen when it is fanned out, stalling in mid-air ready to plunge down to the water surface, and it is a diamond or tending towards more wedge shape which maybe explains the squared end look of the tail when closed. The bird can be told from Townsend’s Storm-Petrel on several features. The wings are sharper and pointier looking unlike the rounded or more bat like effect of Townsend’s wings. It has a squared off tail without any real sign of a fork to it, maybe only a small notch, unlike Townsend’s which does have a forked appearance to the tail although less strongly than Leach's. The rump-tail coloration length ratio beyond the wings in this bird is about 2:1 where in Townsend’s is more or less equal 1:1 in my experience, and in photos that I have taken of this species in San Diego County, and as stated in Howell (2012), p. 395. The rear end of Townsend’s just looks smaller in my opinion than this bird and the flight pattern is different with Townsend’s more bat like wing action and less direct flight and speed over the water.

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