Mystery Storm-Petrels: Solutions

by Dani

It was good to see that so many people gave it a go on the admittedly difficult mystery Storm-Petrels!

Here are the answers, with some comments on the ID below each photo. If anyone still has doubts, feel free to leave a comment on the comments section.

1- Band-rumped Storm-Petrel

Most people got this one right. This is a well marked Band-rumped Storm-Petrel, photographed in May off the Canaries.

Amongst other things, long wings, with noticeably long “arms”, lack of foot-projection, and relatively dull upperwing-covert bar, help rule out Wilson´s Petrel.

Leach´s Petrel can also be ruled out because of several features. The bird in the photo shows fairly broad wings, with relatively blunt wing-tips, not strongly angular wings, unlike Leach´s, little tail fork (although, as noted below, beware of this character) and a strong bill.

Although the amount of white in the rump patch is bigger and deeper than usual for Band-rumped (where the white rump is usually quite narrow), the shape is still more BR-like, being quite U-shaped.

On the following pic, of the same bird, note too that the shape of the wide white thigh patches is the typical of Band-rumped, extending to a depth of more than 50%

2- Leach´s Petrel

Definitely, a tricky bird.

First, how can Wilson´s Petrel be ruled out?

To me, one of the most important features to help eliminate Wilson´s would be the noticeably long “arm” visible on the photo, looking almost as long as the hand, and giving the bird a long-winged impression. Wilson´s would have a characteristically shorter, and comparatively broader, “arm”, making it look shorter-winged than the bird depicted on the photo. Furthermore, it seems to show a slight angle to the trailing – edge of the wing (typically straight in Wilson´s), rear-carriage doesn’t look too long, It lacks any foot projection (what could resemble it on the photo is in fact a black line on the sea surface), and, although the angle is not the best one to evaluate it, the thigh patch doesn’t extend to a depth of 75%, like in Wilson´s.

Once Wilson´s has been eliminated, the choice is between Leach´s and Band-Rumped.

Some features do point towards Band-rumped,  as some entries have understandably suggested , such as relatively dull upperwing-covert bar, apparently no strong tail fork, U-shaped white rump patch, etc…

However, some of the previous features, such as tail fork and shape of white rump patch, are variable within both species, and can change depending on the angle and position of the bird. On this photo, it’s almost impossible to evaluate accurately the degree of tail fork or the exact extend/amount/shape of the white rump patch.

Although seemingly U-shaped, the white rump patch looks quite deep, as in Leach´s, and the thigh patch doesn’t seem to extend that much into the lateral coverts.

Bill looks quite slender and long, thus Leach´s like.

But, most important, there´s an objective feature, easily seen in the photo, and first pointed out to me some years ago by ID and Seabird Master  Killian Mullarney, that helps establish the ID as Leach´s and not Band-rumped.  Chin.  The strikingly pale chin of this bird, contrasting a lot with the dark face,  is a feature associated with Leach´s, and not with Band-rumped.

This Leach´s Petrel was photographed in Lanzarote in November, during a storm when many other Leach´s Petrels entered the island´s harbours. The photographer, experienced with both species, noted that the bird had the same flight manner (a very useful feature for identifying both species), structure, etc…as many nearby Leach´s, and therefore was identified as such.

3-UNIDENTIFIED: 

Leach´s or Band-rumped Storm-Petrel?

This highly debated  and controversial bird reminds us how some Storm-Petrels, even on good pics, can be extremely difficult, or even impossible, to identify. No wonder how very detailed descriptions, specially for birds seen from land, are needed in order to get accepted by national rarities committees…

Many more photos of this interesting individual, seen off Maine in September, can be found here, as well as different opinions on the bird´s identity

Here and Here

As you can see, and read, the debate is between a fresh juvenile Leach´s, and a well-marked Band-rumped.

Shape of the bird, with long wings, and especially long “hand”,  long bill, and white tips to the scapulars, amongst other things, rule out Wilson´s.

However,  the bird shows a mix of characteristics that makes it difficult to assign with certainty to either Leach´s or Band-rumped.  For example, the brightly coloured and contrasting upperwing-covert bar, strongly angular wings at the carpal joint and sharply pointed wing tips are more Leach´s like. On the other hand, there seems to be almost no tail fork, the white thigh patch extends well into the lateral coverts, overall colour is quite blackish, etc… All pro Band-rumped features.  Shape of the white rump patch looks intermediate…

So, what´s this bird? If I had to say something, I would probably go for Leach´s,  and the skilled birder who took the pictures (Luke Seitz), also thinks the same. But Steve Howell is in the Band-rumped camp, so best then to leave it unidentified…?

Here´s another pic of the bird, where, to my eyes, it looks quite Leach´s like

Note the sharply pointed wing-tips, and strongly angled wing at the carpal joint, as in Leach´s.

Aren´t these Storm-Petrels fun?  To improve your skills on its identification, just join any pelagic, and see them for yourself in the sea.

Thanks to everyone for commenting on them!

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