The Possibilities and the Challenges.
Separating Leach´s from Band-rumped Storm Petrels (the Madeiran Petrel in ‘old money’) has always been one of the greatest challenges for the seawatcher.
As with many other seabirds, these Storm Petrels aren´t as easy to study in detail as are gulls or shorebirds. And that means distant birds, seen briefly or in poor conditions, as from a moving boat in rough seas, are sometimes best left unidentified.
Having said that, it´s true that the basics of their ID are already well-known, and covered in all field guides, especially so in the excellent Multimedia ID Guide by Bob Flood and Ashley Fisher. So you could think a bird shown in a good photograph should be easy to identify. However, it´s not always the case. Not even for the experienced seabirder. And especially if such important things as flight manner or structure can´t be evaluated, like in photos. Sometimes, even separating it from a Wilson´s or a European is tricky. See it for yourself…
Here are 3 Photos…
What do you think are the species? (feel free to guess it on the comments section)
And this one?
What about this bird, facing away? Band-rumped? Wilson´s? Or an atypical Leach´s?
OK then… You´ve done your pelagics. You´ve studied the guides and the birds in the field. You started feeling a little bit confident on the ID of these 2 species…
Until recently, all of a sudden, and as if this wasn´t difficult enough, things got even more complicated.
Thanks to the pioneering studies of several highly skilled birders and ornithologists, including those by Magnus Robb and Killian Mullarney/The Sound Approach (see their fabulous “Petrels Night and Day” book for more information), the Band-rumped Storm Petrel complex in the North Atlantic has preliminary been divided into 4 taxa. Mainly based on differences in vocalisations on the breeding grounds and on different breeding timings (summer breeders and winter breeders), as well as on biometrics and moult. So now, according to them, we have:
-Grant´s Storm Petrel: Widespread winter breeder
-Madeiran Storm Petrel: Summer breeder from the Canaries and Madeira
-Monteiro´s Storm Petrel: Very rare summer breeder from the Azores
-Cape Verde Storm Petrel: Winter breeder from the Cape Verdes
However, most of the criteria considered for their separation is of little use to the field observer. Although several subtle plumage features have been suggested as more indicative of certain taxa. For example, Monteiro´s is supposed to have a deeper tail fork, and more prominent carpal bars than the others. But…
There´s a big problem that shouldn´t be forgotten for at-sea identification: VARIATION.
As Flood and Fisher indicate, no consistent plumage features have been found to assist on the at-sea identification of the various proposed forms.
Our own observations, mainly off the Canaries, confirm this. We´ve seen birds from presumably the same population showing features associated with birds from all other populations! We think variation within the same populations is the explanation… Of special interest is that, amongst presumably summer breeders (“Madeirans”), off Lanzarote, we´ve seen, roughly speaking, 2 types, including one that resembles Monteiro´s ! This has also been noted off Madeira by H.Shirihai, B.Flood et al.
a – Summer breeding, Monteiro´s-like birds: Birds with quite deeply forked tail (apparent from all angles and positions), pointed wings and prominent pale carpal birds. We think these birds are definitely not Monteiro´s, of course. But they look superficially like them at sea. And this would make identification of out-of range Monteiro´s impossible, on current knowledge
b - Summer breeding, “classic”, almost European Storm-petrel-like birds. Birds with shallow or no tail fork and square-ended tails, (but note that the degree of fork can change depending on angle/position) – being more accentuated when the bird faces away), relatively blunt wing tips, broad wings and quite faint upperwing covert bars.
Moult would be then the only visible field feature that could help, sometimes, to divide the complex into 2 wide categories: “Summer” breeders and “Winter “breeders. On actual knowledge, this is in my opinion the best approach.
Steve Howell´s studies on moulting birds off North Carolina are very interesting and help shed some light on the issue.
However, It must be stressed that at certain times of the year, birds on certain states of moult, can´t even be ascribed to one of the 2 main categories. And this is because there can be some overlap amongst different age groups of the different forms.
See the state of moult of the following birds, presumably of the 2 different populations, from the Canaries:
1-Presumed winter breeder (Grant´s)
2-Presumed summer breeders (Madeiran)
Several very worn birds, with no signs of moult, were seen in Lanzarote last September. Shape and wear of primaries would probably indicate they are juvenile. Could these birds be worn first cycle Winter breeders?
Second prebasic moult of summer breeders should, in theory, start by July. Thus, these birds, with no visible wing moult, shouldn´t belong to that population. But can some first cycle summer breeders start it later…? More work is needed in order to answer this, and other questions related to these fascinating Storm-Petrels.
Over to you. Did you have a go at identifying the ‘mystery petrels’ ONE, TWO and THREE?