The Band-rumped puzzle

22 09 2014

“The real voyage of discovery consists not in seeking new landscapes, but in having new eyes.”

 – Marcel Proust

The concept “cryptic species” is interesting and worrying at the same time. It’s quite scary to think we are overlooking some rare taxa among the common stuff. But… are all these cryptic species actually unidentifiable? Some papers have already dealt with the Band-rumped storm-petrel complex identification and there seems to be some features noticeable in the field, especially with good photos.

Since it’s one of the commonest species seen in all the Lanzarote Pelagics expeditions to La Concepción Bank and I’ve just led 3 of these expeditions together with Juan Sagardía, I took the chance to take a proper look at the subject and put some light on the contradictions existing between different authors, especially when it comes to distribution of winter and summer breeders and to both structural and coloration features. Of course there will be always some tricky birds, but I hope this post can be useful for people birding in the Macaronesia and for the brave ones joining us on next year’s expeditions.

bill shape grants

Grant’s storm-petrel (winter breeders) are just arriving to the Canary Islands after having spent the summer in the other side of the Atlantic. Both adult and 2cy birds might show fresh flight feathers since they’ve just moulted them during late spring, but, since 2cy are meant to moult earlier, their primaries should be worner than those of adults. Actually, we usually see two types of birds: some showing a higher level of contrast between primaries and secondaries and some others showing an evenly jet black flight feathers. Secondaries should be always somewhat newer than primaries since they are moulted later, but the difference in the wear between primaries and secondaries is sometimes too high to be explained by a natural moult timing. Moreover, we see some birds here in the Canaries that are actively moulting secondaries, what demonstrates they can suspend the moult after having moulted the primaries. The bird below is an example of a suspended moult, but surprisingly it’s between P5 and P6. If this bird is gonna finish the moult or not it’s still a mystery, but if it doesn’t, it could lead to a very strange looking bird on its next cycle. In conclusion, the contrast could be due to an earlier moult of primaries (that of a 2cy bird) or a suspension in the moult before pre-breeding migration. So, how can we age them? Worn primaries (never as worn as in a summer breeder by this time of the year) might be indicative of 2cy but a high contrast between secondaries and primaries might indicate just a suspension. A proper assessment of primary’ wear and shape is essential for an accurate aging.

suspension primaries

One of the still unanswered questions about Band-rumps is why we see that many worn birds (that should be summer breeders) in La Concepción Bank if they are meant to be much scarcer than the fresh winter breeders. I confess I was identifying all the worn birds as summer breeders, but nowadays I actually think some of the worn birds we see are just 2cy winter breeders with an earlier primary moult. However, when you think you’ve found the answer, some new questions do emerge: some of this “worn” winter breeders (like the bird below) do suspend the moult before starting with the secondaries, what makes no sense if primary moult takes place earlier. The primaries in the bird below are older than the secondaries, but they are still broad and black, definitely not as worn as they should be in a summer breeder. In my opinion, this individual could be an example of 2cy Grant’s, with a really thick bill and a plain upperwing impression supporting this id.

cf adult madeiran

More questions: Then, how does a summer breeder look like right now? Birds moulting primaries right now should be all summer-breeders and the age would depend on the number of the primaries moulted. As happens in Grant’s, 2cy birds moult earlier so they should be now really advanced in their moult, like the bird in the top below (only 3 old primaries, p7 growing). In the other hand, adults have just started or are about to with the primaries. The birds we’ve seen during this week in La Concepción Bank were moulting p1-p3 (bottom below). In addition, secondaries in these birds will be always newer than primaries (as happens also in Grant’s), but worner than in the winter breeders that have just moulted them. It’s worth to keep in mind that the secondaries in a summer breeder are right now several months old.

Lanzarote 2274

adult summerbreeder

Apart from moult timing, some authors describe a difference in the extension and coloration of the white rump band, being narrower and somewhat dirtier in summer breeders. The silvery band in the GCs is also meant to be less extensive and contrasted in Grant’s (winter breeder), not reaching neither the elbow nor inner secondaries. Structural differences include a thicker bill and an overall bigger size in Grant’s. The bird below shows a perfect combination of features for being safely identified as Grant’s: thick bill, broad clean white rump band, plainer impression of the upperwing, heavy structure and fresh flight feathers.

moult grants

This bird lacks the GCs that should be grown soon. The upperwing band looks therefore restricted to the inner secondaries. Note how bright and black flight feathers are, indicating a late bird in terms of moult.

growing GCs

Some birds show yet another contrast between body and head feathers. When this happens, the head is usually blacker than the belly. The reason could be either an earlier moult of belly feathers leading to a worner brown area or a late moult (not yet moulted) of belly feathers. What comes first? Head or body? Two facts make me think head is moulted earlier and hence birds showing a worn brown underparts still need to moult this area: some birds show a newer head than body and some birds are uniformly jet black, but I’ve never seen a bird in September with the head browner than the body. In my opinion, this means they moult the head first (if you see them by then the head is much blacker than the rest of the body) and then, not a lot of time later, the rest of the body (what gives them the jet black appearance). The bird below shows such a contrast. It would be interesting to catch it in order to check if it’s actively moulting body feathers or if it just suspended the moult after the head.

band rumped moulted head

Moreover, I have the feeling that birds with a contrasting blacker head are commoner in August, whereas wholly jet black birds are really abundant now in September. The bird below also shows a very nice combination of Grant’s features after a complete renewal of both body and flight feathers: note how nicely evenly black the whole plumage is. Thick bill and less-contrasted upperwing band also supports the identification of it as a winter breeder. However, note the mottled rump, in my opinion more usual in winter breeders than previously described.

jet black grants

jet black grants under

 

Yet another winter breeder bird showing even a more extensive dark marking in the rump, almost approaching Leach’s. This kind of rump it’s been reported also in Monteiro’s, as this bird in Richard Bonser’s blog.

stripe dividing rump

 

Another feature meant to be unusual in Grant’s and typical of summer breeders (especially Monteiro’s storm-petrel) is the forked tail. However, it seems to be quite common also among winter breeders. The bird below is a classic Grant’s in every respect but shows a surprisingly deep fork in the tail. This bird also shows a classic Grant’s plumage in terms of color and wear (note for instance the upperwing band not reaching the elbow) but a mottled rump and a forked tail.

frok tailed grants

And yet an even trickier bird: even deeper fork in the tail and much more extensive and contrasted upperwing bar. Although hard to judge because of the position, the bill also looks thinner… The whole plumage is completely new so the only option of it being a Monteiro’s would be a 1cy recently fledged. Sadly, chicks don’t fly until October and moreover, for such a young 1cy, I would expect the usual extremely new white tips in primaries and secondaries. Hence, I stand with a winter breeder showing Monteiro’s characters.

frok tailed grants2

Finally, [Caution: what follows it’s a complete mindtrapp!] this bird with a really deep fork in the tail, a thin bill and a worn plumage (just starting post-breeding moult by inner primaries). Everything fits with a Monteiro’s and I don’t think anybody can say it’s not for other reasons but the range and the abundance.

monteiros like

A deeper analyse of the subject would appear in the forthcoming Macaronesian Birds magazine. More news on that soon!

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