snake eagle

Identification of second and third plumage in Short-toed Snake Eagle

OLLÉ A. &  MONTRÀS-JANER T. 2020. Identification of second and third plumages in Short-toed Snake Eagle.

Short-toed Snake Eagle displays four plumages that can be identified in field, namely juvenile, 2nd, 3rd and adult plumage. Juveniles (1cy) and adults (+ 4cy) are the easiest to recognize. Intermediate plumages from immature birds are somehow trickier. However, with good photos and some practice they are relatively easy to age too. 

Immature birds are in general brighter than adults. Moreover, an irregular margin along the wing’s edge reveals two different generation of feathers and two (or even three) moult fronts, differing from the juvenile plumages (which have none). The extension of the synchronic moult in both wings determines the age. 


JUVENILE. This is the plumage the bird acquires at the nest, and the plumage the bird will keep until the end of the winter or beginning of the spring (i.e. in its 2cy). At that stage, the moult has not started yet. Thus, there is only one generation of feathers, no moult fronts and the margin along the wind’s edge is uniform. Juvenile birds have a pale brown or cinnamon coloured hood, upper part of the chest and, up to a higher or lower degree, lesser and median underwing coverts. The flight feathers (primaries and secondaries) on this plumage are very different from the ones on an adult plumage, displaying only a subtle dark fringe along the margin of the wing with up to two (at the most) very thin fringes (often partially or totally missing). The tail shows much thinner stripes in juveniles than in adult birds.

Photo 1. Juvenile plumage 1cy. September. © Víctor Estrada. Uniform wing margin, with no moult fronts and in good condition. This individual presents a typical design of a juvenile plumage, with cinnamon upper part of the chest and underwing coverts. Primaries and secondaries have no fringes nor obvious dark margin.

Immature birds are those with the so-called 2nd and 3rd plumages, between 2cy spring and 4cy summer. They are non-breeders and the plumage differs from that on adult birds as follows. Immatures are very bright, even in long distance, with no uniform pale brown or cinnamon hood nor large dotted area across the underwing covers. The fringe along the wig’s margin is not uniform but very irregular. However, and opposite to adult birds, the moult is synchronic in both wings. Many birds in their 1cy and even 2cy summer remain in Africa. A few of them fly to Europe, although they tend to remain within the Mediterranean region. Accordingly, individuals marked with transmitters have shown many of them only reaching to North Africa and not crossing into Europe. These individuals often occupy flat plains with high foraging resources (differing from the mosaic habitats occupied by the European breeders), wandering around with no fidelity nor roots for any special site. Immatures in their 3rd plumage (3cy or 2nd summer) are still non-breeders. Nonetheless, some of these birds, especially those in their 4cy, might join the breeding birds and perhaps even support them in some way before finding a territory of their own.

SECOND PLUMAGE. This is the plumage juvenile birds acquire when they start moulting. The first feather to moult is the innermost primary (p1). The moult usually starts sometime during the first couple of weeks of April, although it has also been registered starting from end of March and as far as beginning of May. At this age, the spring migration starts later than for adults, probably due to that these birds have no rush to reach anywhere in Europe and occupy a territory. In that sense, some birds start moulting in Africa, suspend their moult during migration, and restart again once they reach Europe, showing one or even 2 moulted primaries (p1/p2). Other birds though, migrate a bit earlier and so they have no moulted feathers yet during their journey. Once they reach Europe, the moult starts. The moult progresses orderly towards the outer parts of the wings. In October 2cy, four to six inner primaries (p1-4/6) are already moulted. At this point, the autumn migration starts and the birds suspend the moult. In this plumage, very few secondaries are moulted. At the most, the most external (s1-2), the central (s5), and the most inner (tertiaries) feathers which usually, will not be moulted until at least the two or three innermost primaries are moulted. The tail feathers normally moult completely, starting at the centre of the tail (r1). 

Photo 2. Second plumage 2cy. April. © Alex Ollé. This individual shows a typical moult for those birds that just have arrived from Africa, with a moulted p1 and a growing p2. On the left wing, we can see a moulted greater covert which indicates that the moult of the secondaries have started (at the s5). All tail feathers are still juvenile feathers. The lesser and median upperwing coverts are worn out (showing a typical sand colour).
Photo 3. Second plumage 2cy. May. © Alex Ollé. Very similar moult to the photo above, with growing p1 and p2. The rest of the flight feathers are still juveniles. The tow central rectrices have fallen off indicating that the moult may have started. The juvenile pattern on the secondaries show triangular tips without dark margin. The fringe along the wing’s margin is still uniform.
Photo 4. Second plumage 2cy. August. © Alex Ollé. The four innermost primaries (p1-4) are already moulted and the p5 is growing. The outermost secondary (s1) is growing and the innermost secondaries are replaced. The two central and external rectrices (r1 and r6) are moulted and the rest are growing. Notice at this age, the bird has still many juvenile feathers, translucid and with pointy tips.
Photo 5. Second plumage 2cy. August. © Fran Trabalon. Very typical moult extension concerning the age and time of the year. The moult front is located in p5. Notice s1, s5 and the innermost secondaries are already moulted. At least the central and outer tail feathers have already been replaced.

THIRD PLUMAGE. During the 2cy winter, from November to February, most birds winter in Africa. The conditions there are optimal to keep with the on-going moult of the remiges. Now is mostly the turn of the secondaries so that, in the 3cy spring almost all of them will be new. All tail feathers will moult to the ‘second generation adult type’, with a much wider subterminal fringe. As for the primaries, birds can moult either none of them or up to two of them, carrying on where the moult was suspended in October 2cy. In this way, in spring 3cy all these birds reach Europe with a moult that varies extremely from individual to individual. In general, they are very bright birds and the ageing would be reflected in the extension of their moult, with two moult fronts (still synchronic) on the inner and outermost primaries. This age is difficult to assess. If we can identify that the bird has none (or very few) juvenile secondaries in spring, that would separate this from a 2nd plumage individual. In spring, one of the outermost primaries is usually moulting (p8-9 or 10). At the same time, another moult front appears in the innermost primaries (p1-2). The moult will progress during the 3cy. In October 3cy, all individuals will have no juvenile feathers, the inner and outermost primaries completely new (reaching around the p3 or 4) and the central primaries somehow worn out.  

Photo 6. Third plumage 3cy. May. © Alex Ollé. This individual can be confused with a 2nd plumage. However, notice the secondaries. The secondaries are new and with rounded tips (discarding juvenile feathers). As for the primaries, the p10 is slightly browner than the rest, especially p9 that appears newer, greyer and with a white tip. p10 is then a juvenile feather and we are looking at a moult front. The innermost primaries (p1 and p2) are also new, with wide bright margins. This is another moult front. The tail feathers are not juvenile feathers either which indicates, given the date, that this is not a 2nd plumage bird.
Photo 7. Third plumage 3cy. May. © Maxime Pastore. Very typical individual. The two outermost primaries (p9-10) are juvenile feathers, browner and clearly worn out. Both wings have lost the p8 (signalling that a new one is growing). Also in both wings, the p1 is new (third generation feather). Therefore, there are three generation of feathers in the primaries (1ª p9-10; 2ª p2-7; and 3ª p1) with two different moult front; one located in the p1 and the other, in the p8. The secondaries are of 2nd generation. The empty spaces are juvenile feathers that have fallen off as new feathers are growing underneath. Notice the irregular margin along the wing’s edge.
Photo 8.Thirds plumages. May. © Maxime Pastore. In this photo we can appreciate how different 3rd plumages may look like amongst individuals, depending on the extension of the moult during the second winter in Africa. The bird on the left hand side is the same individual as in the previous photo 7. The bird on the right, is more intensely coloured but yet, not an adult. Notice the bright throat and the active moult, symmetric in both wings, with a growing p9 (the second outermost primary). The two innermost primaries (p1-2) are new. Altogether, discard a juvenile and a 2nd plumage. Notice there is no juvenile feather amongst the secondaries. The two central rectrices (or tail feathers) are new, but third generation. This is therefore, a 3rd plumage in the spring of 3cy.
Photo 9. Third plumage 3cy. July. © Fran Trabalon. We can easily appreciate in both wings, a simmultanous moult front in p8 (still growing), the two remaining juvenile feathers p9-10; a new p1 and a p2 that has fallen off, creating an empty space. Also in both wings, there are some remaining juvenile feathers in the secondaries, s8 and s10.
Photo 10. Third plumage 3cy. July. © Fran Trabalon. Notice two very obvious moult fronts in the primaries, one at p9 and the other at p2 (both growing). p10 is still a juvenile feather. On the left wing, s8 is also a juvenile feather however, will be soon replaced. Most likely, this bird will have no remaining juvenile feathers by August.

FIRST ADULT PLUMAGE or SUBADULT. This is the previous to the adult plumage. There is no remaining juvenile feather amongst the remiges. The moult fronts are still symmetric, up to a large extend. Some individuals can achieve this plumage already during summer 3cy while others will reach there as late as in spring 4cy. All and all, from the 4cy summer is very difficult if not impossible to age birds in field. However, there are some characteristics that may remain until the 5cy such as a bright throat and a relative simultaneity of the moult fronts.

Photo 11. First adult plumage 4cy. March. © Joan Goy. This bird has no juvenile feathers so technically, it would not be possible to age it. However, during the spring of the 4cy we can still age some individuals. Notice that the most inner and outer primaries are new; the central primaries, old; and we can appreciate two generations of secondaries (no juveniles). This means that during the 2nd plumage, this bird moulted p1-5 and then, during the 3rd plumage, it moulted p6-10 and again, p1-3.
Photo 12. First adult plumage 4cy. May © Fran Trabalon. Throat and cheeks slightly bright. The synchronic moult signals no definitive adult plumage. The innermost and outermost primaries are relatively new, with the central primaries p6-7 older and worn off.
Photo 13. Subadult, presumed 4cy. September. © Fran Trabalon. A priori, this individual looks like an adult bird. However, notice bright head and throat. Notice as well the two simultaneous moult fronts amongst the primaries (an inner one at p4; and an outer one at p8). The outermost primaries (p9-10) are old feathers but not juveniles, which discards a 3rd plumage (if that was a 3rd plumage it would be extremely late considering the date). The moult of the inner primaries into a third generation feathers have reached the p8; p1-4 are fourth generation feathers; and the two outermost primaries (p9-10) are second generation feathers. The secondaries are adult type, with dark nail-shaped margins. As anecdote, p1 seems newer than p2-3 which could be due to either, moulting later than p2-3 (for some bizarre reason) or that p1 is in fact a fifth generation feather.
Photo 14. Adult plumage +4cy. September. © Fran Trabalon. In general, adult birds are browner, with a uniform coloration throughout the throat and chest. The different moult fronts, not simultaneous anymore and with chaotically alternated new and old feathers, give a peculiar ‘piano pattern’ design to the under wing.