Earlier this year I read Molt in North American Birds by Steve N.G. Howell (a Brit!). The book is one of the most amazing books on birds I have read. Howell’s knowledge of bird moult (sorry – I just can’t get used to spelling it the American way!) and the way he writes about it is extraordinary – challenging and inspiring at the same time. There’s so much to take in that I felt the need to try to apply some of what I read using real examples, and this eagle, photographed so well by Neoh Hor Kee, provided just such an opportunity.
Howell describes four basic types of wing moult strategy:
1. Standard Sequential Wing Moult – where primaries are moulted sequentially from inner to outer and secondaries are usually moulted from outer to inner (e.g. petrels, quail, smaller hawks, waders, gulls and terns, woodpeckers and passerines).
2. Synchronous Wing Moult – where most or all flight feathers are shed practically simultaneously, rendering the bird temporarily flightless (e.g. ducks, loons, rails and small bitterns).
3. Stepwise Wing Moult – where succeeding ‘waves’ of moulting feathers start before the preceding one has been completed (e.g. many long-winged birds, including large hawks, boobies and pelicans).
4. Alternative Wing Moults – several moult strategies which don’t fall into any of the above three categories! (e.g. Albatrosses, Barn Owls, some typical owls and some kingfishers).
This close up of the eagle’s primaries shows three groups of different-aged feathers. P1-6 are the newest (as can be seen by their neat outlines), P8-10 are a little older (and a bit more abraded) and P7 is the oldest and most worn of all. So this bird is following a stepwise wing moult strategy.
Howell writes of Stepwise Wing Moult:
Many large, long-winged birds simply do not have enough time between breeding seasons to undergo a standard sequential primary moult, because feathers can only grow so fast. If these birds need to fly to forage, becoming flightless is not an option. A strategy that helps them overcome this problem is stepwise moult, in which multiple waves of moult can be set up in the primaries…How do stepwise moults develop? Basically, before one wave of sequential primary moult (from P1 out to P10) has completed, a second wave starts anew at P1…The period between the start of each wave is usually about a year (p36-37).
What can be seen in the eagle at Chuping is that the first wave has completed (i.e. reached P10) and the second wave has begun, reaching as far as P6.
P7 is an anomaly. It should have been moulted during the first wave, but somehow got missed. This may have happened as a result of moult being suspended during the bird’s first winter. Steve Howell wrote that it may be linked to the bird turning up outside its usual winter range:
The old p7 (a juv feather?) seems like an aberration, perhaps suggesting that migration and molt are both screwed up, which may happen more than we think – if one thing is “off” when other things may be also.
Assuming this is a juvenile feather, this helps us ‘age’ the bird as in its third plumage cycle. In ‘normal schedules’, the first wave starts at about 9 to 10 months of age (in the birds first summer after hatching), the second at 21 to 22 months of age (in the bird’s second summer), and so on each year.
That would make this bird around 2 years old (in its third winter).
The secondaries tend to moult both outward from the tertials and inward from the outermost secondary. Because a big gap in the middle of the wing could compromise efficient flight, the outer secondaries do not start moulting until the new inner primaries have grown in. Typically, when primary moult has reached the middle primaries (say, around when P6 is shed), the outer secondaries start to be replaced…In longer-winged birds there is often another point of initiation of secondary moult at S5, whence moult proceeds toward the body (p34).
This process can be seen in the Chuping eagle, with moult having begun at S1 and S5 moving inward, and from the tertials moving outward.
Two weeks later I saw the bird for the first time. Not much seemed to have changed in terms of wing moult from when Hor Kee photographed it.
This photo was taken about three weeks after the previous one. Here again, with the exception of P7 on both wings, which has finally been shed, there’s not much sign of further moult progression. This is perhaps not surprising if moult is suspended during the winter season. It will be interesting to see, if the bird sticks around, whether the new P7 develops, or whether that will only be replaced in the next wave of moult. Without the obviously worn P7 separating the two waves of moult, it is now much harder to see the two age classes of primaries, and even the gap left by the missing P7 is far from obvious.