Molt Strategy in Large Raptors

Notice that all of the wing flight feathers look new and perfect in this Turkey Vulture. It is probably a first year bird, who has yet to replace any of its first set of feathers.

All birds have feathers, and all feathers wear out and need to be replaced regularly. The process of replacing feathers is called molt. All birds have one (sometimes two) molts each year where they replace their feathers. This typically happens after the breeding season, when food is still in good supply, and after laying eggs and raising young. Production of offspring is critical to the birds in order to pass on their genetic material, and to sustain the species, but it takes second place to the importance of molt. In years when food supply is short a bird can opt out of breeding, and this happens regularly when expected rains do not happen, when weather patterns do not coincide with the critical time to breed for migrant species, or for many other reasons. If an individual bird, or a pair of birds, does not rear a clutch of young, they can always try again next year or later in the season. If a bird does not replace its feathers regularly it will become weakened in its ability to fly, to maintain body temperature, and to migrate if a migratory bird. If any of these things happen the bird will have a high chance of dying. Therefore molt is always attempted, even if it is at the expense of reproduction.

Note in this African Crowned Eagle, a very large eagle, that the outer 7 primary feathers are all fully grown, but that the inner 3 are different. Likely the innermost has been completely replaced (P1) and the next two are almost fully regrown (P 2and 3) on each side. Also some of the inner wing feathers, called "secondary" feathers, are being replaced.

The plumage a bird has after its annual complete body molt is called the basic plumage, and so the molt into basic plumage is called the pre-basic molt. During the prebasic molt most birds replace all of their feathers, both their body feathers and their wing and tail feathers. The wing and tail feathers are collectively called “flight feathers.” Very large birds, including large raptors, have such big flight feathers that it takes along time to grow an individual feather. Raptors are highly dependent on expert flight, and so cannot replace several flight feathers at the same time, and so there just is not enough time in the year to replace all of the flight feathers one after another. For this reason (experts think) large raptors, and some other large birds like albatrosses, geese, and others use a staggered molt strategy, often called “Staffelmauser molt” strategy.

In this African Fish Eagle notice that the outer primary feather on the right wing is long, the next one in is shorter as it has been shed and not fully regrown yet. Counting inward you'll see that the few feathers just inside the short 9th primary feather look brand new, showing very little wear. The outermost feather on the right side has wear on the inner veins, and will be replaced next after the 9th one grows out. This bird has likely also just replaced many of the innermost flight feathers as they look crisp and new, but the two innermost primary feathers look ragged, likely they are older and more worn.

In this strategy after one year of life when the raptor needs first to molt it’s wing flight feathers (remiges) the innermost primary wing feather will usually be shed and replaced first. When this is almost grown out the second from the inner end of the primary feathers is shed, and this continues until after maybe 4-5 primary feathers are replaced molt is suspended. The next year when molt begins again it starts in more than one place at a time. The innermost flight feather is again replaced at about the same time as the nest in line to be replaced going outward from the year prior, maybe the 5th or 6th primary feather. Then each year the molt resumes both at the innermost end, and at the next feather in line from the last one replaced in the prior year. This can lead to two, or in the largest of birds three remiges being replaced on each side.
This strategy allows large raptors to gradually and regularly replace their remiges while maintaining good flying ability at all times.

No raptor and no molt here, just mixing things up with a cool photo of a cloud I like.

It can also lead to scruffy looking large raptors who look like many feathers are missing or shorter than the others. The next time you see a raptor soaring who has several gaps in the wing feathers, remember that this is the normal process of primary flight feather replacement. You can also feel smart if you mention that you know this bird is not a juvenile (who always have all perfect new flight feathers because they were just born this year and have not lost any yet) and that she staggered, or Stoffelmauser molt is underway.
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