Falconry Friday! Feathers, Daylight, and Molting

February 10, 2017  •  Leave a Comment

Hi all!  Now that the hunting season is over and our hawks are starting the molt I thought we'd take a closer look at the molting process as well as feathers in general.


Up until fairly recently , there was quite a bit of "which came first, the chicken or the egg?" debate about feathers: 

Did dinosaurs have feathers? (yes)

Are birds living dinosaurs? (yes)

Who else has the genes for feather production? (pretty much everyone - including humans)



Recent fossil finds, especially in China, show feather prevalence many millions of years before birds took to the air and DNA studies have shown that feather production is genetically well preserved across many non-avian genera (including us).  This genetic prevalence across the animal kingdom supports the notion that the genetic basis for feather growth pre-dates birds (and even dinosaurs).  So, while we all have the genes, we don't seem to have the on-off switches - leaving modern birds the only ones capable of growing actual feathers.


As many of you will recall from the following chart, feathers (15%) are a larger portion of a bird's weight than the skeleton (11%). Growing them initially, and replacing them annually, represents an enormous energy commitment!





Raptor Composition by weight:

  • 15% Feathers
  • 11% Bone
  • 17% Breast muscle
  • 10% skin
  • 10% Fat
  • 10% Leg muscle
  • 9% Wing muscle
  • 18% Combined Organs





The number of individual vaned feathers varies by species - from a low of around 1,000 for hummingbirds to as many as 25,000 for large waterfowl like swans (most raptors fall in the 7,000-10,000 feather range).  Regardless, each individual feather grows in the same way: as a helix, or spiral, from a temporary keratin sheath.  As the feather elongates, new barbs are laid down onto the main vane (rachus) by a germinal collar. As the feather reaches full length, the germinal collar lays down the solid quill (calamus). For the long flight feathers, this process takes around 6-10 weeks.

Here is an illustration of this growth from Thor Hanson's excellent book Feathers :




And here are a few photos of the new feather emerging from the temporary sheath as it grows:


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In this way, subsequent feathers are continuous with their predecessors - the new feather pushes the old one out as it grows in a continuous string. This also explains why, if a mature feather is traumatically pulled from it's soft tissue sheath, it may not regrow -- the feather's artery and germinal collar may be too damaged to produce new keratin.

Alternately, a damage feather follicle may produce a defective feather, either just once or permanently.


A good example of transient damage is the presence of "stress marks" or "hunger traces".  These weak spots along the feather shaft are the result of a nutritional deficiency during feather growth that effect the current feather(s) but not necessarily future ones.


More permanent defects can sometimes be caused by a West Nile virus infection during feather growth - these feathers will often slough at the keratin sheath with replacements in subsequent years bearing a defect at that stage of growth (as in Stanley the Hellgate Osprey's wonky flight feather).

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While the growth of each individual feather is the same, the pattern of growth is very different for adults as compared to hatchlings.




The first feather growth (natal down) actually occurs while the embryo is still developing within the egg. This whitish fluff dries quickly upon hatching and serves as the hatchling's initial insulation.  This is quickly (around day 10) supplemented with (but not replaced by) the grayish true down.  Once the legs and wings have grown long enough to support feather development, the vaned feathers appear -- pushing out the natal down feathers as described above.  This is the only time in a bird's life she'll grow all her feathers at once. By 6-8 weeks of age, growth is complete (full summed) and the calamus is solid (hard penned).  For most raptors, this first set of feathers is somewhat longer (by 1/3-1/2" apiece) than the adult feathers will be and are usually dullish browns on top and mottled whites underneath, thought to help camouflage  these inexperienced youngsters - both from predators (above) as well as prey(below).


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Adult molting


Adult raptors molt once a year, usually starting in June and finishing in September or October.  Three things seem to be necessary to initiate the molt, all of which can be manipulated to start the molt early or even, as is often the case with demonstration birds, reverse molt:

  • adequate body condition - usually 10-15% above ideal flying weight (remember, this is an energy-intensive process)
  • moderating temperatures - losing down=losing insulation so warmer temps are part of the equation
  • increasing daylight - this seems to be the primary trigger for initiating the molt (around 15 daylight hours)


Adults typically do not replace 100% of their flight feathers in any given year -- it is usually not until a raptor's 3rd or 4th year that she has a complete set of adult primary feathers.  Interestingly, this trait allows some degree of "aging" a raptor between the full juvenile and full adult plumages in those first few years of life.  Jerry Ligouri has a nice pictorial at Hawk Watch International using this method to age Golden Eagles.

Here is a motly looking Dora, I think at her second molt, showing a mix of juvenile, adult, and emerging adult primaries:


Bird nerdiness: Kestrels will actually undergo a partial molt around 6 months of age (November-December) and can look very similar to adults even to experienced birders.




Adult raptors replace their primary feathers a couple at a time, so as to be able to continue flying and hunting in the interim.  The next one will not start until the first one is 2/3 grown.

There is supposedly a pattern to how they replace the primaries but I've never seen any individual bird actually follow the "rule" we all had to memorize.  Here is said pattern, in case your bird can read the textbook:

  • Body feathers -- fairly random, about a quarter to a third at a time
  • Secondary feathers -- start in the middle and progress to both ends
  • Tail feathers -- start in the center and progress outward
  • Primary flight feathers (falcons) -- start and #4 and progress in both directions
  • Primary flight feathers (hawks) -- start at #1 (body side) and progress outward


note the emerging red tail feathers as well as reddish body feathers:


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Anyhow, after a couple molts one ha quite a few feathers lying about (for imping, of course):






You gotta be quick though as they LOVE to play with molted feathers:




here is a comparison of Harris' Hawk tail and wing feathers, with the juvenile pattern on the left and adult on the right:

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Finally, since we are talking about plumage afterall, here are a series of juvenile and adult hawks:


Red Tail, juvenile, second year, 10+ years:







Harris' Hawks















and Prairie Falcon  (juvie right)








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