The original point Stephen made was that molting was tightly linked to
breeding. He has just proposed four quite different mechanisms to explain
how molt could be DECOUPLED from breeding! Was the original point more to do
with the suppression of molt DURING breeding, rather than the onset of molt
being coupled to the END of breeding?
It's very valuable to get some input from Stephen, seeing things from a
different perspective to those of us who see more the end result. I'd be
interested to hear what Stephen has to say about the following:
a) An adult Common Tern starts molting P1 in July while still on the
breeding grounds. The P molt progresses slowly from P1 to P10, apparently
with a break during migration, but not finishing until February. About
February, P1 molts again, and several of the inner Ps are molted as the bird
aquires its alternate plumage. Clearly, the external stimuli under which the
different primaries are molted must vary greatly. If P1 is shed at a time of
increasing thyroidal and decreasing gonadal activity, surely that won't also
apply to P10, or to the 2nd molt of P1.
b) In a Western Gull, the first P molt starts in April, when the adults are
getting into their breeding activities and when the daylight is increasing
fast. Successive P molts start later, until the adult pattern is achieved at
age 4, when the P molt starts in July, at a time of decreasing daylight. On
the face of it, it would seem the P molt of a Western Gull is NOT tied to
either breeding or daylength! It's easy to imagine that P molt of a Western
Gull may be stimulated by an abundance of food, while being suppressed
during actual breeding.
c) I understand there are birds which show active molt WHILE they are
actively breeding. If this is so, wouldn't it make more sense to presume
that suppression of molt during breeding, while very common, is probably
only a consequence of selection favoring birds which put more energy into
bringing up their kids. So there may be cases where food is just so abundant
for a short period of time that selection will favor those who can take most
advantage of it by getting their molting done while the abundance lasts.
Overall, Stephen, do you see the physiological mechanisms having any
predictive value for molt timing?
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