Annie Rogers wrote:
?The moult scheme of Humphrey and Parkes is explained in detail and it has
been used in the all Plumages and Related Matters sections. In it you will
find that the magic word is 'pre-basic'. ?
Thanks, Annie. For the benefit of people subscribing to Birding-Aus, HANZAB
Vol. I says:
?Adult birds that have one plumage per cycle almost invariably lose and
renew their plumage in a complete moult. This is called the basic plumage;
it is renewed in the the pre-basic moult. The pre-basic moult can also be
recognised as a complete moult in adult birds that have two plumages per
cycle. These birds have a separate moult, usually partial, termed
pre-alternate plumage.?
Let?s examine Annie?s excellent descriptions of Grey and Rufous Fantail
breeding, moulting and movements:
?Grey Fantails spend about 9 to 10 months of the year in southern Vic (some
don't leave at all). Grey Fantails are far more widespread in Vic than
Rufous Fantails but where they overlap they share the same habitat. Grey
Fantails breed from early Sep to late Dec, possibly later. Grey Fantails
moult everything (not just primaries) from Jan to early April. Most leave
at the end of May, but a few remain all year - probably immatures. Grey
Fantails almost certainly raise more than one brood a year.?
This is a clear example of moulting being tied to breeding.
Predicted Physiological Mechanism:
Good breeding conditions stimulate excessive and prolonged gonadotrophin
secretion necessary for recrudescence (development) of reproductive organs,
production of sperm and ova (unfertilised eggs), onset and maintenance of
reproductive behaviour sufficient for the production of more than one
brood. High gonadotrophin levels and reduced food intake by parent birds
(as a result investing their time and energy in rearing their broods)
suppresses thyroxine secretion. A physiologically-stressed bird also
secretes corticosteroids (commonly referred to as ?stress hormones?) from
the adrenal gland. Breeding is physiologically stressful, corticosteroid
secretion is high and, in turn, this has an added suppressive effect on
thyroid activity. Prolonged breeding activity would have a great abrasive
effect on the feathers, hence, the need to moult and grow new feathers
immediately after breeding.
At the conclusion of breeding, excessive thyroid activity is stimulated as
a result of rapid and excessive drops in gonadotrophin and corticosteroid
secretion, and increased carbohydrate intake (as a result of an abundance
of food and more time to feed). The gradual lengthening of days also
stimulates further thyroxine secretion. High levels of circulating
thyroxine results in a complete body moult.
?Rufous Fantails spend only 5 months of the year in Vic, arriving,
conveniently, about one week before the twitchathon (end of Oct). Rufous
Fantails breed from Nov to Jan.
Rufous Fantails do not moult anything before they leave at the end of March
and none as far as we know stay behind. We did catch a moulting adult at
Paluma in northern Queensland in May but, of course, have no idea if it was
a migrant or not. Rufous Fantails that breed in Vic probably don't produce
more than one brood a year. A possible reason could be that Rufous Fantails
are basically 'warm weather' birds that take advantage of the the warm to
hot weather in the south when they arrive in late spring and they leave in
early autumn to avoid the cold conditions later.?
Predicted Physiological Mechanism:
Same hormonal cycle as for Grey Fantail during the breeding period.
Gonadotrophin secretion is not as prolonged, perhaps circulating levels not
as high, as in the Grey Fantail. Feather abrasion as a result of breeding
is perhaps also not as great as in the Grey Fantail.
I agree that the Rufous Fantail is essentially a warm-weather bird. The
late finishing to its breeding season means that there is insufficient time
to complete a post-breeding moult in Victoria before the onset of cooler
weather. To undergo a moult at this time would be a thermoregulatory risk.
Therefore, I suspect that thyroid activity is elevated at this time as a
result of reduced levels of gonadotrophins and corticosteroids, to a level
that is high enough to elicit migratory restlessness, but not high enough
to stimulate an immediate feather moult.
Voelker and Rohwer (1998) indicate that some western North American and
trans-Saharan migratory passerines moult during post-breeding migration,
which is subsequently completed at wintering grounds. They maintain that
moulting is supressed at the breeding ground, just prior to migration,
because of insufficient food. However, the birds feed in areas along their
migratory route that provide enough food to stimulate moulting. It would be
interesting to know if moulting in Rufous Fantails is also initiated during
post-breeding migration.
Chris Corben asks:
?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??
Yes, you?ve pretty much hit the nail on the head regarding the original
point. I have to admit that I?ve softened my attitude a tiny bit about the
relationship between breeding and the timing of moult, since the start of
this discussion. Nevertheless, I still maintain that breeding activity
causes excessive feather abrasian in most species, but reproductive
hormones, corticosteroids and lowered food intake suppress thyroxine
secretion. A sudden drop in these hormone levels towards or at the end of
breeding triggers thyroxine secretion unless it is overidden by the
inhibitory effect of low food abundance.
Western Gull:
?a) 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.?
It would appear that gulls have a classic physiological response. Basal
levels of thyroid hormones are higher in young birds than in adults because
of the roles they play in growth. Conversely, basal levels of reproductive
hormones are higher in adults than in immature birds. Therefore, the
lowering of basal levels of thyroid hormones, coupled with the increased
suppressive effects of reproductive hormones would tend to delay the annual
moult as young birds mature.
Given the pugnacious behaviour of gulls during the breeding season, I would
imagine that reproductive hormone (especially testosterone) and
corticosteroid levels would be extremely high in breeding birds, thus
exerting a suppressive effect on thyroid activity. Immature birds would not
have this suppressive constraint, so would be free to moult during the
breeding season when there is an abundance of food.
I also suspect that breeding birds would have a significantly reduced food
intake becuase of the time spent defending the nest site, incubating eggs,
and rearing the progeny.
As an aside, it would be interesting to know if gull species modify
moulting patterns if they feed on large quantities of junk food provided by
humans. Junk food is typically high in carbohydrates, and I would expect
that significantly high consumption would stimulate thyroid activity.
Common Tern:
?b) 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 acquires 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.?
I'll engage in pure guess work on this one. A seven-month moulting period
(July-February) suggests that food may be a limiting resource outside the
breeding period. In the early stages of this period, I would say that
lowered reproductive and corticosteroid hormone levels would be sufficient
to induce increased thyroid activity. Increasing daylength in the immediate
post-migratory period would further induce thyroid action. Towards the end
of the primary moult and the start of the 2nd moult, the main stimulus may
be increased food abundance, but decreasing daylengths and, perhaps early
rises in reproductive hormone secretion (in preparation for the breeding
season) may stifle thyroid activity. Hence, the partial (rather than
complete) moult.
?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.?
This may be so, but I suspect that these moults begin in the latter part of
the breeding cycle rather than at or near the beginning. Perhaps you or
someone else may be able to prove me wrong. Adult reproductive hormone
levels peak in the early part of the breeding cycle and then start to
decline when there are nestlings. This is because the production of
reproductive hormones is also energy expensive, and so levels start to
decline once the necessary reproductive condition has been obtained and the
appropriate reproductive behaviour has been exhibited. In species that
produce more than one brood in a season, these hormones will peak with each
brood production. Therefore, I would not expect a significant increase in
thyroid activity until there is a decline in reproductive hormones, either
between broods or in the latter part of the breeding cycle.
?Overall, Stephen, do you see the physiological mechanisms having any
predictive value for molt timing??
Only to a limited extent. Precise measurement of environmental stimuli in
the field (e.g. food abundance) is often difficult. Physiological responses
to stimuli are almost instantaneous; measurable effects thyroid hormone
secretion occurs within 15 hours of the onset of external stimuli
(Assenmacher 1973). I suppose birds need to take immediate advantage of
optimal conditions while they last.
Cheers,
Stephen.
References:
Assenmacher, I. (1973). The peripheral endocrine glands. In: Farner, D.S.,
King, J.R. & Parkes, K.C. (eds). Avian Biology, Vol. 3 (Academic Press, New
York).
Voelker, G. and Rohwer, S. (1998). Contrasts in scheduling of molt and
migration in eastern and western Warbling-Vireos. The Auk. 115(1): 142-55.
************************************************************
Dr Stephen Ambrose
Research Manager
Birds Australia (Royal Australasian Ornithologists Union)
Australian Bird Research Centre
415 Riversdale Road,
Hawthorn East,
VIC 3123.
Tel: +61 3 9882 2622
Fax: +61 3 9882 2677
Email:
1997 Australian Bird Research Directory is on Birds Australia's
home page: <http://www.vicnet.net.au/~birdsaus>.
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