Ken and Annie Rogers say:
?Indeed, there are quite a few species that don't have amazingly worn
primaries after breeding. This brings us to our third point; there are
exceptions to the rule that moult happens soon after breeding ...
? ...We suspect that the list of exceptions would be pretty large if we worked
hard at it. So, explanations please. It is insufficient to say, ?... one
ocasionally comes across a species that hasn't read the rule book?! ?
I think you have misinterpreted my point, this is probably my fault for not
fully explaining my ?rule book comment?, and because we are viewing the
causes of moult from different perspectives.
The responses of animals (and plants) to their surrounding environment can
be explained by sequences of biochemical, physiological and behavioural
processes. These processes are continually governed by environmental
stimuli and inhibitors acting upon the animal?s body. It is the role of an
environmental physiologist to accurately record the environmental
conditions to which an animal is subjected, and to determine what body
processes (i.e. physiological mechanisms) and behaviours are exhibited by
the animal to adapt to those conditions.
Closely-related taxa nearly always employ the same physiological mechanisms
when subjected to the same environmental stimuli and inhibitors, even
though the timing and magnitude of this response may vary between species.
Therefore, environmental influences on the thyroid gland, and the overall
mechanisms and effects of thyroid hormone action on other parts of the body
(an example of a physiological mechanism) appear to be the same in all
birds and mammals.
At a functional level, a physiological process is the product of
interacting biochemical, electrical, and physical processes within the
body. In making the statement ?some animals don?t read the rule book?, I
was flippantly referring to the fact that the occasional species may use a
modified physiological mechanism to respond to the same environmental
conditions as other closely-related species. There are several reasons for
such modifications, but it is often as a result of the species lacking a
specific enzyme to facilitate a chemical reaction in a biochemical pathway,
and so other biochemical routes (which, in turn, may lead to other
physiological processes) being employed.
The hormonal influences of moulting have been studied in a broad range of
bird species, from penguins through to finches (not just ducks and chooks).
To the best of my knowledge, these birds respond to the same environmental
stimuli and inhibitors, and use the same hormonal mechanisms, that control
moult. However, I concede that very few Australian passerines have been
studied, and the few studies of other physiological attributes (e.g.
thermoregulation, water and electrolyte metabolism) have produced some very
surprising results. Drs Lee Astheimer and Bill Buttemer, of the University
of Wollongong, recently started investigating hormonal cycles, including
reproductive and thyroid hormones, of Yellow-plumed Honeyeaters living in
the arid zone, and it will be interesting to hear their conclusions in a
few years time.
I?m willing to bet that all the birds Ken and Annie listed conform to the
physiological model described in my last message (13/5/98), rather than
being ?exceptions to the rule?. It is a question of the timing and
magnitude of stimulatory and inhibitory effects of moulting and the
physiological sensitivity of the bird to those effects.
For instance, let?s take an example provided by Ken and Annie:
?... Rufous Fantails that go north before moulting unlike the
closely-related Grey Fantails which moult first?.
I have already mentioned that thyroid activity increases as reproductive
hormone levels decrease. One can speculate several reasons for this
(1) reproductive hormone levels may not subside to basal levels before the
onset of migration (thus having a suppressive effect on thyroxine
secretion) in Rufous Fantails, whereas they do in Grey Fantails.
(2) thyroxine (T4) is being secreted in Rufous Fantails, but much of it is
being deiodinated to triiodothyronine (T3) to help break down carbohydrate
stores in the body in preparation for migration. If Rufous Fantails migrate
longer distances than Grey Fantails (and I suspect many of them do), then
this process would be more important for Rufous Fantails than Grey Fantails.
(3) Dietary carbohydrate of Grey Fantails may increase at or near the end
of breeding, thus stimulating moult, whereas Rufous Fantails may have to
wait until reaching the end point of migration to find extra carbohydrates.
(4) Grey Fantails may be more sensitive to the stimulatory effect of
increasing daylength than are Rufous Fantails, thus responding earlier.
Daylength would be increasing at the conclusion of the seasonal breeding
period, but daylength may need to be longer for Rufous Fantails than for
Grey Fantails for moulting to be stimulated.
I?m sure that similar principles apply to the Holarctic waders, upon
migrating to the Australian region at the conclusion of breeding.
David James states:
?Feathers are dead, and so the physiological stress of breeding can have no
effect on them. I cannot see how breeding would increase the rate of decay
from U.V. radiation or bacterial attack. Possibly abrasion may be increased
in sitting birds but I am not convinced that it would be significant. Maybe
lice attack is more serious in incubating and brooding birds (it certainly
is in chooks). I would expect that degree of feather wear more closeley
correlates with age of feather than anything else, but this is just a hunch.?
I?m not suggesting that there is a connection between the physiological
stress of breeding and feather wear. I?m saying that behaviour associated
with breeding usually causes significant feather abrasion. You have
mentioned the possibility of feather abrasion and feather lice attack in
the nest and I, too, believe that this would contribute to feather wear.
However, I suspect that increased foraging activity associated with feeding
incubating birds, nestlings and young fledglings will also significantly
increase feather abrasion (e.g. ground-foraging birds searching through
leaf litter, birds probing underneath bark, etc). Additionally defence of
the breeding territory (e.g. boundary disputes) may occasionally lead to
loss and tear of feathers. An increased proportion of a bird?s time spent
foraging and in territorial defence may also lead to less time devoted to
feather hygiene (e.g. grooming the plumage).
Ken and Annie ask:
?First, what is ?intensive breeding?? We have never heard of it. Birds are
either breeding or they are not; there wouldn't seem to be much room for
This is a term often used to describe a huge opportunistic breeding
response to optimal breeding conditions. Most Australian arid-zone species
will invest a good proportion of their breeding activity to these optimal
times (intensive breeding response) and have reduced breeding activity at
other times. Australian pelicans, stilts and swans are good examples ...
some individuals may breed only once every few years when inland lakes fill
with water (e.g. Lake Eyre). However, during this period they produce
multiple broods, whereas seasonal breeders under less optimal conditions
may only produce one.
I would imagine that the feathers of these birds would be extremely worn
after the production of multiple broods. I would expect them to be moulted
soon after breeding given the right environmental and physiological stimuli
(as discussed for fantails above).
?I suppose if birds began breeding as conditions improved, and young fledged
at or just after conditions reached an asymptote then moult could also
begin when conditions were favourable?.
Yes, I would agree with this. I appreciate your concern that breeding and
moulting are energy-expensive. The energetics of some components of the
avian life cycle are discussed by Walsberg (1983). The expenditure of
energy in breeding varies considerably between species, depending upon the
breeding system (clutch and brood size, number of breeding attempts,
parental investment, etc) employed, environmental conditions, etc.
Therefore, it is not pertinent to pick an average figure for birds.
However, testicular growth alone at the start of each breeding season
requires the energy equivalent of up to 40% of a bird?s basal metabolic
rate. Moulting raises a bird?s basal metabolic rate by up to 46%.
Therefore, it is amazing that a species like the Grey Fantail can breed,
moult and then migrate potentially long distances.
?...A minor point is that linking the terminology of moults to the breeding
cycle (ie "post-breeding moult") creates in this instance something of a
circular argument. For instance, sandpipers BREED, MIGRATE, MOULT, MOULT,
MIGRATE and then are ready to breed again. Calling these two moults
post-breeding and pre-breeding respectively is a bit misleading, but that
is what they are most often called. For this reason (and others) Humphrey &
Parkes (1959) proposed a terminology for moults that is based on the cycle
of plumages and moults itself rather than on the seasons, the breeding cycle.?
A worthy point to make, but there are a lot of SEDENTARY bird species which
moult immediately after breeding. How does the terminology of Humphrey and
Parkes (1959) refer to this form of moulting?
Hmmm ... I?ve rattled on too long, once again. I?m running late for this
evening?s German lesson, so I must end here and make haste!
Walsberg, G.E. (1983). Avian ecological energetics. In: Farner, D.S., King,
J.R. & Parkes, K.C. (eds). Avian Biology, Vol VII (Academic Press, New York).
Dr Stephen Ambrose
Birds Australia (Royal Australasian Ornithologists Union)
Australian Bird Research Centre
415 Riversdale Road,
Tel: +61 3 9882 2622
Fax: +61 3 9882 2677
1997 Australian Bird Research Directory is on Birds Australia's
home page: <http://www.vicnet.net.au/~birdsaus>.