Firstly, I would like to congratulate Danny for providing a convincing and
well-researched response to the discussion of moulting patterns in
Australian birds. Much of what he says explains what was observed in
Spinifexbirds ... more about this in a moment.
However, as a person who has a background in avian physiology, I must point
out that moulting is tied more to the breeding cycle than Danny or David
may believe. Feather moult and the growth of new feathers is associated
with increased blood levels of thyroxine, a thyroid hormone (Chandola et
al. 1982; Kar & Chandola 1985) and falling levels of gonadal (reproductive)
hormones (Gupta & Thapliyal 1983).
Thyroxine levels increase with increasing daylength or natural photoperiod
(Bentley 1982; Pathak & Chandola 1983) and gonadal hormones decline at the
conclusion of breeding activity (see Wingfield and Farner 1980). Therefore,
you would expect thyroxine levels to be high enough at the end, or near the
end, of a normal breeding cycle to stimulate moult and new feather growth.
It would be interesting to know if the same mechanism exists during an
opportunistic breeding cycle. This type of breeding response may occur in
the arid zone as days are shortening (e.g. at the end of summer), thus you
may expect that circulating thyroxine levels may be declining rather than
increasing. However, the magnitude and rate at which reproductive hormones
drop at the conclusion of a very intensive opportunistic breeding response
may be enough to elevate thyroxine levels to a point that stimulate feather
moult. This would be a fascinating study, but not an easy thing to do. One
would have to choose an arid-zone bird species that could be caught
repeatedly throughout the breeding cycle, and large enough to allow you to
take sequential blood samples to enable the measurement of gonadal and
thyroid hormones. There is just so much to learn ...
Back to Spinifexbirds....
There is no question that Spinifexbirds were undergoing wing moult in both
July 1982 and February 1993. Danny has indicated that a complete moult
cycle takes a long time and it would be difficult for an individual to go
through two complete moults in a single year. If one accepts that moulting
is under the hormonal influence of the breeding cycle, as I?ve outlined
above, then I see three options:
1. The birds have a shorter period of moult than most other birds, thus
allowing two moults to occur in one year.
2. The birds were going through a partial moult in July 1992 (when
resources were perhaps limiting?) and a complete moult in February 1993
(when there were enough resources available to allow this to occur).
3. The birds which bred seasonally in July 1992 were different to those who
bred opportunistically in January/February 1993.
I don?t accept a 4th possibility of a single long moult cycle lasting from
July 1992 to February 1993 ... it is just too long and the pattern of moult
was so different. Point (3) seems to be the most sensible explanation.
I also accept Danny?s suggestion that moult in Spinifexbirds may also be as
a result of living in an abrasive habitat. Wing and tail feathers of birds
we handled always looked tatty. Anyone who has tried to walk through thick
Spinifex (Triodia) habitat will have experienced the prickly and abrasive
nature of these grasses.
The dumping of all secondaries at once did not seem to affect the mobility
of Spinifexbirds. At best, they are weak fliers, seldom flying more than 50
m before landing on or near a Spinifex clump. The work that my research
group was involved in necessitated recapture of individual Spinifexbirds
within 2 or 3 days of their release (after their first capture), and so
radio transmitters were placed on some of them to follow their movements.
We were extremely amazed at how skillfully these birds moved across the
ground or flew low between clumps without being seen. Therefore, to use
Danny?s term, Spinifexbirds certainly do live a ?skulking existence?.
Bentley, P.J. (1982). Comparative Vertebrate Endocrinology. 2nd Ed.
(Cambridge Univ. Press, Cambridge).
Chandola, A., Pathak, V.K. & Bhatt, D. (1982). Adaptive roles of T4 and T3
in avian seasonal phenomena. In: Suzuki, S. (ed). Phylogenic Aspects of
Thyroid Hormone Action (Centre for Acad. Pub., Tokyo).
Gupta, B.B.P.& Thapliyal, J.P. (1983). Role of thyroid and testicular
hormones in the regulation of basal metabolic rate, gonad development, and
body weight of Spotted Munia (Lonchura punctulata). Gen. Comp. Endocrinol.
Kar, A. & Chandola, A. (1985). Seasonality in birds and reptiles: the
involvement of T4 and T3. In: Follett, B.K., Ishii, S. & Chandola (eds).
The Endocrine System and the Environment (Japan Sci. Soc. Press, Tokyo).
Pathak, V.K. & Chandola, A. (1983). Seasonal variations in circulating
thyroxine and triiodothyronine concentrations in the Spotted Munia,
Lonchura punctulata. Gen. Comp. Endocrinol. 50: 201-4.
Wingfield, J.C. & Farner, D.S. (1980). Control of seasonal reproduction in
temperate-zone birds. In: Hubinot, P.O. (ed). Progress in Reproductive
Biology (S. Karger AG, Basel).
Additional reading that some may find interesting:
Ambrose, S.J., Bradshaw, S.D., Withers, P.C. & Murphy, D.A. (1995). Water
and energy balance of captive and free-ranging Spinifexbirds, Eremiornis
carteri North (Aves: Sylviidae) on Barrow Island, Western Australia. Aust
Ambrose, S.J. & Murphy, D.A. (1994). Synchronous breeding of land birds on
Barrow Island, Western Australia, after summer cyclonic rains. Emu. 94: 55-8.