Dear all,
I don’t agree with many of the points raised by David James’ emails, and
actually think that they mis-represent and further misunderstand the data and
conclusions of my earlier Emu paper (103:21-8) considerably. For these reasons,
I feel compelled to respond in some detail here. I’m not convinced that
Birding-Aus is necessarily the appropriate venue for this debate, particularly
given so long has passed since the publication of the research (’93 and ’03).
Given this, I’ll send a more detailed account addressing each of David’s many
points to him directly, but will forward that response to anyone interested.
Regardless of what your conclusions are, the point is that we still have much
to learn about Brown Falcons. I would encourage all birders to read HANZAB,
David’s emails, my responses, the Emu paper in question and have a look at this
website (http://tinyurl.com/BrownFpics). More importantly, I implore birders to
continue to look at Brown Falcons in detail, wherever you may be. My main
points are outlined below:
- HANZAB lists the brown, rufous and dark morphs, with intermediate forms
existing. HANZAB discusses the many proposed races, but rejects all bar the
ones in Australia and New Guinea, respectively, precisely because plumages are
so poorly understood.
- HANZAB is extraordinary in the detail in which it describes plumages, and I
have never stated that these data were gathered in a ‘quick and dirty’ fashion.
However, science is iterative, HANZAB should not be seen as a final word, but a
keynote of what is known at the time of print. Any study is limited to the data
at hand. Museum specimens are indeed valuable (see Emu 111(3):i). However, in
this case they can only provide a point sample, as the bird has obviously been
killed during sampling. Museum data thus poorly represent age-related effects
in a system where individuals are so variable. This is not a criticism of the
James’ study, HANZAB or museums in general, it is just a fact of life - skins
don’t age.
- Morphs should be stable, that is whilst there is likely some relatively
small changes in plumage over time within birds, if morphs represent true
phenotypic variance we should not observe individuals ‘changing’ between morphs
(e.g. a grey goshawk moulting into white plumage or vice-versa). Yet, this is
what we see at Werribee in the case of falcon morphs. See the pictures of a
female breeding as a ‘dark’ morph that later assumes a more typical ‘brown’
morph adult female plumage (http://tinyurl.com/BrownFpics). James questions the
sampling rigour of the Emu study, this mis-represents how probability and
statistics operates (or indeed was conducted in the paper). If 160 is a
cripplingly small sample size, than what is the probability of me detecting a
rare, aberrant female changing morphs? The odds are remote in the extreme I
would argue. While it would take an enormous sample to categorically prove that
birds don’t move between morphs, it takes only a sample of one to prove that
they can.
My argument for age/sex effects is summarised below. Birds that I measured show
one of two main patterns:
a. bare parts become increasingly yellow as birds age from blue to
blue-grey at fledging. Only in males does this change lead to entire yellow
across all bare parts routinely, a point missed in previous emails. In females
there was often some yellow in adults, but typically this was less plentiful
than a dominant blue-grey colour (http://tinyurl.com/BrownFpics).
b. plumage lightened with increasing age, to a point where females have
whitish bellies with a breast band of speckled light brown, whereas the entire
ventral surface of males lightens. The breastband seems to be characteristic of
adult females, and as such many retained similar looking plumage throughout the
project, even when as old as 13 years. In contrast, males that were in their
late teens had virtually completely white underparts and rufous dorsal surfaces.
- Together, these patterns match what others have described as dark
(young birds), brown (intermediate males/adult females) and rufous morphs
(adult males). Given this, I suggested that for this population, colour morphs
are not the most parsimonious explanation, but rather age/sex differences are
the likely mechanism behind this variance. Importantly, many intermediates
exist in both spectra. Age and sex differences can therefore also explain the
variation and intermediate forms between so called ‘morphs’.
- Extrapolation to other regions is speculative, granted, hence it’s
place as the last paragraph of the Emu paper. I would argue that it is valid in
this case however, as we recognise only 1 race of Brown Falcon in Australia.
Given this, is it really accurate to suggest that the observed patterns are
isolated to the Werribee region alone? Birds move between the Werribee
population and Tasmania, and partial-migration patterns are described in HANZAB
(p240), notably autumn-winter visitors to the north being predominantly
juveniles, that is dark birds to a region where dark birds predominate.
- My hypotheses (again, stressing it is just that, will gladly be proven
wrong when data are collected), is that disproportionate representation of
birds of different colour in these regions cannot be used as evidence of
different morphs. These are point samples of skins or records of unmarked birds
from a distance (age differences are subtle, see Weatherly paper). Given the
movements that exist in this species, it is possible that dark birds
predominate in the north and light birds in the centre if (dark) young raised
in the centre move to the north post-fledging, leaving only (light/red) adults
behind. This cannot be disproven or proven with rhetoric, what we need is other
longitudinal studies in these areas or, perhaps more accurately, a molecular
study to determine how genetically isolated different populations are. Relying
on plumage or size differences (both of whom are confounded by both sex and
age) will not help to resolve the issue.
Finally, I want to highlight some important misconceptions of my study as
re-stated by James in his previous emails:
- Birds were molecularly sexed after that Emu paper was published (and
indeed 40 of the 160 putative sexes were confirmed in the paper, as it states
in the Methods). Thus, there is no circularity or doubt over sexing techniques
in the Emu paper.
- Colour was not used to age birds, rather the plumage characteristics of
underparts as described in the Weatherly paper, thus there is also no circular
reasoning behind age effects.
- It is nonsensical to argue that 500+ point samples collected from
museums provides an n=500, whilst my study, a combination of point samples
(n=160) and recaps (n=14) only has an n of 14! By that logic HANZAB has an N of
0…
- Further, as I believe females don’t change once they reach a certain
plumage type (again a hypothesis), I would NOT expect everyone of the 14 birds
recapped to have changed in colour. Thus the application of statistics as a
8/14 being non-significant is erroneous and simply incorrect. More damning is
how proponents of the morph hypothesis can explain birds moving between
different colour morphs, as an n of 1 in this instance effectively kills that
phenotype-based argument.
To re-iterate, we need more data and more people watching Brown Falcons. The
Emu paper is not meant to throw out HANZAB data or any other study, rather it
builds upon that foundation and interprets information at hand given the new
lines of evidence available. I argue that my conclusions stand up to the
current level of data, and are defended in more detail in another email for
those hardy souls interested. I’d be quite happy to be proven wrong, but the
point is that we require data to prove this, opinions are not sufficient either
for or against.
Cheers,
Paul
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr Paul G. McDonald
Lecturer
Zoology, School of Environmental and Rural Sciences
University of New England
Armidale NSW 2351
Australia
Ph: +612 6773 3317 Fax: +612 6773 3814
Publication list: http://publicationslist.org/paul.mcdonald
Thompson ISI Researcher ID: http://www.researcherid.com/rid/A-5928-2010
Web: http://www.une.edu.au/staff/pmcdon21.php
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