Part 1.
Chris Watson’s original question about the meaning of yellow ceres in Brown
Falcons has sparked a lot of responses. Whilst the question has been answered,
more or less, there have been some conflicting answers, and a lot of peripheral
misinformation about variation in plumages and bare parts. This stems mostly
from a study of Brown Falcon by Paul McDonald (2003: Emu 103, 21-28) that,
despite being a good study, reaches unsupportable conclusions due to
extrapolation of the findings beyond the capacity of the data set, some
circular arguments and rejection of prior studies without justification. On
this latter point, McDonald (2003) rejected virtually every aspect of the Brown
Falcon ‘Field Identification’ and ‘Plumages and Related Matters’ accounts in
Marchant & Higgins (1993: HANZAB Vol. 2, pp. 237-8 & 248-51). As the author of
that plumages section in HANZAB I wish to correct some matters that have been
dismissed or overlooked in this
Birding-Aus discussion so far.
I agree with Paul McDonald on the relationship between cere colour and age and
the lightening of plumage with age (both these processes were identified
earlier in Weatherly et al. (1985) and HANZAB (1993)). I recognise that Paul’s
study uncovered a significant component of sexual differences in plumages that
was not identified in HANZAB or previously. However, I disagree with the
assertion there are no plumage morphs or “phases” in Brown Falcon and plumage
variation is all due to age and sex.
First, let me establish that the Plumages texts in HANZAB were not quick and
dirty studies. They involved careful and systematic observations of plumage,
bare parts, measurements, structure and moult of museum specimens from
throughout the species’ ranges, with large sample sizes when available.
Variation was systematically assessed to partition the contributions of age,
sex, season, plumage wear, morphs and geography by sorting the specimens by
each of these characters to reveal apparent patterns.
In the bare parts section of Brown Falcon in HANZAB (V.2, 1993 p. 250) I wrote,
for all morphs, that "Cere and orbital ring usually pale grey to pale bluish
grey; in a few, yellow, yellowish grey, or greenish yellow; yellowness develops
with age, may be more common in males, and is associated with pale iris...
[etc]. This largely answers Chris' question, though Jeff Davies is collecting
evidence that it is particularly prevalent in rufous morph birds from arid
regions. Condon (1951: Emu 50, 152-74) wrote about yellow ceres too, but I
don't recall whether he made the association between bare part colours and age.
Weatherly et al. (1985: Emu 85, 257-60) looked at changes in captive brown
morph birds from Tasmania and noted the association between yellower bare part
colours and age.
McDonald, in his 2003paper and in recent postings to Birding-Aus, has proposed
a hypothesis that I will couch as follows <there are no plumage morphs in Brown
Falcon but rather birds get paler with age, and males are paler than females
and this gives the illusion of different morphs as erroneously described in
HANZAB>. This hypothesis contradicts all previous studies of the subject.
However, it is an extrapolation beyond the applicability of McDonald’s data
set, because it takes findings from a study at a single small location (Western
Treatment Plant in Werribee Vic) over a short 3 years and extrapolates them to
the entire continent of Australia. Considerable evidence is published that
conflicts with this hypothesis. The descriptions in HANZAB were based on
examination of approximately 500 specimens (many more than the 160 (or 14,
actually) birds studied by McDonald) from the entire continent, not just a
single site, that were accumulated by
collectors over more than a century. This evidence has been dismissed by
McDonald as under sampling. Some particular problems with McDonald’s hypothesis
are:
1) Most juveniles are brown with extensively buff underparts, so if they
lighten with age how would dark birds described as the dark morph in HANZAB
come to exist? Reliably aged juvenile skins exist that show entirely dark
underparts and upperparts consistent with the descriptions of dark morph in
HANZAB; at this age there can be no lightening of plumage due to age, and both
males and females occur in this plumage. Therefore, juveniles show different
morphs.
2) Juveniles from rufous parents (collected together in central Australia)
resemble juveniles from brown parents in their underparts, but differ in having
consistently broader and more rufous fringes to the brown feathers of the
upperparts at the same age. Therefore juveniles show different morphs before
any lightening with age occurs (and this holds throughout their lives).
3) There are no rufous birds (rufous morph) or dark birds (dark morph) in
Tasmania; this means there is by definition a geographical component to the
variation across the country as a whole, not just age and sex components.
4) Brown birds are more common in humid coastal regions, rufous birds more
common in arid interior regions, and dark birds only prevalent in tropical
northern areas. Once again this is a geographical component to the plumage
variation. However, it is not a simple case of all rufous birds inland and all
brown birds on the coast, all three forms occur widely, so these are not
geographically isolated forms or genetically separated populations (i.e. they
are not subspecies). Nor is it a cline of gradual transition from brown to
rufous moving inland. That leaves only one parsimonious alternative: varying
geographical ratios in the expression of different phenotypes across the
continent, presumably as a result of differing evolutionary drivers across a
meta-population in a heterogenous environment - or more simply put, different
morphs.
McDonald studied birds in a single location to generate his hypothesis. He
ignored the lack of variation in Tasmania (from where he had no data), implying
that it must be present but so far overlooked. Evidence indicates otherwise.
Similarly the contradictory evidence for geographical variation (higher ratio
of rufous birds in the interior) was dismissed by the contrived suggestion that
Brown Falcons live longer in the inland than they do on the coast so more birds
live to become pale. Where is the data to support that speculation? Where is a
precedent? There are countless precedents for the alternative explanation that
there is geographical variation in the expression of colour morphs in
Australian birds.
It is still the case that the morphs are not distinct but integrade to some
extent, and birds do change (mostly get paler) with age. However, the sample
sizes in McDonald’s study were rather small. Of 14 individuals recaptured after
moulting to a subsequent plumage, 8 got paler but 6 did not. That is not a big
sample, not a uniform trend, not statistically significant, and not
geographically representative. HANZAB had already noted that birds get paler
with age over several moults, without over generalising.
HANZAB did not recognise the differences in sexes at the same age (males
lighter than females) reported by McDonald. This seems to be an important
advancement from McDonald’s study, and an oversight in HANZAB. McDonald’s birds
were sexed by methods that have not been totally declared (described merely as
“behavioural observations and a variety of morphological characters”) and the
HANZAB method of simultaneously comparing a large sample of adult males with a
large sample of adult females (sexed by dissection) did not detect this
pattern. It is possible that I overlooked it. I suggest that the sexual
difference is likely to be true, but technically it needs further evidence
before it can accepted as proven.
Weatherly et al. (1985) was a longitudinal study of a small number of subjects
over a long period of time. HANZAB was a cross-sectional study of many
subjects, each preserved at a single point in time (though also incorporating
the results of Weatherly et al. and others). McDonald (2003) had a bit of both
aspects with mostly one-off captures of many birds and some recaptures of a few
banded wild birds, but it lacked the continent-wide coverage and large sample
size of HANZAB. An advantage with skins over wild birds is you can compare
large numbers simultaneously for extended periods, and even revisit them later.
All studies have their advantages and limitations, so combining the insights
from multiple lines of evidence is the most effective path. My thoughts are
that McDonald does not replace all previous studies, but adds to HANZAB in
demonstrating that plumage changes with age and sex are even more marked than
previously recognised.
(more to come)
David James,
Sydney
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