Syd asked about sexual size dimorphism in Eastern Whipbirds.
As it wasn't my project I do not know the basis for which the PhD student I
was working with used to sex the birds on measurements, although it was
clear there was consistently one sex larger than the other. I do not know
whether DNA samples were taken.
Having looked over HANZAB, they measured museum specimens of each sex and
found a significant difference in wing length, tail length, bill length(? -
BILL S in HANZAB), and tarsus between adult male and female and first
immature male and female. This was for the nominate subspecies occurring in
SE Australia. For subspecies lateralis of Nth QLD, only adult skins were
measured and the differences between the sexes was found in wing and tail
lengths only. In all cases where a significant difference occurred the mean
for the male was larger than the mean for the female (i.e. male was the
larger sex). Weight of males was also larger but this was not statistically
significant. All museum specimens would have been sexed on reproductive
organs.
So while a significant difference doesn't preclude overlap in measurements
between the sexes (it indicates that the difference in the means of the two
or more groups being tested are greater than what could be expected by
chance), when you have a pair that are inferred to be a pair from
behavioural observations then the larger bird can be reliably sexed as the
male (and the colour-banding allowed the pairs to be clearly identified
through consistent association such as foraging together, calling to each
other with vocalisations other than song, and also through duetting - this
study was a great use of colour-banding by the way, it just yielded so much
data).
Laurie replied to me personally and described an experience with Eastern
Whipbirds responding to his imitation of the introductory whistle (that
precedes the actual whip 'crack'). When we were working in the field we also
observed that the birds may sometimes reply to imitations of the whislte
(none of us could do the crack very well!). I think it was also observed
that neighbouring birds may reply to an introductory whistle, and males
sometimes gave this without the whip crack.
Cheers, Dean
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