Hi All,
Dean Portelli commented on the significance or otherwise of sex size
differences reported in HANZAB. A word of caution is in order. A
significant difference does not mean the same thing as a large difference.
It is one thing to say that males are larger on average when looking at
large samples of each sex. It is quite another to say what the probability
is that a single bird of known size will be a male. This is only possible
if the difference between the means (averages) is large relative to the
common (between sex) standard deviations (these measure size variablity
about the average. A ratio of difference to common SD of about 2 is
required if size is for reasonable numbers to be sexable on size. The
HANZAB figures give a ratio of 1.3 and 1.7 for adult wing and head lengths
respectively so I wouldn't be too confident on any sexing of this species
based on measurements. I would be very hesitant to use the superficially
more attractive tail lengths were it not that long tails of ground dwelling
birds are subject to shortening due to abrasion and other damage. Also,
despite the general excellence of HANZAB's measurements of skins - much less
measurer error than with live birds - samples are too small to support any
firm conclusions - other, of course, than this one.
Cheers,
Ken Rogers
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