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Re: Eastern Whipbird Antiphonal Song - measurements

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Subject: Re: Eastern Whipbird Antiphonal Song - measurements
From: "Dean Portelli" <>
Date: Sat, 24 May 2003 18:21:22 +1000
Hi All,

Ken Rogers wrote:
"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. ".

I echo this word of caution, and in fact stated briefly what a stastical difference between means implies: "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)".

I did not state that any individual whipbird can be reliably sexed by itself but rather when comparing a known pair (behavioural definition and ASSUMED to be ONE MALE & ONE FEMALE) then the measurements seem to me at least to be reliable, as in this case you are not viewing a single individual but rather comparing two birds you assume to be of the opposite sex: "...... 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......"

Ken also wrote: "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". I would tend to disagree somewhat. Certainly the sample sizes can be viewed as relatively small for EWhipbird (26 adult male, 18 adult female nominate subspecies), however the results of the statistical analysis for a comparison of adult male and female nominate (to which I will restrict myself here) showed significance for wing, tail and tarsus at the P<0.01 level (the 'standard' accepted level of a two-way analysis is generally P<0.05, which is what was obtained for bill). So, the results are highly satistically significant, despite the relatively small sample sizes. Statistical power (roughly equating to the ability of the analysis to detect a difference between males: a smaller sample is more likely to fail to find a difference between means that is real, than a larger sample, this is termed a type I error) increases with sample size, so the fact that this result was obtained with a small (but not inadequate) sample in my mind at least indicates that the difference is real and a firm conclusion of sexual size dimorphism (again, it is not absolute and can include overlap in the measurements of the sexes) is supported. This is also despite any possible confounding effects of geographical variation in size as specimens were taken from NSW, QLD and VIC. I also note that with most of the pairs that I worked with intensively (behavioural observations) I could readily see the difference in size between the birds and could confirm the male was the larger by knowing the colour-bands. I realise this argument is somewhat circular as measurements are the basis for the sexing. I also did not always know the colour-band combination when first observing the bird (so this did not always bias my scrutiny of size). One last point I wish to make is in regards to the way measurements are analysed themselves. Currently, most methods focus on univariate measures (either using wing, tail or tarsus for example by themselves to conclude with a degree of confidence the sex of any individual bird), it is likely that multivariate methods will prove much more powerful in resolving differences between the sexes, and that this method should be adopted more often (for example, these types of analysis will combine a number of univariate measures to maximise the separation between the samples, i.e. male and female. Principal Components Analysis and Discriminant Function Analysis are two examples of multivariate methods that can be used in such cases, and have been used with some species with success, e.g. Little Penguin DFA). Unfortunately, you have the catch 22 problem of needing to know the sexes first in order to accurately test for differences (without assuming, whether based on no data or a small amount, that the larger mean belongs to the male or female sex).

Anyway, while I agree with most of what Ken says, and respect that he is well-versed in the science of bird morphometrics, I felt the need to clarify what I was saying.

Cheers, Dean

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