On Mon, May 12, 2003 at 06:12:28PM -0700, Penn Gwynne wrote:
> G'day Andrew, bird deaths caused by an unnatural event such as being
> snared by a longline or snared in a tree due to being trapped by a
> legband are NOT a NATURAL event and therefore will not appear in the
> databases of those that snare. How can they appear? so the data being
> used is very flawed and dare I say lopsided? an informed guess~timate
> at best is made. That's my beef, how incorrect the assumed death rates
> are from banding.
How can they not appear. When you estimate mortality rates from
subsequent obervations of marked individuals, all causes of mortality
will be included in the estimates.
The re-observations are translated into estimates of mortality and
other population parameters using various statistical models. If you
really want to know how biologists analyze such data read chapter 4 of
"Population Parameters - Estimation for Ecological Models" by Hamish
McCallum.
Separating causes of mortality would generally requires data which is
more difficult to collect, e.g. you might have to radio-track individuals
over long periods.
Biologists definitely consider whether the methods of marking or
observation might influence the data - Its not just deaths - e.g. color
bands might enhance a male's chance of reproductive success.
You seem to be alleging that biologists doing such studies are cruel,
deceptive and/or incompetant. I've met a few and they are smart
people who go to lengths to avoid killing or injuring animals.
Andrew Taylor
Birding-Aus is on the Web at
www.shc.melb.catholic.edu.au/home/birding/index.html
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