Re: birding-aus Re: Migration of noisy friarbirds

Subject: Re: birding-aus Re: Migration of noisy friarbirds
From: Andrew Taylor <>
Date: Tue, 20 Jul 1999 13:12:21 +1000 (EST)
On Tue, 20 Jul 1999, Judy Caughley wrote:
> When we see concentrations of birds or other animals (e.g. kangaroos, mice
> etc), we tend to jump to the conclusion they have migrated from somewhere.
> But one day I sat down and thought if all the animals within a radius of 5
> km moved to the central 2 km  what would the effect on density be. The
> answer is quite amazing and suggests that local movements can really explain
> many of the variations in density we see. I'm not denying migration in birds
> like yellowfaced honeyeaters etc  - the winter exodus from Canberra to the
> coast is spectacular - but for species that are mobile, concentration on
> food sources is not unlikely.

Well the maths is simple so I'll throw it in.  If animals distributed
within a circle of radius of 5km of a point move to within a circle of
radius 1km, density increases by (5/1)^2.  In otehr words, density goes
up by a factor of 25.  Its a good point.

Andrew taylor

To unsubscribe from this list, please send a message to

Include ONLY "unsubscribe birding-aus" in the message body (without the

<Prev in Thread] Current Thread [Next in Thread>

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the birding-aus mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU