Re: Bird mortality: North vs. South

[John Bois <jbois@umd5.umd.edu>]

On Sun, 6 May 2001, Ronald Orenstein wrote:

> At 05:15 PM 06/05/01 -0400, John Bois wrote:
> >Birds in the northern hemisphere generally live shorter lives than
> >southern birds.  They also have larger clutches and are more willing to
> >defend their brood.

Not defend! I should have said "feed" their brood.  And it's more complex
than I said: Northern birds are more wary of _offspring_ predators (e.g.,
bluejays) and reduce nest visitation rates presumably to maintain
concealment.  But they are _less_ likely to reduce visitation rates than
southern birds when confronted with an adult predator (e.g., hawk).

> The potential 
> for longevity may be no different; it is the survival rate in the wild that 
> differs.

I agree with that.  But the authors have, I think, done a good job of
controling many of the interhemispherical variables.  As well as
reviewing published data, they did their own study of a comparison of
three pairs of species closely matched phylogenetically and ecologically
in two study sites--one in Arizona (conifer forest) and the other in
Argentina (mixed subtropical forest).  All of the relationships held
up: nth/sth fecundity and mortality differences; and the behaviors
predicted from these.