Re: Hadrosaur nesting strategy...(was Re: The Life of Birds- Pa

[Casey <TUCKERCJ@muohio.edu>]

I would argue that one of the prime reasons that Ross's & Snow geese are
proposed not to migrate into more temperate climates is because they would
have to compete with the Canada geese which fill a similar niche.  

I would argue that there is an abundance of arctic predators that would
feed upon the geese & their young, including: Arctic fox, Arctic wolves,
Polar Bears, Grizzlies, Peregrines, Goshawk, mink, Snowy owls, etc.  Note
that compared to the first list of temperate predators most of the
predators in this list are larger in body size, & would be capabe of eating
a large number of eggs/young/adults, thus having a comparable impact on the
population as the aforementioned predators.  Especially since the temperate
birds would be better hidden in various plant material.  So perhaps the
supposed adaptation for anorexia & high energy storage allows for larger
population sizes to accommodate predator saturation.

I would also like to point out that Ross's geese & Snow geese do migrate
into temperate climates during the harsh winters.  Being a bird-watcher I
have had the opportunity to see them as far south as Cincinnati, Ohio. 
They come down, after many of the Canada Geese have migrated south for the
Winter so that competition is lessened.

Just my 2 cents

Casey Tucker
Dept. of Zoology
Miami University
Oxford, Ohio


----------
From: John Bois <jbois@umd5.umd.edu>
To: Dann Pigdon <dannj@alphalink.com.au>
Cc: dinosaur@usc.edu
Subject: Re: Hadrosaur nesting strategy...(was Re: The Life of Birds- Pa
Date: Saturday, July 31, 1999 1:03 AM



On Fri, 30 Jul 1999, Dann Pigdon wrote:

> As far as the arctic geese are concerned, arctic summers are very
> brief so many plants grow and bloom rapidly at about the same
> time, providing a sudden abundance of food that is highly predictable.
> Lack of predation is probably just one of the advantages of nesting
> in a polar or sub-polar region.

This must be true, of course.  However, I argue it is a prime advantage.
here are my reasons.  Some goose species nest in areas where there is
little food (see reference in earlier post).  Indeed, their anorexia is
apparently an adaptation to _low_ or unpredictable food supply available
to vast numbers of birds! Also, if food were a prime concern, and not
predation, one would expect nesting sites to be in more temperate regions
where there is lots of food.  Why then, do they not nest in these places.
The reason for this is straightforward: given the choice between food and
no predators, and food and plenty of predators, the geese have, by and
large, opted for low predator density.  It is very expensive to fly to
these places.  And, in cases where geese have found themselves on islands
without predators--tropical places, they have on occassion lost the
ability to migrate.
This also suggests it is the predators which select for long distance
flight.

I agree that this is a difficult issue and that specific adaptive
"reasons" may be be complex, obscure, and numerous.  I also agree that
different species may have very different reasons for migrating.  But
geese have several things in common which makes migration an imperative.
They are large, ground-laying, noisy and really obvious to those who would
eat them.  In non-nesting times this is OK: they can simply fly to some
other foraging site when a predator comes along.  But in nesting season
they cannot do this without losing their eggs.
In this sense they are like hadrosaurs.  But hadrosaurs can't fly to
remote places.  They and the dinosaurs that would eat them were relatively
homogenous with regard to locomotory ability.  My guess, then, is that a
hadrosaur colony served a very different function than a goose colony.