Re: Hadrosaur nesting (long-winded response)

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

On Thu, 29 Jul 1999, Casey wrote:

> Essentially migration occurs because of a need for food.  In Spring in
> North America food is more readily available so migrating birds travel
> back into the North to begin laying eggs & raising young.  

This discussion is pretty much about geese.  For it to have relevance we
should specify the species.  I didn't see the episode (Life of Birds).  In
any case, to falsify your above statement see: Gloutney et. al. 1999.  
_Use of
supplemental food by breeding Ross's geese and lesser snow geese: evidence
for variable anorexia_  The Auk 116(1):97-108.

Basically, these geese breed independently of food availability.  And, in
any case, productivity in the tundra is poor (snow geese breeding in large
colonies only were able to harvest 1.4 g of forage per hour.
Final sentence: "...adaptations for ample nutrient shortage and anorexia
apparently enable persistence of these species in breeding areas at
immense population sizes."

And, if they're not going for the food, and they are breeding there--it's
the low predator density that is the attraction (if you could get inside a
goose's head).

> Northern climates have quite a few avian predators including, but not
> limited to, Raccoons, Opossum, Colubrid snakes, Owls, Falcons,
> Accipiters, _H. sapiens_, cats, rats, squirrels, chipmunks, Icterids,
> other aves (House Sparrows, Starlings, etc), weasels, & the list goes on
> of the various species that prey upon not only adults, but eggs &
> young.  So predator avoidance is not a good reason to migrate,
> especially since many predators are coming out of hibernation (like
> small mammals & snakes) during the Spring & are ready for the influx of
> avian prey that is returning with the insects & plants.

I think you would find that many of these predators are scarce or absent
from most geese colonies.  Most sites are further North than this.  
Or they are remote in some way (islands etc.).