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Re: Avian extinction ref.



On Wed, 6 Oct 2004, MICHAEL HABIB wrote:

> I am also curious what continental strategies you are thinking may have
> disappeared.  In in particular in mind?  Flightlessness is not nearly so
> prevalent on the mainlands to begin with (the penguins notwithstanding)
> so I am not certain what strategies would "disappear" on the mainland.

I am guessing that flightlessness was not the most predisposing factor to
oceanic island extinction--I don't mean that flightless species weren't
vulnerable, just that most birds that became extinct also flew.

> Slow life histories might be less prevalent.

Yes.  The luxury of developing a better developed baby (and I don't mean
precocial)--i.e., bigger eggs, fewer eggs, longer incubation times--is
more affordable on predator-free islands.  In fact, I think T.E. Martin
has done a study comparing N hemisphere vs. S. hemisphere birds.  S. H.
birds are less preyed upon and have the above luxuries.  So, I would
suspect island birds would be even further along that continuum, whereas
the more relaxed species would have been eliminated from the main
continents.

> Ground nesting might be in a few places, but overall ground nesters are
widespread on both islands and mainlands

OK...but on mainlands there are many more restrictions on species that
ground-nest.  I have argued here before that size is a limit on continents
because it allows discovery...and so this body plan is only workable in
few mainland biomes--e.g., areas of low predator density ("islands" of low
predator density) such as the ostrich in the arid savanna.

> Ground nesting appears comparatively rare on mainlands because of the
> predominance of arboreal passerines, which do not regularly reach
oceanic islands).

In my view, it is their stealthy nature that has allowed them to flourish.
Why do they not make it to islands?  Size?  Conservative dispersal/nesting
behavior?  Thinking aloud: the same thing that keeps them off the menu on
the continents also keeps them from long distance/sweepstakes dispersal.


> The effect shouldn't be limited to mammals preying on birds, either.
> Other birds would also be limiting, as would some squamate reptiles.

Yes...except that fewer birds were stowaways/pets...and where snakes made
landfall the results were as you suspect, e.g., _Boiga boiga_.

> In fact, I think it would be quite interesting if someone looked into
>  similar impacts on island faunas with introduced birds.

Tag team.

> And again, it is not only a matter of different strategies (such as
> flightlessness).  The confinement of geographic range size is very
> important.  Species with small geographic ranges appear to die much
> more readil y than those with large geographic ranges.

Yes...more places to hide, more niches to slip into...although, if
predators have a continental range this should still be limiting on
specific strategies.

> Definitely a lot of things to think about, and several which presumably
> relate to Mesozoic extinctions.

These effects should be more important in the Mesozoic for two reasons:
many egg-laying taxa were unable to conceal their eggs; and (in
general) the larger an egg-laying adult, the smaller the egg/hatchling is
in relation to the adult.