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Re: Nocturnal crocs?




On Sun, 26 Aug 2001, Michael Bruce Habib wrote:

> I don't see how your mammal predation model produces 
> selective bird extinction, either.

It doesn't.  My prediction is that that neornithine birds outcompeted (or
preyed) enantiornithines; and that this has nothing to do with the
boundary.  Indeed, I would argue that recent findings in Antarctica
support that position (i.e., old birds were already absent from large
areas of the planet--though I recognize other scenarios are possible).

> Tree and cliff nesting does not 
> stop egg predation. In fact, all oviparous species today, 
> be they crocs, birds, lizards, snakes, or turtles, suffer 
> significant amounts of nest predation.  These are 
> acceptable losses.  Predation of nests does not, and 
> probably did not, drive species rapidly into extinction.

Nesting sites are selected in order to _reduce_ predation.  When predation
load on such sites increases, losses become "unacceptable".  Such has been
the case with increased predation in the Pothole region of Canadian
Prairie, on Arctic islands opened up to foxes by trappers (these have
reduced goose populations to point of collapse--now recovering due to
extirpation of predators).

> There are, of course, a number of mechanisms utilized by 
> modern egg-layers to defend nests (otherwise losses would 
> be too high). These include concealment and burial, active 
> nest defense, 

Except for crocs, and birds on islands who only have to face similar-sized
birds, active nest defense is never the prime strategy: it's concealment,
then defense upon discovery.  But, many of these species are "aware" of
the likelihood of winning such a defensive action--and they will abandon
or fight on the basis of some decision.

> I honestly do not see there being a 
> correlation with nocturnal habits and nest survival.

My argument is limited to only one oviparous class: a parent who, like
most/many crocs uses active defense as its _prime_ strategy (in other
words, it doesn't even attempt to conceal and is therefore constantly
discovered).  In such a case, the ability of the predator to see at night
is a significant advantage over a parent who cannot.

> I find it 
> hard to believe that mammal nest predation compares much 
> to egg losses due to fungal and bacterial infections, or 
> that of insects.

I agree that these sources of loss arre significant.  I argue
_summation_, increased predation load to unacceptable levels.  As
evidence I cite a) extant big egg layers which suffer 90% to 95% losses
(ratites) and which remain in the black curtesy of a concealing medium not
available to similarly-sized non-avians; b) inability of larger species to
become viable other than on islands with low predator density.  But I'm
repeating myself--I appreciate the list's tolerance.  And will desist
unless novel criticisms appear.