Re: Brontornis

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

On Tue, 7 Mar 2000, Larry Febo wrote:

> What would be more advantageous against mamalian predators speed or size?
> Probably both, speed at a smaller size range, and sheer mass in the larger
> forms. Both would have their advantages in open areas where hiding was not
> an option. ...

The tactics of mammalian and bird, nest and hatchling predators apparently
make hiding essential.  Ostriches which may easily drive mammalian
predators (jackals, anyway) during the day, are easily harried off their
nest at night.  So here is the dilemma for big open-field egg
layers: bigness helps them defend against predators of adults, it doesn't
seem to protect against predators of immature forms.  On the other hand,
bigness makes hiding difficult--hiding, at least, for a 3 month stretch in
one location--the nest.  Speed is valuable out of reproductive season but
it is useless for an incubating adult (except to save his own
feathers).  These apparent reproductive imperatives may explain the
current distribution of our biggest birds, i.e., they all seem to be
limited by predation.

I believe the rise of the grasslands explains the rise of big birds in the
fossil record--not that they didn't exist at other times.  That
is, savanah grasslands are an ideal breeding ground because they provide
cover and productivity at low predator density.