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> Thus it has been speculated that birds and bats coexist because they
> have, by and large, partitioned nocturnal and diurnal niches between them
> (yes, yes, I know there are nocturnal birds)

> Ronald I. Orenstein  

But - did you know that there are diurnal bats? OK, everyone knows that
some megabats are pretty day-active, but there are diurnal microbats

That bats first evolved as nocturnal animals seems logical, given the
lifestyles and adaptive 'style' of small mammals, but it was argued
recently that bats first took to the skies in the daytime, during Eocene
times. Of course, predatory birds were taking off in a big way at this
time too (there were predatory rollers as well as owls and hawks), and made
short work of the flying mammals. A tendency in bat evolution may
therefore have been to become progressively more nocturnal. Contra 
this are the reduced visual abilities in microbats, and the specialized 
echolocatory skills, throwing the features of megabats into the equation
just confuses everything (read on), but behavioural evidence in extant 
microbats endorses the view (it is alleged). Some modern island in the  
Mediterranean lack birds of prey (nothing to do with the Maltese, BTW) 
and here the microbats are diurnal! Of course, raptors have now evolved 
the accipitrid BAT-HAWK.. a crepuscular predator of the little things.. poor
mammals just get beaten by dinosaurs every time..

I maintain that the monophyly of Chiroptera remains controversial,
though the cranial synapomorphies are good 'weighty' arguments. I'll
explain when I have more time.. 

The origin of bats is probably more 'obvious' than in birds and
pterosaurs, as known arboreal mammals are good bat ancestors. Crude
echolocatory skills are seemingly primitive in mammals, and a
quadrupedal branch-leaping archontran listening out for big moths is
reckoned to be such a suitable ancestor. 'Quadrupedal branch-leapers',
they're called.. QBL.. I can just see the Dinosaur Conventions 10 years
from now:

"So, were you ever into BCF?"
"Oh no, we were always BAMMers"
"Aw gee, that's just BADD"
"Hey, it's BB! Outside - now!"
Fist-fight ensues..

Check out the (continually evolving) Rolf Harris web page!!