Nice article in the Post today about chimney swifts.  Both it and the  
Cornell Lab page state that there is only one nesting pair per chimney.
 
allaboutbirds- 
The Chimney Swift is gregarious, with large numbers of swifts roosting  
together in a single chimney or air shaft during the nonbreeding season.  Nonbr
eeding swifts will roost together in the summer too, and this behavior has  
fooled people into thinking that the Chimney Swift nests in colonies. In 
fact,  only one pair nests in a single chimney. The pair may tolerate other 
swifts  roosting in their chimney, though, further confusing people watching 
the swifts  from the ground.
 
Doesn't seem there would be enough chimneys to support the  population.
 
 
 
Tom  Beal
Glenn Dale, MD PG Co
(just west of  Bowie)