Concerning names for flocking birds, here is a link that may be of some
help. There are in fact different names for flocks of different birds. Of
course, flock can work for most everything, but there are more specific
names you can use as well, such as "a siege of herons" which is an
expression I find particularly appropriate and wonderful.

Hope it helps.

http://birding.about.com/hobbies/birding/library/weekly/aa032700a.htm

Jim Lynch
Olympia WA