Raft vs flock

[lavatera at w-link.net]

Hi all,

A writer friend asked me a question I couldn't answer.  When does one
call a group of birds on the water a raft and when is it a flock?  Is it
a matter of numbers, cohesiveness, or what? Are there any other terms
for groups of water birds?

For the record, my collegiate dicitonary defines raft as "an aggregation
of animals (as waterfowl) resting on the water" and flock as "a group of
birds or mammals assembled or herded together.

Thanks,
Maxine Centala