[accounts of flocking in YRWAs and speculation about whether they migrate
together snipped]

For those interested in the question, here's a tidbit:

Moore, F. R. (1990) Prothonotary Warblers cross the Gulf of Mexico
together. Journal of Field Ornithology 61: 285-

As I recall the article, observations made of warblers arriving at a site
on the U.S. Gulf coast led the observer to conclude that several (3-6?)
Prothonotaries there had migrated from Venezuela (or wherever) together.
The evidence was that the PrWas all had some unusual greenish mud smeared
around their bills and faces. No other migrants arriving that morning
were stained, and the mud staining was apparently not of local (U.S.)
origin, so the observer concluded that the PrWas had foraged together,
migrated together, and arrived together. I may be corrupting some details
- I read it when it came out, which was quite a while ago. As I
recall, the account was more convincing than my summary here. Dennis, do you
have it on the bookshelf? The article should mention if there is other
evidence for passerines migrating at night in conspecific flocks.

Chris Hill
Seattle, WA
cehill at u.washington.edu