On Tue, 25 Apr 1995, Dennis Paulson wrote:

>
> I may not have made it clear that I was writing about *nocturnal*
> migration, not just the moving through the landscape that birds do while
> they forage and that is often in the direction of their normal migration.
> It makes all the sense in the world for birds to flock (lots of literature
> on that), but it's hard for me to imagine a tight little flock of one
> species of warbler moving through the sky at 1,000 meters or so. I was
> wondering if Yellow-rumps, which so obviously flock when on the ground, do
> so when in the sky. We have no idea, but I'll bet some of the raw data
> from TV-tower mortality might shed some light.
>

I doubt if the usual TV tower data would really help, at least if the
data is collected daily, as I think it is. Let's say you had the
following kills hit the tower:

April 25: 5 Yellow-rumped warbler, 1 Common Yellowthroat, 6 Hermit thrush

April 26: 55 Yellow rumped Warbler, 3 Common Yellowthroat, 2 Hermit Thrush

Could you distinguish between the two possibilities, that either a flock
of YRWAs hit the tower the second night, or that it was just a good night
for YRWA migration, and lots of single birds were killed? Probably you
would need to be sitting under the tower all night to see who was killed,
minute by minute. Sorry, I didn't realize this would turn so macabre.

Some other ideas: on some islands, there is a visible migrant fallout at
dawn when migration is heaviest. The night-migrating birds find
themselves over water at first light, and, spotting an island (or
peninsula or coast, even), make for it. Since the birds must fly a bit
further, they land later than usual, at a time when there is enough light
to identify them. Perhaps an observer in such a spot would be able to
tell if birds were arriving at random or in single species flocks.

You could also go at it from the other end, i.e. takeoff. Follow one of
these Yellow-rump flocks on an afternoon when departure seems likely
(clear weather and following winds?) until they actually do take off and
start to ascend. See if they do it individually or in flocks.

To me, passerines migrating in flocks seems quite plausible - why should
it really be harder for them than for geese (and yes, geese do migrate at
night)? In particular, I find the reports of calling nocturnal migrants
suggestive. On Long Island (coastal New York), one can count Swainson's
and Hermit thrushes in the hundreds passing over if you know the call
notes, as they call while flying. Why would they call if not to keep
flocks together?

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