The other day I was birding at Theler Wetlands in Belfair (you ought to try it),
and was watching Barn and Cliff Swallows gathering mud. The difference in
behavior was striking: while the Barns tended to work alone, calmly, with wings
folded, the Cliffs tended to gather in groups of six or more, constantly held
their wings almost vertically, and "quivered" their wings strongly. The overall
effect was a large mass of motion. In those cases where a Cliff was gathering
mud alone, he also quivered his wings.

I mentioned this behavior in the Birding forum on CompuServe (EARTH/BIRDING) and
received a reply which mentioned that Bent received communications from a
Canadian that the purpose of the behavior was to keep the wings clean from mud
and to also help prevent the birds' sinking into the mud.

I don't have access to the prior messages by file, but will include, below, my
response to that message.

Does anybody here have any information about the "quivering" behavior????

Thanks... Jerry Blinn -------->

***** --- my response to last CompuServe message ----*****

Well, I wonder about the explanation passed on to Bent. The wing quivering
would certainly apply no upward force to the birds - in fact, I'd consider it
neutral, aerodynamically. Thus there would be no force to "keep them from
sinking ino the mud." As for keeping their wings clean, in this particular case
there was no need - things were not that juicy - which doesn't rule out that
modality, though. The Barn Swallows seemed to be frequenting similar mud in the
same mud hole, and they kept their wings folded.

The mud collection seemed to have more social content with the Cliffs. While
occasionally one bird would stand off alone collecting (and quivering), the
more likely scenario was a group of six or more in one quivering mass of
mud-collecting animation. Now, I don't know if they stayed in the group because
that particular mud area had the most desirable content, or if they just liked
to do this thing together. Maybe the loner Cliffs just didn't know good mud.

While I'm sure there will be a more sensible, and probably obvious in
hindsight, explanation, I still have this feeling that the behavior is either a
unique social thing, or that the quivering mass presents a less likely target
for predators when the birds are in this particularly exposed condition.

But then, why don't barn swallows do it? Well, Barn Swallows are not nearly as
gregarious as Cliff Swallows, thus they would be unlikely to develop protective
behavior that relies on group participation. The fact that there were several
Barn Swallows at the mud hole seemed to be simply because there was mud there -
but with the Cliff Swallows it seemed to be more a group thing. Also, there
were Barn Swallows at other mud sources, but the Cliffs stayed with this one
mud hole - their picnic spot.

I also have this feeling that maybe the wing posture and quivering prepares the
birds for quicker escape from a predator. Maybe Cliff Swallows are particularly
slow off the mark compared to other swallows, and they developed this behavior
to quicken their escape. (Like some nervous base runners leading off first)

Jerry


E-mail from: Jerry Blinn, 01-Jun-1996