Ed (tweetsters),

You asked for advice on whether or not to knock down a used swallow's nest.

For many years Barn Swallows (Hirundo rustica) have nested on wires
supporting the awning of the Latona Tavern, here in the north end of
Seattle. Early-on the owner would knock down the nest each year because he
didn't like the building materials spilt at the tavern entrance. Finally,
one year, he asked me what he could do to reduce the spillage. I suggested
he leave the nest up, so the birds wouldn't have to build (and spill) each
year.
For years now, the swallows have happily raised young in the same nest each
year and the debris coming down has been reduced to some feces from the
young just around fledging time. Most people love the birds and are willing
to take the occasional splattering in stride.
You can tell the regular customers from the occasional ones. The regulars
always look up when entering/leaving the place ;)

My suggestion: leave the nest.

Sincerely,
Martin Muller, Seattle