Expert: Roger Lederer - 6/29/2010

Question
Hi -- I just read your article about swallows and the migratory bird act.  I live in a modern one story Spanish style house outside of Los Angeles, CA.  The house has a classic Spanish-style red tile roof.  While hosing off the front of my house I noticed what appear to be large mounds of mud on my roof.  One is sitting on the peak of the roof of my attached garage (About 20 feet high) and I just found a tiny dead baby bird on the concrete driveway in front of the garage, immediately below what appears to be a mud nest.  

Then I noticed what may be 3 more mud nests stuffed into the end of a few rows of the rounded red tiles, on the same side of the house (facing west) but about 20-30 feet away from the nest on the garage.

If these are nests, they do not appear to be small nests, but rather each one appears to be closer to the size of a football!
I often find a lot of bird droppings on the pavement near these possible nests. I could not identify the bird from the tiny corpse, as it was hard and rotted.  Very small, with brown and gray feathers.  

Q -- Are swallows the only type of birds which build these round, mounded mud nests?

Q -- How long will the nests remain there if they are not removed?  I hope that the 3 which are stuffed into the end of the row of red pipe/style roof tiles, will not block drainage of water or rain off of the roof.

I have sometimes seen neighborhood cats sitting up on my roof as it is hot and sunny up there, with no trees or nearby buildings shading the roof.

Thank you for any advice and info you can offer me about these possible nests and these birds.

Answer
I can't be sure, but I think what you are seeing are wasp nests. Cliff Swallows and Black Phoebes build mud nests but they are no larger than about the size of a canteloupe; certainly not the size of a football. Maybe there is a nest or two that belong to swallows considering you found a dead bird, but the larger ones are most likely wasp nest. Send me a photo or have someone else confirm before you wash them off.