Yesterday a swarm of swallows were circling the house. 20 to 30 birds were swooping all around. I've had a serious problem with these birds in the past. They like to make mud nests up in the peak of the roof under the eaves. I'd seem them flying around outside and upon closer inspection I'd find 10 of them constructing a colony of mud nests. I'd hose the nests down with the jet spray and by the very next day they'd be back at it. They are stubborn birds.
Here's a pic of a mud nest I found on the internet.
The nets cost about $120.00 to have the exterminator put them up for us.
After about 2 weeks of this I finally called in a professional. The exterminator told me that the roof peak was ideal for the birds and unless we did something this would always be a battle. So we ended up attaching netting from the wood eave to the stucco. It changed the shape of the space and didn't provide the solid third wall the birds required for nest construction. It was an immediate success. While each year the birds come back and circle the house as they did yesterday, they move on because they can't find a good place to build.