We had some in the vent for out bathroom last spring and I actually did what Hanni suggested. A couple cans of compressed air got the birds out and the chicks near the edge where I grabbed them and moved them. I kind of wanted to let them move out on their own but when examining the vent I found a dead one so the eviction notice was posted and they lost their damage deposit.
I used one of those wire picker things that mechanics use to pick up nuts and bolts that fall into tight spots to pick out all the nesting.
Once that was all cleaned out I put one of these over it from Home Depot.
