After a ride I usually hang my helmet on the porch to dry, and the other day after it had been up a couple hours I noticed a little wren buidling a nest in it. I went out and found an old cruddy helmet that I don't use anymore and hung it up.
Watch out for crows, snakes and squirrells. They love bird eggs among a million other animals out there. We had something similar and we they were all gone within a week... The next time we put some chicken wire around it(so only small birds can fit through) and ended up with 2 little babies around for awhile..