Into spring and summer birds and squirrels are looking for places to nest. Unfortunately they may chose and opening in your house for their nesting site.There are easy ways to prevent this.Make sure all gaps at the roof line and soffit are filled. Hardware cloth can be purchased at a home improvement store. Cut the hardware cloth to the correct size fold it to a V shape and insert it in the gap.If your house has gable vents check to see that the screening is in tack. I if it needs replacing it can be done from inside the attic for some people this means hiring a handyman.