We have left a bag hanging on our mailbox because a tree swallow has decided to nest inside. We have a small hole one side of the mailbox, and thus it makes a nice protective home to lay her eggs.
Sorry the picture of the eggs inside is not clearer. The eggs are each just 1/2 inch long and white. She laid one egg each day over three days.
Now to learn how long before they hatch? Hmmm... Now to look that up.