bird convention

There was a bird convention near our house. This picture doesn't show it well, but I'm sure there were millions of birds, flying in hoards from one end of the field to the other over and over. Occasionally a few of them would pop up to the power lines and take a break. Their warble filled the air. The convention happened for about a week across from the girl's daycare, and I puzzled over it every day when I drove by to pick them up. What do you suppose they were convening about?