We once had a swallow in a building. Basically, we darkened the building and opened one door. This is only good during the day, as they would expect it to be dark at night.
Birds do not like a dark place, and usually will fly to the light to get out. This worked for me in the past in another building when a starling got in.
The higher the exit the better. If you have a rollup door that is very tall, that is good as birds try to stay high.
"...everything can be taken from a man but one thing: the last of the human freedoms--to choose one's attitude in any given set of circumstances, to choose one's own way." Viktor E. Frankl
"Life's like a movie, write your own ending." The Muppets