I was just in Portland and went with my son and niece to watch the Vaux Swifts funnel into a chimney at Chapman Elementary School. There must have been over 50,000 swifts flying in formation, diving at the chimney, and then flying off again for 2 hours. It was a beautiful dance where the audience (thousands of people) would ooh and ahh as the birds got closer to the chimney and then flew off again.
Then they somehow knew it was time to swirl and funnel into the chimney.