For 5 years or more we have went to Rio Lindo Academy to watch the Vaux Swifts. Vaux Swifts flock to a chimney located on the Academy's property. This year on her visit it was estimated by my birder husband that there were about 6,000 little birds whirling into the chimney. They fly in an hoover in they sky until the timing is right and all of sudden it begins the decent into the chimney. It looks like a funnel as they drop into the chimney. They do this through September and into October, the birds migrate to warmer locations.