At the end of summer and beginning of fall, Ruby-throated Hummingbirds start migrating south toward their wintering grounds. Adult males are the first to leave. Adult females are the next, followed shortly by juveniles. Some males leave as early as July, but most hummingbirds depart towards the end of August and beginning of September.
By leaving earlier than the females and immature birds, the males won't be competing for fading fall blossoms and other food sources. Newly-fledged birds need the time to mature and gain fat reserves before flying off on their first migration south. More food will also be available along the migration trail when hummers leave at different times.
The number of ruby-throated hummingbirds migrating south in the fall may be as much as twice the number that return in the spring, as it includes all the immature birds that hatched out this past summer.
Wikimedia: Jeffrey W. of Illinois
