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Responding to Changes
In fall, there is an internal clock that tells hummingbirds when to fly south. The drive to migrate is caused by hormonal changes triggered by changing day length. You don't have to worry that keeping your feeders up will encourage the birds to hang around too long, because the shorter hours of daylight are their cue to fuel up and head south. Hummingbirds leave even when flowers are still in bloom and feeders are full. They instinctively know when it's time to go. Keep your feeders up for two to three weeks after you see your last hummer for the season. Stragglers and late-fledging juveniles who begin flying south after others still need nectar to fuel their long journey.

Dick Daniels
Hummingbird: Responding to Change