In the fall there is an instinctual clock that tells the hummingbirds when to head south. People still disagree over the precise mechanism within the bird that causes this. Most sources say that that food supply is not a factor and there is no reason to take down hummingbird feeders to stimulate migration. Birds that are born late in the season are vulnerable. Leaving your feeders up may provide a critical opportunity for these hummingbirds to build reserves and “catch up.”
Just before they answer the call to travel south, they eat in excess and build a layer of rich fatty fuel just under their skin. You can notice the extra fat along the back, belly, and throat. A hummingbird gains 25 – 40% extra body-weight to have enough fuel to travel 1,400 miles – with no wind of any kind. A headwind of only 10 miles per hour will cut that distance down to 600 miles and more than 20 mph will push them backward. However the ruby-throated hummingbird does take advantage of tail winds constantly. Southbound ruby-throats rebuild their reserves in the early morning, travel about 23 miles during the day and forage again in the late afternoon to keep up their body weight.