Hummingbirds migrate by instinct. Fewer hours of daylight trigger hormonal changes that cause the urge to fuel up and fly south. Your backyard hummers will leave while nectar-rich flowers are still in bloom and feeders are full.

• Fueled by nectar, hummingbirds double their weight as they prepare to fly hundreds or even thousands of miles. Watch your hummingbirds go on a feeding frenzy before leaving their breeding grounds.