When they can’t get off the ground.
To survive their epic migrations, Rufous Hummingbirds may double their body weight by converting energy-rich sugar into energy-rich fat.
Seriously, a wild, free-living hummingbird can’t get too fat. Fat is fuel for migration, and they pack on the grams as necessary to prepare for travel and shed them just as quickly when the journey is over.
Life, Birds, and Everything 