I want the world to know that the standard 1:4 ratio (by volume) of sugar to water in syrup for hummingbirds is 1:5 by weight, and weight is what counts in everything but booze.
And if hummingbirds are still coming to the feeder, keep on filling it. They know when to migrate (if they do -- some don't), they don't need an empty feeder to tell them.
Also, let the damn syrup cool before filling the feeder, unless shattered feeders and syrup on the floor are your idea of fun.
@ImagineThat I sidestepped the issue by planting flowers the hummingbirds like instead. The flowers have their season, and by the time they're done, the birds have generally started to migrate, anyway. I may look into some that bloom a bit earlier and later than the current ones to keep the feeding season going longer, though...
@HaikuHedgehog Around here (South Puget Sound), in fact all down the US west coast, Anna's are resident year-round. I don't know how they do it. I know they go into an energy-conserving torpor when necessary, but it's been below freezing most nights and some days this week, and torpor doesn't seem like much.