To be more specific, change the feeder syrup (and clean the feeder) when it starts getting cloudy. That's bacteria growing. The birds carry it in from the flowers they visit, so there's no way to prevent or delay it. Boiling won't help. Also clean the feeder, inside and out, at any sign of black mold.
And remember, you don't have to take feeders down to persuade the hummingbirds to migrate. Some have been migrating since early July. The instinct to head south is much stronger than hunger. I leave at least one or two up until freezing becomes a problem.
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