Here are my two cents concerning hummingbird food preparation,

In Tennessee where I grew up mildew tends to build up very quickly in
hummingbird feeders. In the hottest summer months the feeders have to
be cleaned out sometimes twice a week.

My father claims that boiling the water will often give him a few
extra days before the feeder needs to be scrubbed out.

He also simmers the water after boiling for 10 min to help evaporate
the chlorine from the water more quickly so that the hummingbirds
aren't subject to the chlorine levels we normally consume from tap
water (whether or not they notice the chlorine or are harmed in any
way by it is unknown to me). I know chlorine will evaporate from
standing water over the course of several hours or that the process
can be accelerated with heat.

Whether or not boiling is necessary, it at least keeps things more
sterile and less chlorinated. Those are the only reasons I can think
of as to why boiling might be desirable.

Nathaniel Peters
Seattle, WA
ncpeters at u.washington.edu