[cobirds] Hummingbird feeders

I have always heard that we should take down hummingbird feeders in the Denver area around the end of September or beginning of October. My nephew, who is scientifically trained, met up with a PhD student who claims it's claims it's fine to leave your hummingbird feeders up all winter long. Rather than get into the specifics of the debate we're having, I would appreciate it if any of you have any information that I can forward to him to refute what he heard I from the rather irresponsible the graduate student who is apparently studying hummingbirds. Thanks.
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