Won't a hummingbird die in the cold?
Many western hummingbirds are remarkably tolerant of sub-freezing (and even, for a few days at a time, sub-zero) temperatures. This makes sense, since many of them nest in cold climates, including elevations near timberline in the Rockies or Cascades, or as far north as southcentral Alaska. Rufous hummingbirds may return to their nesting grounds in Alaska or British Columbia as early as April, when the snow still lies deep in the forest. Anna's hummingbirds routinely winter as far north as British Columbia, coming to snow-covered feeders.
One important adaptation these western hummingbirds possess is the ability to go into a deep, hibernation-like torpor on cold nights, as a way of saving energy. The hummingbird will find a perch for the night, fluff its feathers and dramatically lower its body temperature from a normal high of about 102 F to just 54 F. To all appearances, the hummer looks dead, frozen to the branch, but come morning it will begin shivering, rousing itself to normal activity within a few minutes. (Ruby-throated hummingbirds possess this ability only to a degree, although they are hardier than many people assume.)
And while most people assume hummingbirds feed only on flower nectar, a significant majority of their diet year-round consists of insects and other small arthropods, supplemented in cold weather by tree sap lapped from sapsucker wells. Hummingbirds wintering in the East are adept at finding dormant insects, spiders and other small organisms even in cold weather – and on all the but the coldest days, they can usually find tiny, cold-hardy insects like midges and springtails that remain active throughout the winter.
Selasphorus hummingbirds like rufous and Allen's, as well as Anna's and calliope hummingbirds, routinely weather mid-winter snowstorms and bitter cold in the Northeast and mid-Atlantic region, including spells when the thermometer will drop below zero overnight for a few days. Such weather holds obvious risks for these birds – but no more so than for any small, insectivorous bird like kinglets or creepers with a high metabolism and a huge appetite. In 2010-2011, for instance, Pennsylvania's first Anna's hummingbird successfully overwintered in Berks County despite a week when nighttime lows dropped to at least -8F. In Jan. 2014, a banded rufous hummingbird near State College, PA, survived air temperature lows of -9F and wind chills of -36F -- a record for any hummingbird.
Although fall and winter hummingbirds may visit feeders, not all of them do. Nor are the feeders preventing them from migrating – bear in mind, these birds have already migrated thousands of miles from the Northwest. They are no more apt to be "trapped" by the offer of food than a siskin or grosbeak is by a seed feeder. Migration is an overpowering instinct, and unless the hummingbird is ill, or has some injury that prevents it from migrating, it will leave when its instincts tell it to.