Migration
They are migratory, many of them migrating through the Rocky Mountains and nearby lowlands in July and August to take advantage of the wildflower season there. They may stay in one spot for considerable time, in which case the migrants, like breeding birds, often aggressively take over and defend feeding locations. Most winter in wooded areas in the Mexico state of Guerrero, traveling over 2,000 miles by an overland route from its nearest summer home—a prodigious journey for a bird weighing only three or four grams.
This is the western hummingbird most likely to stray into eastern North America. In the United States, there has been an increasing trend for them to migrate southeast to winter in warmer climates like Florida or on the Gulf Coast, rather than in Mexico. (They do arrive at the Turks and Caicos Islands.) This trend is the result of increased survival with the provision of artificial feeders in gardens. In the past, individuals that migrated eastward toward Canada and the northern USA in error would usually die, but now they often survive as they seem to spend more time in the warm Gulf Coast and Florida. Provided sufficient food and shelter is available, they are surprisingly hardy, able to tolerate temperatures down to at least -20°C, so they can be seen in late fall in places like the Upper Midwest, Great Lakes, and upper New England. As winter comes birds in these areas normally head to the warmer Gulf coast and Florida.
Most hummingbirds that migrate east are juvenile birds and may occasionally be adult females but are very seldomly adult males. Since juvenile or female are essentially indistinguishable from Allen's Hummingbirds unless they are examined in hand, many of the eastern vagrants are classified as "Rufous/Allen's Hummingbird". However, a majority are believed to be from the Rufous species.
Read more about this topic: Rufous Hummingbird