The Fate of Hummingbirds That Stay Behind in Winter

It is 18 degrees at dawn in December and a hummingbird hovers at a feeder rimmed with frost. These birds should be 2,000 miles south, but thousands appear every winter across northern states, juveniles mostly, knocked off course by navigation errors in their first migration. 📖 The book I mention in the video — The Birds You're Missing: bill.myreads.store A bird built for 90-degree days now faces nights cold enough to kill it in minutes. A 3-gram hummingbird burns energy 10 times faster than an Olympic sprinter, adjusted for mass. It needs half its body weight in sugar daily, feeding every 10 to 15 minutes. Winter offers no natural nectar and no insects. At 20 degrees Fahrenheit the math says it should freeze or starve by morning. But every night the bird drops into torpor, heart rate falling from 1,200 beats per minute to 50, body temperature plunging 30 to 40 degrees, metabolism slowing to one fifteenth of normal. A 12-hour night costs less than 1 calorie instead of 8. Within weeks of cold exposure they grow denser plumage, 15 to 25 percent thicker, and store subcutaneous fat most hummingbirds never develop. Not all species survive. Ruby-throated hummingbirds see winter survival below 10 percent without feeders. Rufous and Anna's hummingbirds, evolved in climates with cold nights, hit 66 to 67 percent survival with feeder support. Banded individuals return to the same winter territories for years. Reports of winter hummingbirds in northern states are up 340 percent since 2000. What looked like navigation mistakes 30 years ago might be the leading edge of a range expansion.