Spring 2004 Migration of Ruby-throated Hummingbirds

Spring 2004 Migration of Ruby-throated Hummingbirds
As with most migratory birds, hummingbirds apparently evolved to their present forms during the last ice age. They were (and largely still are) tropical birds, but as the great ice sheets retreated from North America, they gradually expanded their ranges to exploit rich temperate food resources and nesting space, filling unoccupied niches in the U.S and southern Canada while evading intense competition in the tropics. Some songbird species have adapted completely to our variable North American climates, in part by becoming vegetarians in winter, and don't migrate. But hummingbirds are carnivores (nectar is just the fuel to power their flycatching activity), and depend on insects that are not available in subfreezing weather, so most of them must retreat back "home" to Central America in the winter or risk starvation. A few Ruby-throated remain along the Gulf coast each winter instead of continuing to Central America, perhaps because they are too old or sick to make another trans-Gulf flight or too young (from very late nests) to have had time to grow fat and strong enough to migrate; their survival chances depend on the severity of each particular winter, and many perish in unusually cold years. Another small population winters in the Outer Banks of North Carolina.