While many people are visiting Rocky Mountain National Park during the spring and summer to enjoy the majesty of mountains, wildflowers, and wildlife, broad-tailed hummingbirds (Selasphorus platycercus) are visiting the park to breed. The breeding season for these western hummingbirds begins shortly after their arrival from the Mexican highlands in April and lasts for about two months.
Females build the nests on the horizontal limbs of aspen, cottonwood, willow, and coniferous trees along streams and rivers near meadows and forested areas. The nests are a diminutive two inches in diameter and seldom occur higher than 21 feet above the ground or running water. These luxuriant miniature nests are constructed of soft and silky materials such as thistle down, hair, and feathers woven together with spider webs and caterpillar silk. The outside of each nest is decorated with lichen, bark, leaves, and other plant materials, rendering the nest nearly invisible to the casual observer.
Having mated with one of the many assertive males in her area, a female needs between one and three days to lay two white eggs measuring about .5 x .35 inches. Two weeks after the eggs are laid, naked chicks emerge and begin demanding food. At about age 18 days the birds begin to practice flying, leaving the nest around three days later.
Shortly after the youngsters have fledged, the broad-tailed hummingbirds head up toward treeline and alpine meadows where nectar-producing flowers are still blooming in profusion. The young birds have a short amount of time to learn their life skills before they migrate back to Mexico in September.