|Species:|| Regulus calendula|
Adults have greenish grey upperparts and whitish underparts, a teardrop shaped white eyering, slightly broken at the top and bottom, and olive lores. Males have a ruby crown patch, while females haven't. Black at base of secondaries, contrasting with white lower wing bar. Immatures have pointier tail feathers than adults, a few immature males have orange, yellow, or olive crown patch. 
Ruby-crowned Kinglets seem nervous as they flit through the foliage, flicking their wings nearly constantly.
Females build their nests in five days. They usually build them in trees, near the trunk or suspended from small branches. Nests are made of cobwebs, grass, moss and feathers. When completed, the globe-shaped nest is 4 inches wide and 5-6 inches deep, and requires regular maintenance to keep it from disintegrating. Inside, it's about 3 inches across and 2 inches deep. The nest is elastic enough that it can stretch as the brood grows. Breeding pairs stay together for two months, until their chicks fledge (after about 16 days). 
Ruby-Crowned kinglets feed on small beetles, flies, leafhoppers, true bugs, caterpillars and spiders. They forage at all levels from treetops to low brush. 
The ruby-crowned kinglet's vocalizations are remarkably loud and complex for its size. Its song can be divided into three main parts: a series of high pitched notes (zee-zee-zee or tee-tee-tee), two to five low trills (turr or tu), and a repeated three note "galloping" phrase (tee-da-leet, tee-da-leet). However, there is variation in the songs of a given individual, and they often contain only one or two of the three parts. The third part is only sung by male birds; an abbreviated version is heard from the females. Other vocalizations of the ruby-crowned kinglet include alarm calls, simple contact calls, and begging calls produced by chicks.