Formerly breeding almost exclusively in the remote, boreal spruce-fir (Picea-Abies) forests of North America, the diminutive Golden-crowned Kinglet has been expanding its breeding range southward at lower elevations into spruce plantings in Pennsylvania, Illinois, Indiana, and Ohio and into white pine-hemlock (Pinus strobus-Tsuga) forests in eastern Tennessee, northeastern Georgia, and the western Carolinas. Nesting high in conifers, it lays its large clutch (5–11 eggs) in a cup-shaped nest built by both parents. Most pairs produce 2 broods a year, despite the short breeding season at northern latitudes. Much remains to be learned about the breeding biology of this species; only one study (northern Minnesota; Galati and Galati 1985, Galati 1991) has focused on this phase of its life history.
The winter habits of the Golden-crowned Kinglet are better known. It joins mixed-species flocks, wintering throughout its breeding range and south across the United States and into northeastern Mexico, in a variety of habitat types, both coniferous and deciduous. This species tolerates colder temperatures than the Ruby-crowned Kinglet (Regulus calendula) and winters farther north, in areas where nighttime temperatures may fall to below –40°C. Individuals may reduce overnight thermoregulatory costs in winter by huddling in small groups, but apparently do not use torpor.
Phylogeography of kinglets needs study. DNA sequence and bioacoustic data suggest the Golden-crowned Kinglet is a sister taxon to the Goldcrest (Regulus regulus) of Europe and Asia and is not closely related to the other North American Regulus species, Ruby-crowned Kinglet, but evolution of these biogeographic patterns in Regulus is not well understood. The position of Regulus in passerine phylogeny also remains controversial. Traditional analyses place kinglets within the superfamily Sylvioidea, closely allied to Old World leaf warbers (Phylloscopus), but more recent molecular work lacks agreement on this.
This species exhibits a strong affinity for old-growth dense conifer forests during the breeding season, where it is sensitive to logging and other habitat disturbances. This sensitivity may explain population declines in the western portion of the range, where remaining old-growth forest is limited. Populations in eastern and central portions of the range are apparently stable and appear to have benefited from reforestation of spruce in the eastern states.