The Ruby-crowned Kinglet, one of North America's smallest songbirds, has a loud, complex song and, with up to 12 eggs, lays the largest clutch of any North American passerine for its size. Males and females are nearly identical in plumage -- olive green-gray on the upperparts with two strong white wing-bars and a broken, white eye-ring. The male has a scarlet crown patch, which is usually concealed unless agitated.
During migration and winter, the Ruby-crowned Kinglet occupies a variety of habitats and is often recognized by its constant wing-flicking. As a breeder, it inhabits spruce-fir forests of the northern and mountainous western United States and Canada. Its nest is hidden, often near the trunk and up to 30 meters above the ground, making reproductive data difficult to gather. Much remains to be learned about the breeding biology and behavior of this species.
This kinglet breeds farther north and winters farther south than the congeneric Golden-crowned Kinglet (Regulus satrapa), and the northern boundary of its winter range appears to be influenced by temperature. Its breadth of habitat choices during migration and the nonbreeding season has allowed the species to escape the influences of most human disturbances, although a resident subspecies on Isla Guadalupe off Baja California is extinct, and western populations may be affected by logging activities.
The systematic position of kinglets within the oscine passerines has been controversial. Traditionally they have been allied with Sylviidae, particularly Phylloscopus warblers. Recent molecular studies place kinglets as a sister taxon to tits and Old World warblers ( Sheldon and Gill 1996 , Sturmbauer et al. 1998 , Päckert et al. 2003 ). In addition, molecular data ( Ingold et al. 1988b , Päckert et al. 2003 ) also confirm suggestions based on morphology ( Clark 1974b ) that Ruby-crowned and Golden-crowned kinglets are not particularly closely related within the Regulidae.
Many aspects of Ruby-crowned Kinglet breeding biology remain poorly studied, perhaps because of its remote boreal forest habitats and its placement of nests hidden high in conifers. However, habitat requirements have been relatively well studied, particularly on breeding grounds. In general Ruby-crowned Kinglets occur in highest abundance in old growth conifer or mixed forest habitats ( Kessel 1998 , Drolet et al. 1999 , Hobson and Bayne 2000c , Cumming and Diamond 2002 ). Migration habitat use and differential migration are also well studied: Ruby-crowned Kinglets are habitat generalists during migration ( Carlisle et al. 2004 ). Males migrate earlier than females in spring, apparently at least in part due to earlier initiation of migration, and later in fall ( Fairfield and Shirkoff 1978 , Swanson et al. 1999 ). Among age classes, juvenile birds precede adults during fall migration ( Benson and Winker 2001 , Andres et al. 2005 , Carlisle et al. 2005a ).