Wintering Biology of Ruby-Crowned Kinglets in the Lower Colorado River Valley

Abstract

The Ruby-crowned Kinglet (Regulus calendula) was studied from October-April during four years, 1975-1979, among six major plant communities in the riparian corridor of the lower Colorado River valley. Kinglets were most abundant in cottonwood-willow and least abundant in arrowweed. Within each community, the greatest density of kinglets generally occurred in areas of tall, dense vegetation. Density differed significantly among the four years in November, the month of peak abundance, and in February, winter's end. Populations decreased significantly through the winter in each year and decreases were not uniform among plant communities. Differences in relative abundances and winter population decreases were related to winter weather conditions. During the study period, kinglets were subject to marked seasonality as determined by decreases in average monthly minimum temperatures, changes in tree phenology, and changes in insect abundance and biomass. Kinglet responses to seasonality were most apparent in population dynamics, distribution among plant communities, foraging substrate utilization and choice of prey. Analysis of these data emphasizes the selective importance of the nonbreeding season ecology to kinglets and implies that these birds may be winter-limited.