Mid-Winter Lipid Reserves of the Golden-Crowned Kinglet

Abstract

Passerine birds have relatively high weight-specific metabolic rates that increase sharply in progressively smaller species (Lasiewski and Dawson 1967). Kendeigh (1970) pointed out that small birds generally are poorly insulated and might be expected to be stressed by low ambient temperature. He suggested, therefore, that energy expenditure may be influential in determining the lower limits of body size of passerines (Kendeigh 1972, but see Kendeigh et al. 1977). High costs of endothermic temperature regulation may be avoided through overnight decreases in body temperature; it appears that nocturnal hypothermia may naturally occur in several species of small passerines (Haftorn 1972, Chaplin 1976, Bucher and Worthington 1982, Lustick et al. 1982). Direct proof is difficult to obtain; hypothermia may occur in small birds in the laboratory because of stress from caging or handling, and may not be a natural part of the daily cycle. Kinglets (Regulus spp.) might be expected to show hypothermia because they are among the smallest Temperate-Zone passerines in the world (see Kendeigh 1972, Chaplin 1982). Two species occur in North America, the Golden-crowned Kinglet (R. satrapa; body weight 5.7 g) and the Ruby-crowned Kinglet (R. calendula; 6.3 g; Kendeigh 1972). These species appear to tolerate cold and may winter as far north as southern Canada and Alaska. To our knowledge, little has been published about the utilization or storage of energy by kinglets (see Gavrilov 1972). Therefore, we undertook to measure the size of lipid depots and the amplitude of daily energy cycles in the Golden-crowned Kinglet. We hypothesized that such a bird might not be able to carry sufficient energy stores to maintain high body temperatures over long, cold, winter nights. If daily cycles of energy storage are not sufficient to fuel overnight survival, this indicates that nocturnal hypothermia must occur if the species is to survive.