Factors in the Energy Budget of Mountain Hummingbirds

Abstract

The abundant flowers of a compressed summer in the mountains provide an energy resource for exploitation by hummingbirds. However, the cold nights constitute a liability in their total energy budget. Adiabatic cooling, clear dry air, and the heat sink of the cold sky result in chilling conditions for the incubation of eggs by a tiny hummingbird. Howell and Dawson (1954) recorded the ability of the Anna’s hummingbird (Archilochus anna) to maintain homeothermy while incubating overnight. I was greatly impressed the first time that I saw an even smaller calliope hummingbird (Stellula calliope) incubating her eggs in the presunrise cold of Jackson Hole, Wyoming. In a later visit I was equipped to record temperatures from two calliope nests. They also maintained homeothermy all night, despite the colder climate (Calder, 1971), stimulating my interest in heat-exchange principles and problems. It was obvious that ornithologists had given little consideration to physical factors in bird behavior, and that further study of hummingbird nesting would be rewarding. The population of broad-tailed hummingbirds (Selasphorus platycercus) at Gothic, Colorado, has been ideal for this. Evidence of the marginal energetic situation for this population may be seen in the occasional recourse to hypothermia during incubation (Calder and Booser, 1973) and the abandonment of live, normal chicks at some late nests when the flower supply declines, simultaneous with influx of competing migrant hummingbirds, in late July and early August (Calder, 1973d). Thus energy conservation in thermoregulation is of major importance.