Oxygen consumption during hover-feeding in free-ranging Anna hummingbirds.

Rates of oxygen consumption during hover-feeding of wild, unrestrained, adult male Anna hummingbirds (Calypte anna) were measured with an artificial outdoor feeder converted into a respirometer mask. A computer sampled changes in O2 concentration in air drawn through the mask, automatically detecting the presence of a hummingbird from a drop in the O2 concentration, and photoelectrically timing the duration over which the feeder functioned as a mask. Birds coming to the feeder were weighed on a trapeze perch suspended from a force transducer. Feeding bouts consisted of sallies which carried the head in and out of the feeding mask about once a second. The volume of O2 consumed per feeding sally was linearly related to the length of the sally. The energy cost of hover-feeding in five hummingbirds, mean mass 4.6 g, was 41.5 +/- 6.3 ml O2 g-1 h-1.