Date of this Version
That birds are homeotherms is demonstrated by their ability to maintain a relatively constant body temperature over a wide range of ambient temperatures. An important com- ponent of this ability to maintain a constant body temperature in a cold environment is the feather layer. Birds have been shown to acclimate to cold by increasing their plum- age 20-30 percent and, hence, increasing their insulation (Kendeigh, 1934; Scholander, et al., 1950; West, 1962). Hutchinson (1954) states that oil secreted by uropygial (preen) glands maintains the ability of the feathers to shed water and, therefore, aids in maintaining the insulating layer. Since feathers are so important for minimizing heat loss, anything that will decrease their insulative efficiency will increase the thermal conductance and the lower critical temperature. Baldwin and Kendeigh (1932) clipped feathers off of Eastern House Wrens (Troglodytes aedon aedon) and observed a decrease in body temperature. Brush (1965) obtained similar results in the California Quail (Lophortyx californicus), thus indicat- ing the necessity of feathers for minimizing thermal conductance and maintaining a con- stant body temperature. Kessler, et al. (1968) observed mass deaths of Cowbirds (Molothrus ater ater), Bronzed Grackles (Quiscalus quiscula aeneus), and Starlings (sturnus vulgaris) after a driving rainstorm with concomitant low ambient temperatures (10°C). Under similar circum- stances Jogi (1968) reported mass deaths of swallows (Riparia riparia, Delichon urbica, and Hirundo rustica) in Estonia. Apparently the driving rain had destroyed the insulat- ing quality of feathers, and with low ambient temperatures the birds succumbed to hypo- thermia. Caslick and Stowers (1967) and Stickley, et al. (1971) used a surfactant to remove the preen gland oil from the Red-winged Blackbird (Agelaius phoeniceus phoeniceus) and exposed them to low ambient temperatures and wetting, which resulted in a large number of deaths. The surfactant reduced the surface tension of the preen oil, allowing water to penetrate through the feathers to the skin, thus destroying the insulative quality of the feathers. More recently this technique of wetting the bird with surfactant has been used as a means of controlling blackbird populations.