A COMPARISON OF METABOLISM AND EVAPORATIVE WATER LOSS OF BALTIMORE AND BULLOCK ORIOLES

1. 1. Both Baltimore and Bullock orioles use about 2·4 ml O2/g per hr at thermoneutrality; this value is approximately 14 per cent lower than the 2·8 value predictable on the basis on mass alone. 2. 2. At 44·0°C, the highes Ta used in this study, Baltimore orioles used significantly more O2 than did Bullock orioles. 3. 3. The rate of evaporative water loss increased only gradually with an increase in Ta at lower Tas, but increased markedly as Ta approached Tb; at 44°C, approximately 100 per cent of the heat produced by metabolic activity was dissipated by evaporative water loss. 4. 4. Both forms showed an increase in Tb at higher Tas; at moderate Tas, the Tb of Bullock orioles is somewhat lower than that of Baltimore orioles, but at 44°C, Bullock orioles appeared to be higher; 25 per cent of the Baltimore orioles tested died at 44°C whereas none of the Bullock orioles died at this Ta. 5. 5. The physiological responses of these two orioles correlate well with their ecological preferences. Where their ranges come into contact, Bullock orioles distributionally replace Baltimore orioles probably because they are physiologically better adapted to hot, dry regions. © 1969.