Temperature Regulation in Two Endangered Hawaiian Honeycreepers: The Palila (Psittirostra bailleui) and the Laysan Finch (Psittirostra cantans)

ABST•ACT.--Theclosely related, morphologically similar Palila (Psittirostra bailleui) and Laysan Finch (P.cantans) are natives of thermally dissimilar habitats.The 34.8-g Palila is confined to the cool, montane forests of Hawaii Island, whereas the 31-g Laysan Finch is restricted to a low, treeless atoll.To study their climatic adaptation, we measured their body temperature, oxygen consumption, and evaporative water loss at stable air temperatures between 3 and 40øC.Bioenergetically, these species are most distinct in their response to heat.The Palila's upper critical temperature (31øC) is 7øC lower than the Laysan Finch's, and its heat-strain coefficient is 31% higher (1.76 versus 1.34 mW'g -•'øC •).The Palila is much less heat tolerant than the Laysan Finch (which, in its heat tolerance, resembles other passerines).Consequently, the Palila probably is restricted physiologically to cool, mountain forests and is not a suitable candidate for lowland introductions.The Palila's lower critical temperature (17.5øC), thermal conductance (0.612 mW'g •'øC •), and nighttime basal metabolic rate (BMRp) (12.87 mW/g) were all within 8% of levels predicted from mass, indicating that its cold tolerance is not unusual for a passerine of its size.The Laysan Finch's BMRc• averaged 15.2 mW/g H20/day, 80% of the value predicted from mass.Its evaporative water loss (rhwe) also was lower than expected, 2.22 versus 3.31 g HzO/day.Reductions in BMR and rhwe have adaptive value for small birds living in waterless environments.