Metabolism and Solar Radiation in Dark and White Herons in Hot Climates

Metabolism, body temperature (Tb), and feather reflectance was measured for dark Louisiana herons (LOU), little blue herons (LB), snowy egrets (SE), and cattle egrets (CE); only LOU nest preferentially in shade. The reflectance of LOU and adult LB was 13%-15%; it was 80%-82% for SE, CE, and immature (white) LB. Tb for all four species approximated 40 C; hypothermy was never observed. Basal metabolism (Ḣb) was 107% of the expected level for LOU, 85% for SE and CE, and 66% for LB. Minimal thermal conductance (C) was calculated as approximately 131% of the expected value for LOU, 115% for SE and CE, and 110% for LB. Metabolism and solar radiation may combine to produce heat stress in hot climates, particularly in dark birds; reduced Ḣb allows LB and other dark birds to nest in exposed sites. High values for C suggest a heat dissipation function; the relatively lower values are found in species with relatively lower Ḣb's, thus preventing or reducing increases in T₁, the lower limit of thermoneutrality. White plumage is thermally advantageous in tropical/subtropical birds nesting in open habitat, but dark coloration is not; other evolutionary explanations must be sought to explain the occurrence of dark birds in hot climates.