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

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Abstract
Metabolism, body temperature ( ), 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. for all four species approximated 40 C; hypothermy was never observed. Basal metabolism ( ) 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 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 , 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.