Foraging differences between white and dark morphs of the Pacific Reef Heron Egretta sacra

Abstract

Among the species of diurnal herons (Ardeinae) showing light‐dark colour polymorphism in plumage, colour phase is age‐related in one species, closely associated with geography in three species, and found in most populations in three closely related species of reef herons. In Pacific Reef Herons Egretta sacra white‐phased birds sought prey by a flight, land, and freeze hunting style in breaking surf, while dark birds pursued prey by actively walking or running on reef flats. On Mangaia, in the southern Cook Islands, more white‐phased birds used the windward side of the island and more dark‐phased birds used the lee side. On Rarotonga, 204 km from Mangaia, reef herons foraged almost exclusively in shaded streams and no entirely white‐phased bird was encountered. These results provide the first evidence of differences in hunting techniques associated with colour morph and only the second observation of differences in habitat use by white‐ and dark‐phased herons at single localities. As such they support Murton's (1971) idea that colour polymorphism serves herons in their role as predators. For reasons discussed in the text such differences are more expected in reef herons than in most other colour‐polymorphic herons.