Foraging and Plumage Coloration of the Galapagos Lava Heron (Butorides Striata Sundevalli)

Abstract

The Lava Heron (Butorides striata sundevalli) is a distinctively dark-plumaged heron that feeds along the rocky intertidal shoreline of the Galapagos Islands. The open shoreline used by the Lava Heron differs markedly from the dense swamps used by most populations of Butorides. Larva Herons feed by standing and by walking slowly at the water's edge, averaging only three steps per minute and catching primarily fish, crabs and prawns. Lava Heron foraging is characterized not only by its use of open shoreline, but by an extreme dependence on standing feeding behavior, frequent shifting of feeding sites following the tidal flux across a broad intertidal zone, and importance of crabs in the diet; but overall its foraging does not differ significantly from that used by Butorides elsewhere. The fundamental Butorides foraging strategy appears to have accommodated the habitat shift from dense bushes to open shoreline. Individuals maintain separate feeding territories, with the nest being placed within the territory of one of the pair. Feeding success was 0.12 prey per minute. In that nesting production is low contrasted with other populations, it appears that food may overall be limiting. The Galapagos population differs from South American Striated Herons (Butorides striata striata) in having a longer and stouter bill, a feature that correlates with the importance of crabs in the diet. Although known for its distinctive dark color, plumage ranges from all black to light grey, similar to South American birds. The dark color has been assumed to provide camouflage against the similarly colored shoreline, but its specific function has not been explained. Dark ventral plumage likely provides camouflage from active fish and crab prey. Dark dorsal plumage likely provides camouflage on the dark shoreline from above. Although the Galapagos lacks predators, four bird species steal prey from herons, suggesting that the dark plumage of Lava Herons may function to provide camouflage against piratical birds. Whereas the complex plumage patterns of most Butorides populations renders those birds cryptic within their usual densely vegetated habitat, dark plumage is similarly cryptic along the exposed dark shoreline of the Galapagos.