Brown Noddy chick predation by Great Frigatebirds in the northwestern Hawaiian Islands

Abstract

Adult female Great Frigatebirds (Fregata minor) were responsible for 64% of the predatory behaviors on Brown Noddy (Anous stolidus) chicks on Tern Island, French Frigate Shoals, Hawaii. Subadults with red orbital rings (presumably females) also preyed on Brown Noddy chicks, although they were observed hunting less frequently than adult females. No adult male frigatebirds were observed hunting in 118.3 hours of observation. Frigatebird predation likely accounted for 95% of mortality observed in Brown Noddy chicks less than 24 days of age. Chicks were as likely to survive to fledging regardless of nest location or down coloration; however, more chicks survived to fledging if hatched between mid-February and mid-May. Unequal sex ratios, sexual size dimorphism, resource partitioning, and breeding strategies of frigatebirds were evaluated to explain why sex differences in frigatebird predation behaviors were observed on Tern Island.