His research has concentrated on the evolution, biogeography, conservation, and extinction of tropical birds, particularly in the islands of the Pacific Ocean. He has also authored over 180 scientific publications. He has conducted a number of digs at prehistoric sites and uncovered widescale extinctions caused by humans in the early stages of colonisation. He has described a number of extinct species of birds and more recently was involved in discovering that the Solomon Islands Frogmouth is a species (instead of a subspecies of the Marbled Frogmouth, as formerly believed). He worked extensively on Easter Island, carrying out the first systemicatic excavations of the island in order to identify the plants and animals that once lived there.