He made systematic revisions of the taxonomy of the birds of Peru and their relatives in other parts of South America, and in his later years combined this with studies of New World flycatchers, preparing the section on the Tyrannidae for Peter's Check-list of Birds of the World.
The Motagua Valley thornscrub forms a habitat for 75 bird species, mainly Columbidae, Tyrannidae, Icteridae and Fringillidae, and it is the only place in Central America where Momotus mexicanus lives.