I am especially indebted to Dr. J. Knox Jones, Jr., under whose direction this field study was undertaken, and to Dr. Richard F. Johnston who aided greatly in the identification of specimens and critical reading of the manuscript. I am grateful also to other members of the field party who helped in collecting many of the specimens reported herein: Ticul Alvarez, A. Binion Amerson, Dr. William E. Duellman, Dr. J. Knox Jones, Jr., Thomas E. Lovejoy, III, Jack G. Makepeace, Dwight R. Platt, William C. Stanley, Jerome B. Tulecke, and John Wellman. Dr. K. C. Emerson graciously identified the specimens of Mallophaga and Dr. Allan R. Phillips aided in identification of some of the birds. In addition, I wish to express sincere thanks for quarters provided our field party at the Campo Experimental Forestal "El Tormento" at Escárcega, Campeche, by Ing. Hector Flores S.; at Pueblo Nuevo X-can, Quintana Roo, by Pablo Alimilla; at Felipe Carrillo Puerto, Quintana Roo, by Fernando Esquival Montono; and at Pisté, Yucatán, by Luis V. Polanco. The field party benefited also from arrangements and helpful suggestions made by Eduardo C. Welling of Mérida and Marynoll Father Joseph Early of Felipe Carrillo Puerto. Wherever we camped the local Mayans were exceptionally helpful, especially as guides and collectors. Our field party was composed of two units. One unit, working on a survey of Middle American vertebrates and their ectoparasites, was supported by the United States Army Medical Research and Development Command (Contract No. DA-49-193-MD-2215). The second unit, composed of students in the Field Course in Vertebrate Zoology from The University of Kansas, was supported in part by the Museum of Natural History and by a grant from the National Science Foundation (G 20939, Special Projects in Science Education). The late Ing. Luis Macías Arellano, Departamento de la Fauna Silvestre, Dirección General de Caza, México, D.F., generously provided the necessary permits for collecting vertebrates. PHYSICAL FEATURES The Yucatán Peninsula is a northeastern projection of Central America lying between the Gulf of Campeche, the Gulf of Mexico, and the Caribbean Sea. Its area is approximately 143,500 square kilometers and includes the Mexican states of Campeche and Yucatán and the sparsely settled Territory of Quintana Roo, plus parts of British Honduras and the Petén region of Guatamala. The peninsula has a mean breadth of about 300 kilometers and a coast line of about 1100 kilometers.