Avian Distribution in Central America

Abstract

As Central America is not a naturally delimited geographic entity, I define it arbitrarily for present purposes as the area from the United States-Mexico border and the lower Rio Grande valley southward through Panama and including a small section of Colombian territory southwest of the Gulf of Urabfi. A more' exact demarcation of both boundaries is given below. For the sake of simplicity, Baja California and all offshore islands are excluded from consideration. However one defines Central America, the analysis of the distribution of its avifauna remains an intriguing problem. During most of the Tertiary, part of the region was crossed by seaways that blocked the spread of many organisms and interrupted the ranges of others. The long physical isolation of North and South America permitted the evolution of distinctive biotas on those continents. The partly submerged and sometimes frag-mented portions of Central America were for some individuals or groups an absolute barrier to northward or southward dispersal; for others they were a filter bridge; for still others, an obstacle that was nevertheless crossed in both directions, perhaps repeatedly. Around the end of the Pliocene an isthmus linking th'e northern and southern continents developed. This resulted in new and extensive invasions of previously separated sec-tions of the Central American region by the biotas of North and South America, a process that is still continuing. Climatic changes in the Pleistocene influenced the extent and direction of these movements, and the last glacial period in particular brought about a strong southward thrust of the northern biota into what is now the tropical zone and doubt-less caused halts and retreats in th'e dispersal of many northward-expand-ing forms of southern derivation. This rough outline of historical events is well-known to all students of