The Number of Bird Species in Some Tropical Forests

Resident birds were censused in seven plots in Costa Rica ranging from 0 to 6 months dry season and from sea level to 2,380 m in elevation. Included were one highland and two lowland sites that were dominated by one or two species of trees. All stands were tall, undisturbed forests, but some were surrounded by agricultural land. The number of species of birds recorded was not measurably affected by the tree species diversity or the length of the dry season. However, a larger proportion (about 90%) of the total forest species known to occur in the region as a whole was found in the drier areas, whereas only about 50% were recorded in the wetter areas. This suggests that "between—habitat" diversity" may be greater in the areas with less severe dry seasons. The highland sites and markedly fewer species than the lowland sites, and a greater percentage of the species foraged by moving actively than was the case in the lowland sites. The large number of tropical lowland forest species that hunt by sitting and waiting for prey to move can probably be attributed to lack of wind, larger leaf size, larger biomass and higher recruitment rates of insects, more large nocturnal insects and more arboreal reptiles and amphibians. Many species of birds were limited to narrow vertical ranges in the forests, but the great increase in numbers of species in the tropics is only in small part accounted for by finer vertical subdivision of the forest. The greatest contribution is made by the addition of species with no temperate counterparts, suggesting that the greater range of resource types permanently above threshold values in tropical forests is the major cause of increased bird species diversity.