Tropical countries have many times more species of most taxa than temperate ones, and small areas in the tropics have a smaller multiple of the number of species of small temperate areas. Where many species are present, abundances tend to be more equal and geographic distributions more spotty. Most tropical environments are less seasonal and more productive, and the dry areas and mountains which are relatively more seasonal and less productive have fewer species. The species which have reached offshore islands are often much commoner there and occupy expanded habitats.
To account for these relations, the following general hypothesis seems necessary:species interactions are important and the tropics have a head start on speciation. The head start, or greater rate, allows extra species to pile up in the tropics, but because of the importance of competition, no single area becomes as greatly enriched. Rather, faunal differences between areas increase. The lesser excess of tropical species in small areas is largely due to greater productivity and reduced seasonality which make marginal ways of life profitable. With more overlap in resources, the closely packed tropical species have more uniform abundances and the coexistence of these species is more precarious, causing the spotty geographic distributions.
Neither the species diversity of the food supply nor the longer breeding season (supposedly allowing staggered nesting seasons with an early shift and a later shift) is relevant to bird species diversity.