In this paper four aspects of seabird biology are reviewed for that part of the tropical Indian Ocean lying west of India: title breeding seasons, the composition of the avifauna, the density of seabirds at sea, and lastly seabird migration.

In species composition, there is little overlap between the seabird faunas of the Arabian Sea and the area south of the equator, where it is almost identical to that of the other tropical oceans. Breeding seasons south of the equator are mostly protracted if not continuous throughout the year, whereas annual breeding during the southwest monsoon is the rule around the periphery of the Arabian Sea. Exceptions are discussed.

Besides the high degree of endemicity in the Arabian Sea, the avifauna there contains, during the southwest monsoon, a high proportion of non-breeding migrants both from the southern hemispheire and the northern Pacific Ocean. In addition, several other species migrate within the Indian ocean concentrating off Arabia and in the Arabian Sea in the northern summer, at which time bird densities are many times higher than in the winter, than in any area further south.

These regional differences can best be interpreted as the results of seasonal changes, acting through the food supply, which accompany the reversal of the monsoons, for these are most marked in the north. While it has not been possible to measure the seasonal changes in abundance of food organisms available to seabirds, the summer breeding and concentration of seabirds in the Arabian Sea occur at a time when the standing crop of zooplankton is highest. The unique Arabian Sea avifauna is presumably adapted to the seasonal changes.