The annual cycle of the Yellow-vented bulbul Pycnonotus goiavier in a humid equatorial environment



This two-year study of the breeding and moulting cycles of a small bird, Pycnonotus goiavier, was made on Singapore Island, which is near the equator and has a very uniform climate. There are well-marked breeding and moulting seasons in the population, and it is suggested that these protein-demanding processes are timed to occur within the period of increased insect abundance in the first half of the year. Seasonal changes in the weight of the flight muscles indicate a cycle in the protein level of the body. This cycle might serve as an internal regulator of the reproductive cycle.