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