SUMMARY.1 In the remarkably unvarying tropical climate of the northern New Hebrides (Pacific Ocean), the Passerine birdPachycephala pectoralisis as seasonal in its reproduction as are the birds of temperate climates.2 In the breeding‐season the mean weight of the testes of adults rises to over forty times as much as in the non‐breeding season.3 The growth of the testes and the start of the egg‐season occur while the days are shortening.4 There is no seasonal change in diet.5 Moulting takes place after the end of the egg‐season.6 The environmental change controlling the breeding‐season is not known.7 The egg‐season ofPachycephala, spp. in the tropics of Australia comes at quite a different season of the year from that of P.pectoralisin approximately the same lat