The Moult and Gonad Cycles of Three Species of Birds at Five Degrees South of the Equator.

Summary. Histological and moult data are given for series of three species collected at fortnightly intervals over one year at Amani, an evergreen locality in Africa at 5 °. The species are Colius (Mousebird, a small non‐passerine) and two Pycnonotidae (Bulbuls), one, Phyllastrephus , living inside the forest, and the other, Pycnonotus , outside. Moult is regular, concentrated and seasonal in both Pycnonotidae; “early” stage and even “mid” stage of wing‐moult are found in both these birds concurrently with enlarged testes and free sperms. These conditions do not seem to have been reported before for small passerines. In Colius moult is irregular and proceeds indefinitely irrespective of testis‐state: continuance of wing‐moult through the breeding season seems hitherto to have been recognized only in large birds dependent on full wing‐efficiency for their livelihood. In all three species testes are biggest from September to March, when they regress sharply and do not approach the maximum again till August. These results accord with field observations on nests. In the resting period, which covers the “long rains” and the subsequent cool season, the internal state of the testes is like that of temperate‐zone birds in winter. Recrudescence takes place when there has been practically no increase in temperature above the annual minimum and day‐length is increasing at less than 20 seconds a day. Lines of further research are suggested. The annual increase in testis size is about 30 times in the Pycnonotidae and 8 in the Colius. Other data for tropical and temperate‐zone species are quoted, from which it appears that the latter tend to have a much greater increase in testis size (up to 2,000 times). In Amani birds spermatogenesis commonly takes place in testes that have not attained full size. In the ovaries atretic follicles, rare in temperate‐zone birds, are abundant and may be an adaptation to high nest‐mortality.