Accounts are given of the detailed structure of the reproductive systems of male and female starlings (Sturnus vulgaris L.). It is shown that there are differences between the sedentary British starling and the migratory Continental bird in the seasonal variations of these systems. The gonads of the first-year British and Continental starlings begin to grow in February, but the rate of growth in the British bird is greater than that in the Continental. The gonads of the adult British starlings do not regress so far in summer as those of the Continental birds, and they start to grow precociously in early autumn. The gonads of the adult Continental starlings do not begin to grow until January or February, the time when the gonad growth of the British birds is accelerated. In February and March the gonads of the adult British birds grow much more rapidly than those of the Continental birds. These differences in the degrees of regression and in the times and rates of growth of the gonads of the British and Continental starlings are reflected in the accessory sexual organs. In the adult British males the rete testis and vas deferens show signs of growth in autumn, and although the other accessory sexual organs, apparently requiring a higher rate of sex-hormone secretion, do not begin to grow until January or February, their growth is earlier and faster than that of the accessory sexual organs of the Continental birds. In the adult British female the oviduct enlarges in autumn, and in some individuals the various organs of the Wolffian system also show signs of growth. It is apparent that the ovary of the adult British female in autumn secretes both male and female sex hormones, but the amount of male sex hormone secreted by different individuals is very variable. The secondary sexual characters of the starling are either permanent or vary according to the state of the reproductive cycle. The browner colour of the iris of the male and the yellowish colour of that of the female are permanent distinguishing characters, and, in addition, the throat and breast feathers of the first-year or adult male are narrower and more pointed than those of a female of the same age. The colour of the beak is a varying secondary sexual character. Owing to the secretion of male sex hormone by the testes and ovaries of the adult British starlings in autumn, the beak turns from dark grey to yellow during this season. No similar change in the beaks of the Continental birds is noted until January or February. When the beak is yellow, a grey base to both mandibles is a distinguishing feature of the male. The behaviour of both races of birds was studied, and it was found that, coincident with the growth of the gonads in autumn, the adult British starlings showed sexual behaviour and close attachment to their nesting sites. The habit of roosting in nest-holes is seen all the year round in some birds of this race, and it becomes almost universal in early January. No similar behaviour was seen in the Continental birds in this country, and none is described in the case of those birds which remain on the Continent. At the end of February, when the gonads of the Continental starlings are actively growing, the males begin to sing, the northward migration commences, and on the Continent interest in nesting sites is first reported. The question of the relation of several aspects of bird behaviour to the activity of the gonads and the secretion of sex hormones is discussed. It is evident that, as these two distinct races of starlings live together in the British Isles in autumn and winter, the differences in their reproductive cycles must be inherent and not dictated by environmental variations. The question of physiological species and subspecies is discussed, and on the basis of the differences described in this paper, it is proposed to rename the British race of starlings as Sturnus vulgaris britannicus.