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they could be brought into breeding condition at any season of the year, regardless of the temperature. In addition, an increase in sexual maturity as shown by increased size and pigmentation of the sex organs, could be induced by substituting periods of activity without light, for periods of artificial illumination. Thus a travelling band kept the birds moving about the cages for long periods of time as compared with " control " birds which rested in the normal manner. When killed, the birds subjected to continuous movement had the more developed gonads, or sex-organs. Interesting confirmation of this effect of physical activity on the sexual maturity of birds was obtained by a study of the starlings which roost in inner London. As is well known, thousands of starlings from the area surrounding London use the ledges of the larger buildings such as the National Gallery, several city churches and so on, for communal roosts to which they flock during the winter. Perched on the ledges, they are subject to continuous disturbance from the traffic beneath, whilst the amount of light reaching them high upon the buildings is negligible. It was found that the sex-organs of such London-roosting birds were in a more advanced state of maturity than those of equivalent birds which remained in country districts to roost and were undisturbed during the night. Other experiments have more or less confirmed these findings, and there seems little doubt that it is possible to increase the degree of sexual maturity of birds by the effect of light and of physical activity. It seems doubtful if activity alone, without any light, can initiate the enlargement of the sex organs, though it may add to any enlargement already started by light. When we come to apply these results to the immediate cause of migration, however, the situation is far from clear. It was thought at first that the urge to migrate was associated with the onset of gonad or sex-organ development, and that once the organs had reached full maturity, the urge would subside. The results obtained from releasing birds whose gonads had been artificially brought to full maturity, however, did not confirm this view. Thus Canadian juncos which were caught in the United States on their way north during the spring migration, were kept in cages until the time for the spring |
