712 REPRODUCTION AND MIGRATION IN BIRDS 



appears to be slight. The authors state that the resuks give no support 

 to Wolfson's summation hypothesis, but they have misunderstood the 

 hypothesis. Many more experimental data are needed to test the 

 hypothesis for this species. For example, no data are given on the 

 existence or duration of the preparatory phase. From their results, it 

 seems possible that the birds had not yet completed the preparatory 

 phase. Other information is also needed: What is the effective daily 

 photoperiod or dark period during the preparatory and progressive 

 phases? Is there any relation between the daily photoperiod or the 

 daily dark period and the rate of response in these same phases? 



Keast and Marshall (1954) have made extensive studies of repro- 

 duction and breeding seasons in relation to drought and rainfall and 

 found that the gonads may remain inactive for a succession of seasons 

 during a prolonged drought. Desert species can respond quickly to 

 rainfall, or its effects, and nesting may begin within a few days of 

 heavy precipitation, irrespective of day length and light increment. 

 Marshall and Disney (1957) on the basis of an experimental study 

 of the induction of the breeding season in a xerophilous species con- 

 cluded that an internal rhythm of reproduction exists which is modi- 

 fiable by external conditions, including possibly rainfall and social 

 stimulation. They state that it is unlikely that the breeding seasons of 

 truly equatorial vertebrates are controlled by photoperiodicity, and 

 that many equatorial and other species have evolved a reproductive 

 response to rainfall or its effects. Too few data are presented in this 

 preliminary report to evaluate their conclusions with respect to the 

 effect of rainfall on reproduction. In a field study Serventy and 

 Marshall (1957) demonstrated a widespread reproductive response 

 to precipitation, or its effects, in wild birds in western Australia during 

 abnormally wet autumn periods. 



In summary, the relation between day length and the gonadal cycle 

 in tropical species is not known, but the point that must be empha- 

 sized is that day length cannot be ruled out a priori as a fundamental 

 regulator of migration and breeding cycles in the Tropics simply be- 

 cause it is relatively constant. In a similar vein, day length cannot be 

 ruled out at other latitudes, because not all birds are breeding when 

 the days are increasing, or because the gonads begin their growth 

 phase when the day lengths are decreasing or relatively constant. 



