708 REPRODUCTION AND MIGRATION IN BIRDS 



day inhibited the preparatory phase. Data from our laboratory (un- 

 pubHshed) on body weight and fat deposition for two captive bobo- 

 links demonstrate an excellent premigratory physiological state, with 

 much more extensive fat deposition and greater increase in body 

 weight than has ever been observed in the North Temperate species 

 we have studied. Experimental studies with equatorial and trans- 

 equatorial migrants are currently underway in our laboratory and will 

 test further the hypothesis of summation as applied to the preparatory 

 and progressive phases. 



Day Length and Breeding Cycles in the Tropics 



Breeding seasons cannot be explained simply by any one factor in 

 the environment. Moreover, it is not possible to extrapolate directly 

 from experimental studies of the gonadal cycle to breeding cycles in 

 the Tropics, particularly when North Temperate species were used in 

 the experimental studies. Nevertheless, in view of our understanding 

 of the regulation of gonadal cycles in North Temperate species it 

 seems desirable to examine briefly the summation hypothesis and the 

 related interpretations in relation to breeding cycles in the Tropics. 

 The basic premise in the discussion which follows is that breeding is 

 preceded by gametogenesis; therefore, what controls gametogenesis 

 plays a fundamental role in the regulation of breeding seasons. After 

 gametogenesis occurs, or is initiated, other factors certainly come into 

 play to regulate the time and duration of breeding. (See Wolfson, 

 1952a, for a more detailed discussion of day length and breeding 

 cycles in the Tropics.) 



Studies of breeding cycles at all latitudes led Baker (1938) to the 

 conclusion that "the main proximate causes of the breeding seasons of 

 birds in nature are thought to be temperature and the length of day in 

 the boreal and temperate zones, and rain and/or intensity of insola- 

 tion near the equator," Recently, a reexamination of the problem led 

 to the same general conclusion that day length and temperature in the 

 higher latitudes and humidity and rainfall in the Tropics are corre- 

 lated with the breeding seasons (Lack, 1950a,b; Moreau, 1950; 

 Skutch, 1950; Thomson, 1950; Voous, 1950). The significance of 

 these climatic factors is believed to lie in their effect on food supply. 

 Breeding seasons are regarded as an adaptation and occur apparently 



