228 



Light 



began laying eggs during the same week as did snails inhabiting a 

 shallow ditch that underwent rapid vernal warming (Jenner, 1951). 

 Very little additional investigation of photoperiodism among the in- 

 vertebrates has been made, but this reaction to light may eventually 

 be found to play an important role in controlling the life cycle of 

 many of the less conspicuous animals. Among mammals certain 

 forms, such as ferrets, are brought into breeding condition by the 

 long days of spring, but others, such as deer, come into heat during 

 the short days of autumn. 



Photoperiodism in birds was first discovered as a result of studies 

 of migration. It had long been realized that the arrival of birds in 

 the spring was not closely related to temperature or other common 

 aspects of the weather since birds are often caught by late winter con- 

 ditions. A similar difficulty arose in explaining the start of the south- 

 ward migration. Many birds leave the northern regions during July 

 or August when the temperature is high, and when an ample food 

 supply is still available (Fig. 6.23). The length of day was finally 



Pholo by Allan D. Ciuickshank, National Audubon Society 



Fir.. 6.23. Tree swallows flocking previous to migration. 



