230 



I II III IV V VI VII VIII IX X XI XII I II III IV V VI 

 Fig. 6.24. Relative intensity of breeding of the English sparrow in each month of 

 the year at the indicated latitudes. (Baker, 1938.) 



Ultraviolet Light 



The small fraction of the sun's radiation that reaches the earth as 

 ultraviolet hght has certain very special biological effects, and some 

 of these may be of ecological significance. The sensitivity of the bee's 

 eye to this part of the spectrum and its use in orientation has already 

 been mentioned. The population size of certain land animals in north 

 central United States was found by Shelford ( 1951 and 1951a ) to be 

 correlated with the intensity of ultraviolet light, although the reactions 

 that underlie the correlation are not known. The bactericidal 

 action of ultraviolet is familar and causes the destruction of micro- 

 organisms that are excessively exposed to the direct rays of the sun. 

 Ultraviolet radiation produces sunburn, or erythema, in man and per- 

 haps causes harmful effects in other animals under natural condi- 

 tions, although this question has not been investigated from the 

 ecological viewpoint. Ultraviolet also brings about the production of 

 vitamin D with its antirachitic effect. Vitamin D is formed by the 

 irradiation by ultraviolet light of certain sterols, or fatty substances. 



