122 



ANALYSIS OF THE ENVIRONMENT 



when their glands associated with sex are 

 aroused by longer days and toward the 

 south when the sex glands are showing 

 short-day, autumnal regression. Rowan 

 states his argument thus (p. 116): 



"Nearly all animal activities are related in 

 one way or another, directly or indirectly, to 

 the influence of the sun. If a species leaves 

 Alberta habitually in the first week of Septem- 

 ber it may leave (in diflFerent years) with the 

 barometer either high or low; the earth may be 

 sunbaked or the very gopher holes may be 



its of its influence cannot be estimated at 

 present. 



Pelage and Plumage 



The varying hare, Lepus americanus, is 

 one of the numerous mammals that bear 

 brown fur in summer and white fur in 

 winter. The color change in the hare can 

 be prevented by exposure to Ught for eight- 

 een hours daily in the autumn, regardless 

 of temperature, while the change from 

 white to brown can be brought about in 



Transients 



Robins 



Occosional Visitors 



___J Red-breasted Nuttiatch : Present Alternate Years 



Fig. 23. Seasonal changes in the bird population of the beech-maple climax community in 



Ohio. (Redrawn from Williams.) 



spouting water; the leaves may be golden or 

 they may have fallen weeks before; the food 

 supply may be abundant or it may have failed 

 or be completely covered by a fall of snow. 

 Only one factor of the environment would be 

 certainly constant— the length of day. Its de- 

 pendability suggests it as the inaugurating 

 principle." 



Length of day appears to be one impor- 

 tant factor in the complex ecology of 

 periodic migration. The full extent and lim- 



January by a sudden increase to eighteen 

 hours' illumination each day. The brown 

 color will be retained throughout the year, 

 despite occasional moults, provided the 

 hares are exposed to an eighteen-hour day, 

 and reduction to nine hours of hght will 

 bring a moult to white winter pelage even 

 though the temperature remains at 21° C. 

 (Lyman, 1943). 



The pelt cycle of the ferret, Putorius vul- 

 garis, of the mink, Lutreola vison (Bisson- 



