WEST 



(Sturnus vulgaris) . The reason for the difference in metabolic 

 response between laboratory acclimated and seasonal acclimatized 

 birds may be that the temperature conditioning process is sup- 

 pressed by variable ambient temperatures (King and Farner, 1961). 



The ability of birds to tolerate low temperature extremes is one 



of the best indications of seasonal metabolic acclimatization. The 



work of Kendeigh (1949), Seibert (1949), and Davis (19 55) clearly 



shows that the House Sparrow can extend its low temperature tol- 



o o 



erance limit from C in the summer to -31 C in the winter (Fig. 



12). In contrast, the Tree Sparrow does not change its lower limit 

 of tolerance and survives to -28 C in both summer and winter 

 (West, 1960). The ability to tolerate low temperatures depends pri- 

 marily on the length of time that the birds can maintain their max- 

 imum metabolic rates. By subjecting seasonally acclimatized birds 

 to a single low temperature. Hart (in press) shows that winter ac- 

 climatized Evening Grosbeaks, Starlings, and Pigeons can maintain 

 their maximum metabolic rates for longer periods of time than 

 summer acclimatized birds. 



In addition to increased metabolic capacity during the winter, 

 there may be a seasonal shift in insulation since Kendeigh (1934) 

 showed a 29% increase in plumage weights of winter over summer 

 House Sparrows, and West (1960) an increase of 25% of winter over 

 summer plumage weights of Tree Sparrows. 



Many species of birds exhibit annual cyclic thyroid activity 

 while others do not (Wilson and Farner, 1960). Wilson and Farner 

 show a direct correlation between thyroid activity and ambient tem- 

 perature in the Gambel's White-crowned Sparrow ( Zonotrichialeu - 

 cophrys gambelii) . These birds experienced an annual temperature 

 cycle of at least 20 C (0 C to 20 C) in eastern Washington. Sim- 

 ultaneously, Oakeson and Lilley (1960) studied the same race of 

 White-crowned Sparrow both on its wintering ground in California 

 and on its breeding ground in Alaska and in contrast, found no annual 

 change in thyroid activity. Wilson and Farner explain this difference 

 by showing that the amplitude of the cycle of temperature that Oak- 

 eson and Lilley's birds experienced was probably about 5 C, 15 G 

 less annual variation than their own birds received. 



310 



