BIRD ADAPTATIONS 



This brings out an interesting correlation between thyroid 

 activity and metabolic acclimatization in migrant and non- migrant 

 species. From the data cited above and from those of Miller (1939), 

 it may be observed thatpermanentresident species have pronounced 

 thyroid cycles and therefore greater degrees of metabolic acclim- 

 atization because they experience pronounced annual fluctuations in 

 temperature, while migrants, such as the Tree Sparrow or the 

 White- crowned Sparrow studied by Oakeson and Lilley do not have 

 cyclic changes in thyroid activity and therefore little change in meta- 

 bolic acclimatization because they experience similar temperature 

 conditions in both winter and summer. 



Ecological Implications 



The physical and metabolic thermoregulatory mechanisms 

 possessed by a species enables it to adapt to a specific set of en- 

 vironmental conditions, i.e., its distribution is limited by these 

 mechanisms. The habitation of any area is determined in part by 

 the ability of a species to acquire not only enough existence energy, 

 but also sufficient productive energy (Kendeigh, 1949) for carrying 

 on energy demanding activities such as molting, reproduction, and 

 migration. In addition to these physiological limits, morphological 

 and behavioral adaptations impose further restrictions on, the actual 

 distribution of a species. Although thermoregulatory adaptations to 

 specific environments are covered elsewhere in this symposium, 

 it is important to discuss some of the energy requirements for exis- 

 tence and other activities under natural conditions. 



The energy intake of all small wild birds yet studied increases 

 in the winter. The added energy intake is used for existence, which 

 includes maintenance of homeothermy and body weight, acquiring 

 food and drink, the SDA of digestion and assimilation (Kendeigh, 

 1949), and the deposition of body fat. The added fat may be an emer- 

 gency measure against severe winter weather or an aid in total 

 insulation. Permanent resident species of temperate regions, such as 

 the House Sparrow, maintain favorable energy conditions throughout 

 the year by increasing their ability to metabolize energy in the cold 

 (see above). Permanent residents of tropical regions, such as the 



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