BIOLOGICAL ASPECTS 325 



(1934) of a golden- crowned sparrow weighing usually about 28 grams 

 which lost 20 percent of its weight between one afternoon and the next 

 morning. That this change was exceptional appears from their rec- 

 ords showing that several days elapsed before the bird regained its 

 usual weight. Many records show that a decline in weight of small 

 birds occurs over night and that their weight increases during the 

 day. Baldwin and Kendeigh (1938) reported that the average diurnal 

 curves for the weight of several passeriform species changed from 

 about 97.5 to 101 percent of the mean weight between 7 a.m. and 6 

 p.m. Since sparrows weighing 20 grams lost about 10 percent when 

 caged without food during a night, the rate of loss of the captives 

 exceeded the average nightly loss of free wild birds in summer. I 

 am inclined to think that the regularity of weight records for each 

 species at Anaktuvuk, although they were taken at random times 

 during the day, is influenced by the fact that with scarcely an hour 

 of darkness there in early May, feeding is never for long interrupted. 

 It is also my impression that the birds there feed frequently but not 

 for very long periods. 



The daily food intake of small birds is undoubtedly a quite 

 appreciable part of their body weight. Captive English sparrows 

 weighing about 24 grams consumed from 4 to 9 grams of mixed feed 

 daily, according to my calculations from the metabolic heat production 

 described in Kendeigh's detailed study ( 1949) . The fact that average 

 weight characterized each of Kendeigh's sparrows shows that their 

 food intake and its utilization were kept in close relation in captivity 

 when food was abundant. The same close accord between consumption 

 and utilization of food seems to be the rule under natural conditions. 



It has been implied in our discussion that variation in weight among 

 the birds of a homogeneous population represents changes in the 

 quantities of nutritive substances in their bodies disposable for met- 

 abolic purposes. The metabolizable substances include food which 

 has been recently consumed and parts of the tissues themselves. 

 Weights of Anaktuvuk species under consideration differed nearly 

 100 fold from redpolls (Acanthis) weighing only 12 grams to scaup 

 {Aythya) as heavy as 1000 grams. The basal metabolic rate of birds 

 has been found to vary with their size according to a formula given by 

 Brody (1945, p. 371) empirically describing the metabolism of do- 

 mesticated birds : 



kilo-calories per day =89 X weight (in kilograms) ^/^ 



A few wild birds which we have examined in Alaska (Scholander, 

 Hock, Walters, and L. Irving, 1950; L. Irving, H. Krog, and M. 

 Monson, 1955) conformed sufficiently well with this relation to war- 

 rant its use for calculations. A bird weighing 10 grams has a meta- 

 bolic rate at rest which would expend about 4.8 k.-cal. in 24 hours. A 



