214 MIGRATION 



amount ranges from 8 to 14 per cent in dry weight of insects. The 

 results of Stevenson (1933) and of Rensch (1931) are cited to indicate 

 the need for more food with lower temperatures, both daily and 

 regional, and those of Rorig (1905) and of Groebbels (1931) with 

 respect to greater consumption under enhanced activity. Similarly, 

 the work of Price (1929) and of Heape (1931) is assumed to show 

 the need of vitamins in the bird's diet, with a possible correlation with 

 migration. 



To quote from Kendeigh: "The southward migration of the eastern 

 house-wren in the autumn is necessary for the continued existence of 

 the species, while the northward migration in the spring avoids un- 

 favorable breeding and existing conditions in the south. By migrat- 

 ing south in the autumn and north in the spring, the bird maintains 

 itself in a more nearly uniform and favorable environment throughout 

 the year. The regulation of migration as to time is controlled in the 

 spring by rising daily maximum and night temperatures and changing 

 relative proportions daily of light and darkness. In the autumn, de- 

 creasing temperatures particularly at night, longer nights and shorter 

 days, and, for some species, decreasing food supply are most impor- 

 tant. The conditioning factor that may act directly or indirectly as a 

 stimulus for initiating migration is an excessive change in the 

 metabolic or physiological state of the body. Changes in physiological 

 state are induced by and correlated with changes in environmental 

 conditions, both directly and through the intervention of the endocrine 

 system" (page 408). 



Riddle, Smith, and Benedict (1932). A number of the studies of 

 Riddle and various associates on the hormones and metabolism of 

 columbids are of indirect importance, but the one of most direct signifi- 

 cance is that which deals with the basal metabolism of migratory and 

 non-migratory species. "Individuals of a feral, cold-avoiding, migra- 

 tory species of dove, when reared in captivity, have been found to have 

 a higher basal metabolism than that found in related non-migratory 

 domesticated doves and pigeons. At 20° C, and measured at all sea- 

 sons, the mean values found for the two sexes in these several species 

 are: 904 (mourning dove), 792 (ring dove), and 680 (tippler pigeon) 

 calories per square meter per 24 hours. 



"Seasonal changes in the metabolism of these species are espe- 

 cially difficult to interpret because possible responses to hot and cold 

 weather and an extraordinary autumnal involution of the gonads, 

 apparently the equivalent of functional castration, are involved. The 

 highest metabolism is found in September; lower and quite inter- 

 changeable values are found during the breeding season, November and 



