ECOLOGICAL FACTORS 407 



twice as much time and more at the disposal of the day-feeding birds. 

 This is one of the advantages that birds enjoy when they leave the 

 tropics for their summer breeding grounds nearer the poles. 106 The 

 longer feeding hours make possible the production of a greater number 

 of eggs and the maintenance of a larger brood. That better nourish- 

 ment results in greater egg-laying follows from the observation that, 

 in mouse years, marsh owls (Asio flammeus) and barn owls {Tyto 

 alba) lay 6 to 10 or more eggs instead of only 3 to 5. 107 There is also 

 experimental evidence for this fact, American chicken breeders have 

 established that a greater number of eggs are produced with artificial 

 illumination of the chicken coops until 9 p.m. and the result is asso- 

 ciated, at least in part, with the accompanying increase in food con- 

 sumption; on the average, every hen laid 16 more eggs per year than 

 the control animals that were kept without such lighting. 108 This may 

 help account for the fact that birds of the tropics lay very few eggs, 

 rarely more than two. Of 53 avian species in Borneo, for which obser- 

 vations are available, 5 lay only one egg, 33 lay two, 10 lay three, and 

 only 5 lay four or more eggs. 109 In British Guiana, 110 the representa- 

 tives of all groups, birds of prey, parrots, pigeons, plovers, lapwings, 

 toucans, and many sparrows, lay but two eggs even when their close 

 relatives lay more eggs in our region. 111 - 112 



Hibernation and aestivation. — In contrast with the relatively 

 uniform climatic conditions of the rainy tropics, one finds in many 

 tropical regions an alternation of wet and dry seasons which has effects 

 somewhat analogous to the alternation of warm and cold seasons char- 

 acteristic of the higher latitudes. The differences in the two contrasting 

 seasons are not equally great everywhere. In the greatest part of 

 tropical America, the rainy season and dry season are not so sharply 

 distinguished as in India or Africa. Likewise, there are all sorts of 

 gradations between summer and winter. Proximity to the ocean 

 diminishes the temperature difference, affording cooling in the summer, 

 and warming in the winter. The difference between the extremes is 

 accordingly less in coastal regions than in the interior of large conti- 

 nents, where the summer is exceedingly hot and the winter is very 

 cold. Localities with an oceanic climate and those with continental 

 climate may have the same average annual temperature and neverthe- 

 less offer animals entirely different living conditions, because of the 

 differences in extremes. 



The rainy season and summer exemplify at times the optimum, 

 the dry period and winter the pessimum. Under the effect of unfavor- 

 able conditions, animal life declines extraordinarily; but in their effect 

 there is an indication that the deficiency in humidity is more injurious 



