447 



THE COLOURATION OF BIRDS' EGGS. 



By D. Dewar, i.c.s. 



(Read before the Bombay Natural History Society on 21st 



January 1904.) 



A well-known naturalist has declared that he knows of no more in- 

 spiriting subject than the colours of birds' eggs. 



" The moat superficial glance over a collection, " he writes, " reveals 

 hosts of interesting problems which require solution." Nevertheless, 

 I know of no portion of natural history which is in so unsatisfactory a 

 state as that relating to the colouration of birds' eggs. 



Since the fowls of the air are the direct descendants of the reptiles, and 

 these latter mostly lay white eggs, it is generally supposed that the eggs 

 of the most primitive birds were white. 



Three theories have been advanced to account for the fact that so many 

 eggs are now coloured. 



According to one this colouring matter is useful to the specias in that it 

 protects the contents of the eggs from the rays of the sun. 



On the second hypothesis, which at present holds the field, colour has 

 arisen in eggs in order to cause them to resemble their environment 

 and thus afford them some protection from egg-eating birds and 

 beasts. 



The third theory asserts that the colouring of birds* eggs is detern i :ed 

 by the influence of the surrounding tints upon the retina of the female. 



In favour of the first theory is the indisputable fact that excessive 

 light is injurious to protoplasm. There is also the fact that most of the 

 eggs which are laid in dark holes and caves are white. 



The theory is, however, confronted by a number of insuperable 

 difficulties. Many birds — as, for example, ducks, grebes, pheasants, 

 pigeons and doves — which do not nest in dark places, lay white eggs. 

 It is certainly true that some of these birds have the habit of covering 

 up the eggs when they leave the nest and thus protecting them from 

 the sun's rays. But many birds do not take this precaution. 



Again, many birds which nest in dark holes do not lay white eggs. 

 Reptiles' eggs are exposed to the hottest sun, yet they are usually not 

 coloured. 



Further, if eggs were coloured with the object of protecting the con- 

 tents from excessive light, we should expect eggs laid in tropical climates 



10 



