244 



Bird - Lore 



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bird that laid them, though with a Httle experience one can identify at sight 

 most of the well-marked eggs. 



The size of birds' eggs varies with the size of the bird, from those of the 

 Hummingbird, which are about the size of beans, to those of the Ostrich, which 

 are nearly the size of one's head. Precocial birds, whose young are hatched in 

 a much more advanced state than the altricial birds, necessarily lay larger 



eggs since development 

 has to proceed further 

 in the egg and more 

 food yolk has to be 

 stored. The extreme in 

 size of egg as compared 

 with the size of the 

 bird occurs among the 

 shore-birds which lay 

 but three or four eggs. 

 Thus, the eggs of the 

 Upland Plover are 

 about twice the size of 

 those of the Meadow- 

 lark, though the birds 

 are about the same size, 

 and those of the Spotted 

 Sandpiper are about 

 twice the size of those 

 of the Catbird. Preco- 

 cial birds like the 

 Grouse, which lay a 

 large number of eggs, 

 lay relatively smaller 

 eggs than the shore- 

 birds. 



The average num- 

 ber of eggs laid by 

 birds in temperate clim- 

 ates is four and in the 

 tropics half that num- 

 ber. The number has doubtless been fixed through the course of natural 

 selection so that it compares favorably to the number of dangers to which 

 the eggs and young are exposed. Thus, sea-birds, like the Murres, that 

 nest on inaccessible cliffs where there are few enemies, lay but a single 

 egg, while the game-birds and water-fowl whose eggs and young are exposed 

 to a great many enemies lay from ten to twenty. While the number in a 



NEST AND EGGS OF THE SWAMP SPARROW 

 They are more protectively colored than the eggs of most Sparrows, 

 but the pigment is apparently in the process of degeneration and un- 

 spotted eggs sometimes occur 



