1903 Notes on the Nests and Eggs 0/ Bints 135 



each species, though varying greatly in different species. 

 Some large and powerful birds which are colony breeders, 

 such as the gannet, guillemot, razorbill, and puffin, lay one 

 egg only. The black guillemot, divers, and pigeons lay two. 

 The latter family present much difficulty in their breeding 

 habits, and the reason for the black guillemot producing 

 one more egg than its common relative may be a provision 

 of nature, lest with its more restricted range and perhaps 

 less robust frame it should become extinct ere the life-work 

 of the species in preserving the balance of nature is accom- 

 plished, for here we are dealing with a bird whose domestic 

 economy is not disturbed by man. 



We next come to those few birds which lay three eggs at 

 a clutch — the oyster-catcher, cormorant, and sand-grouse. 

 These are all exceptional cases and connecting-links between 

 related families which lay either two or four eggs. 



The size of egg laid by any given bird depends more or 

 less on three things : — 



1. The size of the bird itself, and in a less degree upon 

 its age and health. 



2. Upon the state or development of the young at hatching 

 time. 



3. Upon the number of eggs contained in a normal clutch. 

 The first part of the first rule generally holds good, the 



second part needs the following explanation. The eggs of 

 birds in their first year of laying are usually smaller than 

 those of over-yeared breeding-birds of the same species. 

 This is especially noticeable in thrushes and blackbirds, 

 coots and moorhens. Again, the health or state of the egg- 

 producing organs will influence the size of the egg — the last 

 of a clutch, and especially of a second clutch, frequently 

 being very small, and sometimes yolkless. Trading upon 

 this fact, I have known schoolboys indulge in the practice 

 of constant "robbing" to add "varieties" to their collec- 

 tions. Late clutches laid by those birds which naturally 

 rear two or three broods in a season often afford illustrations 

 of hard-worked organs producing poor results ; actual disease 

 or deformity in the producer will also result in defective 

 produce. Secondly, the size of eggs depends very much 

 upon the degree of development the young attain at the 



