84 The Oologists" Record, December 1, 1923. 



amongst five pairs need survive in order to keep up their normal 

 number, i.e., two young in every fifty, or 4 per cent. It will at 

 once be said in reply to this that all birds do not lay 10 eggs a year, 

 so let us take the csise of Gulls and Terns, which lay only three in 

 a clutch. What percentage of these have to come to maturity 

 to keep the species up to its proper number ? Eveiy year enormous 

 numbers of eggs are destroyed by storms, floods, vermin and other 

 Gulls and birds, so that certainly on an average every bird lays 

 twice, and many lay three and even four times. However, if we 

 take six only as the average laid by each bird, we shall not err on 

 the wrong side. Once having reached maturity, we know that 

 Gulls and Terns are long-lived birds. For instance, a Tern which 

 laid peculiar-coloured eggs year after year from 1900 to 1917 was 

 then hale and well when killed by rats. It is probable, therefore, 

 that once past the first year of their existence these birds average 

 a life of some 12 years. In 12 years a hen would lay 72 eggs, and 

 of these only two need survive to replace herself and the cock 

 bird, i.e., 2-8 per cent. 



The only instances in which mankind can expedite the extinction 

 or disappearance of a bird is when Nature has already brought 

 the number so low that any additional factor may prove to be the 

 last straw. Such instances in England as those of Haliactns 

 albicilla, the White-tailed Sea Eagle, and Pandion haliaetus, the 

 Osprey, are proofs of this, whilst the Common Kite and Buzzard 

 would have been further examples but that bird-lovers awoke in time 

 to the fact that they were fast disappearing, and took measures 

 to protect them, not only from human thieves but from other 

 dangers as well. 



Many birds have been driven from our shores which used io 

 breed in these islands, such as the Ruff and Reeve and similar 

 swamp-loving species, but this is not because of the vast quantities 

 of their eggs taken by collectors, but because the conversion of 

 their swamps and fens into fields of crops took away from them the 

 places in which they loved to bieed. 



One severe winter will do more to destroy birds than generations 

 of egg-collectors. Thus, during the winter of 1917-18 the British 

 form of Dartford Warbler was almost wiped out. Now this little 

 bird had, in a few places, been regularly farmed by certain egg- 

 collectors. These men knew eveiy pair breeding on the ground they 



