196 BRITISH HIRUS. 



BLUE EGGS OF COMMON TERN. 



One day last June I visited the flourishing colony of Common 

 Terns {Sterna fluviatilis) on the Chesil Bank, just opposite the 

 well-known swannery at Abbotsbury. I examined a good 

 many nests, some of which were nearly concealed by festoons 

 of the wild pea {Lathy rus maritimus), which grows in abun- 

 dance there, and in about ten nests there was one pale blue, 

 unspotted egg, but in no nest more than one. I was given 

 to understand that the Terns were carefully protected, and 

 there is no doubt that the number of birds breeding there 

 has increased of late years, so that, as Mr. Borrer suggests, 

 some other exx3lanation for the absence of coloration must 

 probably be sought for, as I do not think the nests have been 

 plundered to any extent. M. Vaughan. 



At the Hook of Holland, where there is a large colony of 

 Common Terns, I saw, on July 8th, 1911, a nest containing 

 one, and on July 22nd, a nest containing two unspotted blue 

 eggs. This colony is not protected. G. J. van Oordt. 



[There seems little doubt that the blue eggs referred to 

 above are due to some weakness of the reproductive organs. 

 The enormous increase in the proportion of blue eggs laid by 

 Pheasants of late years, is ascribed by most practical keepers 

 to the over-production of eggs by hens kept in pens for that 

 purpose, and is generally regarded as a sign of weakness. 

 The blue Terns' eggs have a dull surface without gloss, and 

 appear to be thin-shelled and defective both in coloring 

 matter and shell material. This may be due to premature 

 deposition, or may be a sign of exliaustion of the reproductive 

 organs or that the general health of the parent is below the 

 normal standard. Abnormal eggs of a some\\hat similar type 

 are to be found in most of our large colonies of Black-headed 

 Gulls, and it seems possible that indiscriminate protection 

 may be the cause in some cases, as well as over-production 

 in others where protection is lacking. Such eggs are, of 

 course, conspicuous, and would be more likely to be taken 

 both by egg collectors and natural enemies, such as rats, and 

 egg-stealing birds. Where, however, the colony is protected 

 and such eggs are allowed to hatch off, there is a natural 

 tendency for variations, even when due to weakness, to 

 become perpetuated or to increase. It has been noticed that 

 while similar variations are common both in artificially pro- 

 tected colonies of Black-headed Gulls, and also where they 

 are systematically robbed, they are practically unknown in 



