LARIDAE AND STERNIDAE. 



„which is moreover not very skillful in catching fish, is compelled to choose 

 „other food, and it then seeks by preference young mussels (My/elus edulis) 

 „edibie cockles (Cardium edule), tellinas (Tellina soUdula), and especially 

 „five-fingers (Asferia rubcns), the latter being borne by the turbulent water to 

 „the surface; and all these animals it devours, the first three even with the shells. 



„0n examining the excrement and vomit of these birds, which are usually 

 „seen near the Herring-Gull nests, I saw from the broken remains of the shells 

 „occurring therein which of the Mollusca the Herring-Gull must perforce take 

 „as its food. If therefore the weather compels it to select shellfish for its 

 „food, the colour of the shells of the eggs it lays becomes a dirty ^rey/s/i green. 



„If I am asked as to the cause of the eggshells being coloured greener when 

 „the Herring-Gull has to choose the said Mollusca for its food, I must confess 

 „that I am unable to indicate the cause". 



I cannot conjecture why Dr. Venema here opposes the greyish green 

 shell-colour, exclusively, to the light blue shell-colour, and thus leaves 

 the o//ve;'e//oiv/sA and ftroivn/s/! shell-colours wholly out of consideration, 

 notwithstanding the fact that, together, the latter two occur in at least 

 as large, if not larger, numbers than the greyish green eggs. For 

 the rest, the above quotation naturally contains nothing more than 

 an hypothesis about the effect of food on the coloration of the 

 eggshell, regarding which I wish to make the following observations. 



Whenever the light blue colour occurs, it always does so as an 

 exception only; e.g.. Otto Leege ') saw in the middle of the breeding 

 season of 1909 among about 1100 clutches, and in that of 1910 

 among about 1200 clutches, only a few light blue e^^s, and on July 

 15 none of that colour, from which would have to follow that o/i?iv 

 or^en^fl^us-individuals occasionally eat a small fish as a rare exception. 



Furthermore, — if Dr. Venema's supposition were correct — it 

 would show that Sterna fluviatilis e. g. differs strongly from L. ar^g-cn- 

 tatus physiologically in respect of the influence of food on the 



•) Ornith. Monatsschr. 1911, No. 1, pag. 44. 



