242 



SEA-BIRDS 



Incubation Periods of Gulls and Skuas 



MAX. WING-LENGTH CLUTCH INCUBATION 



Sabine's, Xema sabini 

 Black-headed, Larus ridibundus 

 Long-tailed skua, Stercorarius longicaudus 

 KiTTiWAKE, Rissa tridactyla 

 Arctic skua, Stercorarius parasiticus 

 Common, Larus canus 

 Great skua, Catharacta skua 

 Lesser black-back, Larus fuscus 

 Herring-, Larus argentatus 

 Glaucous, Larus hyperboreus 

 Great black-back, Larus marinus 



in cannibalism, by seizing and carrying off, or devouring at the nest, 

 eggs and chicks left temporarily exposed by the absence of the owners 

 or by the intrusion of visitors. In gannetries, great and lesser black- 

 back and herring-gulls pounce on unguarded gannet eggs and small 

 chicks; in gull colonies the same gulls seize and devour the eggs and 

 chicks of their own as well as other species. When the eggs are lost 

 from any of these causes the female will lay a second clutch after 11- 

 12 days, provided the first clutch has not been long incubated. In 

 colonies where regular egg-collecting takes place for human food 

 (e.g. in black-headed, herring- and lesser black-backed gull colonies 

 in the British Isles) fresh eggs are collected up to the beginning of 

 July. If the eggs are taken as soon as laid the female gull is capable of 

 protracted laying (Paludan, 1951), and egg-collectors assert that she 

 will lay up to fifteen if each egg is collected during the twenty-four 

 hours after laying; but that the nest-site may be changed during this 

 long laying period. Many thousands of gulls' eggs, particularly 

 those of the black-headed species, are collected and sent to large 

 consumer centres, notably London, where they are more often than 

 not sold as seasonal delicacies, and until recently were passed off as 

 "plover eggs." 



So long as one tgg of the original clutch remains the female gull 

 is content to continue incubation. Experiments have shown that a 

 gull will brood almost any object which is only approximately the 

 same size and shape as its own egg although some individuals are 

 more discerning than others (see Kirkman 1937, pp. 82-99). Large 

 gulleries in fact have been convenient grounds for several kinds of 

 experiments. Kirkman moved the eggs from a black-headed gull's 



