302 STUDIES IN BIRD-MIGRATION 



numbers, and next to the Puffin is the most abundant 

 species. The earliest date for its arrival is loth May, 

 and the latest 17th May. It begins to leave late in 

 August, and by the middle of September all have gone. 



Birds from further north occur on passage late in 

 September, always during the night. 



Larus ridibundus. Black- headed Gull. — Is generally 

 seen for a day or two in the middle of summer 

 (Tomison). One appeared on 6th May 19 10. 



Larus fuscus. Lesser Black-backed Gtdl — Not a 

 common visitor. It usually appears in May, and does 

 not remain long (Tomison). 



Larus marinus, Great Black-backed Gull, — About a 

 dozen pairs are resident (Tomison). 



Larus argentatus, Herring Gull. — Some twelve 

 pairs nest and remain all the year round. Many visit 

 the island during the herring season (Tomison). 



Larus glaucus, Glaucous Gull. — Fairly common 

 during November, December, and January. Immature 

 birds arrive in November, adults in December (Tomison). 



Larus leucopterus, Iceland Gull. — According to 

 Mr Tomison, it appears in November, December, and 

 January. 



RissA tridactyla, Kittiwake. — A few pairs nest, and 

 many visit the island on passage before and after the 

 breeding season. The breeding birds arrive at the end 

 of March, and leave early in August. A great number 

 appeared on ist June 1902, on their way north ; and 

 large flocks are seen on their return in July and August. 



Stercorarius parasiticus, Btiffons Skua. — An 

 adult male wa's captured on 9th June 1908, and sent to 

 me. 



Procellaria pelagica, Stor7n-PetreL — Many nest 



