266 SEA-BIRDS 



the colonies where they were bom, but others move to more distant 

 terneries. Palmer (1941b) states that the common tern takes usually 

 three years to reach breeding age. We have mentioned the complete 

 removal of some terneries to a new site in an apparently whimsical 

 fashion. It has been suggested that persecution by man is less important 

 as a factor influencing the change of breeding ground than shortage 

 of food supply, and that colonies suddenly reappear and breed in 

 abandoned, or take up, sites, when the fishing happens to be good 

 in that district. But these statements are very difficult to substantiate. 



The annual moult takes place from July to December; it is of 

 necessity gradual, since it is during the moult that terns perform their 

 great migrations to the south. They do not, however, necessarily 

 move south immediately after the breeding season. Ringing has proved 

 that many individuals wander in the opposite direction. Juvenile 

 Sandwich terns may fly for distances up to 350 miles northwards from 

 their breeding quarters. This is the time of year when terns are wide- 

 spread along the North Atlantic seaboard, appearing on shores far 

 from breeding grounds, especially where there is an abundance of 

 small fish. The arctic tern ranges the whole width of the North Atlantic 

 during July, August and September (Rankin & Duffey, 1948). Its 

 wanderings (see Fig. 23, p. 142) are astounding: it makes a round trip 

 during the year of some 22,000 miles, or, during actual migration, about 

 150 miles per day in a straight line — if we make the generous allowance 

 of 20 weeks for the period of both migrations. The main northward 

 migration is believed to be speedy and relatively brief, consistent with 

 the mass arrivals at breeding grounds. The autumn migration, as we 

 have indicated, is leisurely and drawn out. 



Early in the spring there is a second complete moult, when the 

 adult tern acquires its full breeding plumage. In their first summer 

 (a year old) young sea-terns generally lack the full black hood, but 

 may breed in this plumage (skins of breeding birds in this plumage 

 are in the British Museum), although probably the majority do 

 not, but instead wander northwards more slowly than mature birds, 

 and, as the Marples point out, arrive at the breeding ground too late 

 to do more than familiarise themselves with the site. Lockley (1942, 

 p. 214) found non-breeding common terns roosting on the Desertas, 

 Madeira, in mid-July; this would be near the southern limit of their 

 breeding range in the Atlantic. The marsh-terns migrate within a 

 shorter range of latitude, seldom flying farther south than the tropics 

 in winter. 



