TERNS AND SKIMMERS 257 



pirouetting in a perfect circle. If there is no nesting material handy, 

 within a few yards, a nest may not be lined at all. The Marples made 

 interesting experiments in which they proved that the lining of the 

 nest (with a variety of material) depends entirely upon what is avail- 

 able in the immediate environment. Birds which were incubating 

 eggs in shallow scrapes in the sand, with no nest lining whatsoever, 

 at once built a substantial rampart around the eggs when material 

 (scraps of jetsam) was placed within a few feet of the scrape. They 

 also proved that terns have a very precise sense of geographical position. 

 When the Marples buried the nest so that the site could not be dis- 

 tinguished from a wide environment of smoothed-over sand, the owner 

 of the nest alighted above the hidden eggs, quickly dug down, dis- 

 covered the eggs, and brooded them after scratching them clear of 

 sand. Nor were terns deceived by extraneous alterations to the 

 environment of the nest, even when the nest was again buried in sand. 

 Other experiments showed that when a nest was moved fifteen feet 

 away from high tide, the owner found it, in its new position, after only 

 a little hesitation. Terns would also follow their nests for a consider- 

 able distance if they were moved a few feet at a time. When the eggs 

 of terns were painted in various bright colours (terns accepted eggs 

 painted bright yellow, but were somewhat put off when the eggs 

 were painted bright red or blue), or their shape altered by the addition 

 of plasticine, the owner would peck at the eggs as if to test their 

 genuineness, after which it would settle to brood as if nothing had 

 happened. In these experiments the terns showed themselves more 

 perceptive and tolerant than gulls. This perception is perhaps under- 

 standable in a species nesting so close to the edge of the sea: high 

 tides may flood the nesting area at times, or at least change the super- 

 ficial environment, by depositing seaweed and jetsam, or by altering 

 the contours of pebbles and sand; and so an ability to deal with these 

 hazards is important for the tern's survival. The ability to defend 

 the nest against shifting sand on windy days is probably instinctive. 

 When storms blow the brooding bird will constantly scratch accumu- 

 lating sand away from the nest, or will protect the young chick by 

 nursing it carefully, so long as the sand is on the move. 



Incubation is by both sexes, but the female is believed to take 

 the larger share. With most species, incubation begins with the first 

 egg. There are generally three, sometimes two, and less often four eggs 

 laid at intervals of about forty-eight hours. Nests have been recorded 

 with up to ten eggs, but the Marples have shown that where nests 



