The passing of a black crow causes some of the terns to desist from their piscatorial occupation, in order to mob the intruder. This means that there are terns’ eggs or young ones in the vicinity. Many species of birds betray the presence of their nests by displaying unusual pugnacity at the breeding season. To discover the eggs or young of the terns is not a difficult matter. It is only necessary to land upon the nearest island between which and the river bank there is a sufficient depth of water to prevent jackals fording it. If the island contain eggs or young ones, the parent birds will make a hostile demonstration by collecting overhead and flying backwards and forwards, uttering their harsh cries, and the nearer one approaches the nest the more clamorous do they become. In this manner they unwittingly inform the nest-seeker whether he is getting “hot” or “cold,” to use the expressions employed in a nursery game.
The terns which breed on islets in Indian rivers do not appear to do much incubating in the daytime. There is no need for them to do so, because the sand grows very warm under the rays of the sun. Moreover, the only foes to be feared are the crows and the kites, which the terns can keep at bay more effectually when on the wing than while sitting on the eggs. Very different is the behaviour of the sea terns, whose eggs are liable to attack by gulls and crabs. For safety’s sake the sea terns lay in large colonies, and, to use Colonel Butler’s expression, sit on their eggs “packed together as close as possible without, perhaps, actually touching one another.” He once came upon the nests of a colony of large-crested terns (_Sterna bergii_). The sitting birds did not leave their eggs until he was within a few yards of them. Having put them up, he retired to a little distance. “No sooner had I done so,” he writes, “than both species [i.e. the gulls and terns] began to descend in dozens to the spot where the eggs were lying. In a moment a general fight commenced, and it was at once evident that the eggs belonged to _Sterna bergii_, and that the gulls were carrying them off and swallowing their contents as fast as they could devour them.” River terns do not construct any nest. They deposit their eggs on the bare, dry sand. The eggs have a stone-coloured ground, sometimes suffused with pink, blotched with dark patches, those at the surface of the shell having a sepia hue, and those deeper down appearing dark greyish mauve. The eggs, although not conspicuous, may, without difficulty, be detected when lying on the sand. Their colouring would seem to be adapted to match a stony, rather than a sandy environment, but the fact that the colouring of the eggs is but imperfectly protective does not much matter when the latter lie on a sand island, to which but few predaceous creatures have access; the watchfulness of the parent birds more than compensates for the comparative conspicuousness of the eggs.