l62 SEA-BIRDS 



houses, rocks, and the observer himself — but never on fairly clear or 

 clear nights. 



The Manx shearwater is loudest and most vociferous at its breeding 

 island on dark nights and when there is dense mist. This, we believe, 

 is partly explained by the fact that the incoming bird, unable to 

 recognise immediately the environment of its burrow in thick weather, 

 screams almost continuously so as to attract a response from its mate 

 on duty in the burrow, who answers and so guides the arriving bird. 

 Glauert (1946) believes this to be the explanation in the case of the 

 little shearwater {Puffinus assimilis), in Western Australia where on 

 Eclipse Island these birds called loudly on dark nights and almost 

 not at all on moonlit nights. There was the same amount of activity 

 under both conditions; the little shearwater on Eclipse Island has no 

 predatory enemies. But on other islands where adult shearwaters and 

 petrels are slain by resident gulls and hawks and owls * there is notably 

 less activity on fine clear nights, as well as almost complete silence 

 from the few incoming birds. Obviously if the mate of the bird is 

 already at the nest and answers the cry of the homecoming bird on 

 a dark night there is direct guidance. But why should the incoming 

 bird scream on a dark night, when (e.g. early in the season) there is 

 no mate to reply? We put forward the suggestion, cautiously and 

 merely as a possible line of investigation: that just as some animals 

 (e.g. bats) are extremely sensitive to the echoes of sound-waves and are 

 able to judge accurately the short distance they are in flight from 

 objects they cannot see; so the homing sea-bird, on a dark night, 

 may do so by receiving back from the rocks and cliffs and uneven surface 

 of the breeding-ground echoes, perhaps of its voice"]" even its own 

 wing-beats. 



To return to long-distance homing and migration, it would now 

 appear that the tubenoses navigate great distances over the sea with 

 the aid of the sun and a time-sense which enables them to compensate 

 for the diurnal movement of the sun; as well as, after crossing the 

 Equator at the equinox, a return to early summer. Having once 

 made the annual migration both ways (we have referred to an arctic 

 tern, ringed as a juvenile 8 July 1951 in the Christianshab district of 

 West Greenland which travelled over eleven thousand miles across 



* At Skokholm and Skomer little owls prey upon storm-petrels. 



t As this book lay in page-proof we heard of the successful proof by D. R. Griffin 

 and W. H. Phelps Jr. that the oil-bird Steatornis caripensis of the Venezuela caves 

 navigates in darkness by echo-sounding, like bats. 



