On Size in the Avian Order Tubinares. 315 



XV. — Size in the Avian Order Tubinares. By J. T. 

 Nichols, American Museum of Natural History, New- 

 York City. 



(Text-figure 4.) 



It is believed that the very great range in size among 

 the members of the Avian order Tubinares, consisting of 

 pelagic birds of more or less similar habits, and particularly 

 numerous in the southern hemisphere, is due to intra-ordinal 

 competition in absence of extra-ordinal competition. 



Birds of this group feed off shore on fish, other marine 

 life, and flotsam and jetsam, which they obtain at the 

 surface. An exception, the genus Pelecanoides, which has 

 acquired the diving habits and general character of the 

 AlcidaK of the north^, and therefore does not form so good 

 a unit with other members of the order, has been ex- 

 cluded from the present discussion. To compare the sizes 

 of the remaining species, the British Museum Catalogue, 

 1896, has been referred to, and the total length minus the 

 tail has been used. Counting the length of the tail-feathers 

 would give an erroneous idea in the smaller species, the 

 comparative size of which would be greatly changed by the 

 length or shortness of this member. Measurements thus ob- 

 tained have been divided into groups with a one inch radius, 

 and plotted in a line the height of which shows the number 

 of species of the different sizes (fig. 4). For purposes of com- 

 parison a similar line plotted for the Gull-Tern order 

 Longipennes rises abruptly to a single peak and falls almost 

 as abruptly., but that of the Tubinares is much more extensive 

 and shows at least three significant peaks, or predominant 

 sizes — that of the small Mother-Carey^s Chickens, that of the 

 medium-sized Shearwaters, Fulmars, etc., and that of the 

 large Albatrosses. 



The Tubinares are pelagic birds ^ar excellence, and almost 

 all the aerial pelagic birds of the southern hemisphere 

 below the Tropic are Tubinares. To one familiar with their 

 breeding and feeding liabits, it is perfectly obvious that the 

 great range in size relieves pressure of competition. The small 



