200 BULLETIN 113, UNITED STATES NATIONAL. MUSEUM. 



average 47 by 34 millimeters; the eggs showing the four extremes 

 measure 51 by 35.5, 49 by 36, 44 by 33.5, and 46 by 32.5 millimeters. 



Plumages. — The downy young is " pinkish buff " or " cream buff " 

 on the upper parts, shading off to white on the breast and belly ; some- 

 times it is almost unspotted, but usually it is more or less mottled, 

 streaked or spotted with black on the back and on the head. The 

 juvenal plumage is acquired first on the wings, then on the scapulars, 

 back and breast. This plumage is largely " pale gull gray " on the 

 back, with a decided brownish suffusion, which is due to broad edg- 

 ings of "snuff hrown" and "clay color," deepest on the back and 

 most extensive on the scapulars. These browns gradually fade out, 

 before the bird is fully grown, to produce a soft mottled effect on 

 the upper parts, pale buffy tints on a pale gray ground color. The 

 partial postjuvenal molt begins early in September, and the change 

 into the first winter plumage is rapid. The mottled or dusky marked 

 feathers of the upper parts are replaced by the "pale gull gray" 

 feathers of the adult and the heads become whiter. The young bird 

 in its first winter plumage is much like the winter adult, but can be 

 recognized by its slightly smaller bill and by having more dusky 

 streaks on the crown and cervix, as well as traces of dusky in the tail. 

 I have not been able to trace the first prenuptial molt, but infer that 

 it is complete, and that it probably produces a plumage indistinguish- 

 able from the adult. I have not been able to find any spring birds 

 showing any signs of immaturity. 



Adults probably have two complete molts each year, as in the 

 other terns. They are in full nuptial plumage when they arrive in 

 March ; the postnuptial molt occurs in August and September. The 

 adult winter plumage is like the nuptial; except that the head is 

 wholly white, with more or less slaty-gray in the orbital and auric- 

 ular regions. 



Food. — All observers seem to agree that the gull-billed tern is 

 mainly, if not wholly, insectivorous in its feeding; habits. Wilson 

 (1832) says: 



This new species I first met on the shores of- Cape May, particularly over 

 the salt marshes, and darting down after a kind of large black spider, plenty 

 in such places. This spider can travel under water, as well as above, and 

 during summer at least, seems to constitute the principal food of the present 

 tern. In several which I opened the stomach was crammed with a mass of 

 these spiders alone. These they frequently pick up from the pools as well as 

 from the grass, "dashing down on them in the manner of their tribe. 



Audubon's (1840) spirited drawing illustrates this species in pur- 

 suit of an insect. He says: 



I believe that these birds never immerse themselves in the water, as other 

 terns are wont to do ; nor do I think that they procure fish, as, on examining 

 a number of individuals, I never found any other food in their stomachs than 

 insects of various kinds, 



