LIFE HISTORIES OF NORTH AMERICAN GUIAS AND TERNS. 137 



of immaturity during the third year. The new primaries in such 

 birds are black, but they have only a faint suggestion of the subter- 

 minal white spot on the outer primary or none at all; undoubtedly 

 these spots increase in size with the successive molts. There is 

 more or less dusky in the tertials, and the tail has the black sub- 

 terminal band more or less clearly indicated. The remainder of the 

 plumage and the bill is now like the adult. Such birds would be- 

 come fully adult at the age of 3 years. I have one bird in my series 

 which is quite heavily mottled with dusky on the breast, but is other- 

 wise fully adult. 



The complete postnuptial molt of both adults and young occurs 

 mainly in August and September, but I have seen the molt be- 

 ginning as early as June. The partial prenuptial molt, involving 

 the contour feathers only, occurs mainly in March. The winter adult 

 is similar to the spring adult, except for a few narrow streaks of 

 dusky on the crown and hind neck; these are less in evidence in 

 older birds. 



Food.— The feeding habits of this species make it as fully bene- 

 ficial as any of the gulls. . Throughout the agricultural regions of 

 the western plains, where it is more abundant, it is often seen in the 

 spring following the plow, picking up worms, grubs, grasshoppers, 

 and other insects. It also does effective work by feeding on field 

 mice and other small rodents. Dr. J. A. Allen, according to Baird, 

 Brewer, .and Eidgway (1884), states in regard to their feeding habits 

 in Salt Lake Valley : 



At the period of his visit these birds spent much of their time on the sand 

 bars of Weber River, and at certain hours of the day rose in the air to feast 

 on the grasshoppers, on which they, seemed at this time almost wholly to sub- 

 sist. The stomachs of those gulls that were killed were not only filled with 

 grasshoppers, but some birds had stuffed themselves so full that these could 

 be seen when the birds opened their mouths. And it was a curious fact that 

 the gulls captured the grasshoppers in the air and not by walking over the 

 ground, as they have been said to do. Sailing around in broad circles, as though 

 soaring merely for pleasure, the birds seized the flying grasshoppers as easily, 

 if not as gracefully, as a swallow while in rapid flight secures its prey, of smaller 

 insects. 



I have seen ring-billed gulls hovering over a flock of feeding red- 

 breasted mergansers and darting down at them as they rose to the 

 surface. They were apparently trying to rob them of or make them 

 drop some of the fish they had caught. 



We found this and the foregoing species frequenting regularly the 

 garbage dumps on the outskirts of the prairie towns and acting as 

 scavengers along the shores of the lakes in Saskatchewan. On the 

 seacoasts it does its part with other species in cleaning up the floating 

 refuse in our harbors, and gathers in large numbers where garbage 

 is regularly dumped, feasting on the miscellaneous diet it finds. ., It 



