304 U. S. NATIONAL MUSEUM BULLETIN 211 



determined after the fall molt), or, rarely, a recombination of individ- 

 uals that had previously bred in the colony. 



My study at the river-mouth colony revealed that there is no 

 period of male isolation as in a typical territorial species. The flock- 

 ing behavior of fall and winter gradually gives way as the birds asso- 

 ciate more and more in pairs. The activities of the pair are not 

 confined to any territory (except that later on there is a focus of 

 attention at the nest site) and the birds may at first carry on pairing 

 activities while grouped together in a flock. 



A number of displays and accompanying calls are used throughout 

 the breeding season. 



The ruff-out is employed by both sexes, but more frequently by 

 males. The bird holds the bill nearly horizontal, or pointed some- 

 what upward and ruffs out many of the contour feathers, especially 

 those of the head and neck, the breast, and upper tail coverts (the 

 rump feathers remain flat) ; at the same time it partially spreads the 

 wings downward and fans and depresses the tail. As the ruffing 

 and spreading reaches a climax either a squeee or schl-r-r-r-up is uttered 

 and the display immediately subsides. The notes are never uttered 

 without the accompanying display, though the latter varies greatly 

 in the extent of ruffing and spreading. When used by the female 

 it is less developed and the utterance is more subdued. The whole 

 display lasts only a second or two. It functions as a threat, but is 

 also used in mutual display between pairs, as described below. As 

 a threat it is much more frequently used by the male. 



The male pre-coitional display, employed by the male imme- 

 diately preceding his mounting the female, is a more exaggerated 

 form of ruff-out, but the bill is pointed downward and the display 

 held longer. If it is performed on the ground the wings and tail 

 may actually scrape the earth as the male approaches the female. 

 Sometimes, in this display, the male struts in a half-circle in front 

 of the female before mounting. 



In the female generalized display, the bill is held upward at a 

 slight angle; no feathers are ruffed out; the wings are held somewhat 

 out from the body, drooped, and vibrated; the tail is cocked but not 

 spread. This display is always accompanied by a series of kit notes. 

 The display has a definite attracting effect on the male and also is 

 part of the female's response to his advances. 



The female pre-coitional display, or "copulatory invitation," 

 should probably be considered a more fully developed female gen- 

 eralized display, which it resembles, except that the body is tilted 

 forward and the tail cocked at a steeper angle. It is accompanied 

 by a specific series of soft, low, tapping notes. Before the male 



