MIGRATIONS OF WATERFOWL 



about the same time as the Baldpates' departure, adult males of Blue- 

 winged Teal, Shoveller, and Gadwall also go. 



The adult Mallard drakes have a coming together in early August, and 

 these, joined by juveniles, make up the stubble aggregations that remain 

 through September and most of October. In late September some of the 

 adult Canvasback and Redhead drakes arrive at Delta from the molting 

 lakes; the bulk of these species then make their departure in mid-October 

 at about the same time as the Lesser Scaups arrive in large numbers. 



Some of these early movements are directly traceable to the breeding 

 season. The adult Pintail drakes, for example, depart shortly after they have 

 their new flight feathers ; and the end of the flightless season, of course, is 

 related in time to its beginning, which in turn hinges upon the breeding 

 schedule. It is the same in the Baldpate, and the difference in departure 

 dates between Baldpates and Pintails is about the same as the difference 

 between them in breeding schedules. The Baldpate leaves at a later calen- 

 dar date than the Pintail, but on the same physiological schedule. When 

 Mallards depart in late October or early November they have completed 

 their prenuptial molt and are in full breeding plumage; hence the timing 

 of their migration is one step further removed from the breeding cycle. The 

 same holds for the Lesser Scaup. Weller (1953a) found that nearly one- 

 third of the adult Redhead drakes he examined September 19 and 20, 1952, 

 at Lake Winnipegosis showed some soft primary wing feathers, evidence 

 of their recent flightless period. Perhaps their late-September arrival at Delta 

 is linked with their recovery from the wing molt, which comes later in these 

 diving ducks than in the Pintail. Gotz (1929), noting a general relation be- 

 tween molt and autumn migration, proposed the idea of a cycle : breeding, 

 molting, migration. Delayed breeding equals late molt; hence delayed mi- 

 gration. 



Although there seems a close tie between molt and migration in some 

 ducks, I am not attempting to give evidence showing any cause-and-effect 

 sequence underlying all fall migration. I simply wish to show how important 

 passages are hinged in time to certain physiological as well as meteorologi- 

 cal conditions. 



Some of the male aggregations hold together in flocks of their own for 

 some time after the flightless period of the eclipse, and Dawson ( 1923 ) tells 

 how adult Pintail drakes reach California during August in bands of their 

 own sex and age class. At Delta I have seen migrating flocks of Canvasbacks 

 and Redheads in late September which seemed to be made up wholly of 



108 



