CLASSIFICATION OF WATERFOWL TRAVEL 



are in passage over Delta we hear the contact calls of birds going over 

 steadily; and Lincoln (1939:77) gives the account of one observer (watch- 

 ing with telescope focused on the moon ) who estimated that night migrants 

 passed his point at the rate of 9,000 birds an hour. 



DIASPORIC MIGRATION 

 The molting shift 



The molting shift is the mass transfer of mature ducks, mostly drakes, 

 from the nesting ranges to marshes or lakes where they pass the flightless 

 period of the eclipse molt. The journeys begin at the close of the sexual 

 cycle when the male abandons his incubating hen, and end when he ar- 

 rives at the locality where the wing feathers are shed. The shift may be to 

 another part of the same marsh in which he had his breeding territory or 

 to a place several hundred miles away. The transfer shows no constant geo- 

 graphical direction; some, like the Pintails that go to the Bear River Marshes 

 of Utah or the Baldpates that go to Delta, travel toward the winter range, 

 but others go north or east or west. This is an ecological rearrangement of 

 the population, with a bias toward large marshes on the part of river ducks 

 and toward lakes (sometimes the ocean) on the part of the diving ducks. 

 The shift does not take place in breeding geese or swans, which spend the 

 flightless period with their young near the nesting place ; but there may be 

 a molting flight in nonbreeding yearlings and barren adults (Taylor, 1953, 

 and Scott, 1953, for Pinkf ooted Geese ; Klopman, unpublished, for the Can- 

 ada Goose). 



