CLASSIFICATION OF WATERFOWL TRAVEL 



gether. Although young-of-the-year may outnumber adults in direct mass 

 migration, these passages are essentially the movements of experienced 

 birds. By this act of travel with the adults, the juveniles, as children (in 

 geese and swans) or as unrelated companions (in ducks), accept the non- 

 genetic heritage of the traditional route, which thus becomes ancient be- 

 yond the experience of any living individual and is passed on to succeeding 

 generations by social inheritance. 



If inexperienced juveniles arrive at the wintering grounds independently 

 of experienced companions (as some do every August and September), this 

 probably is the result of the undirected wanderings of young-of-the-year, 

 vagabond travel that carries other juveniles of the same kind and at the 

 same time to marshes north or east or west of the natal range. 



Mass fall migration accounts for the direct passage of waterfowl by 

 hundreds, thousands, or, in late fall, hundreds of thousands at a time, moving 

 over a region many squares miles in extent. By such passages most individ- 

 uals of a species or, at the time of deep frost, all waterfowl may evacuate their 

 summer range overnight, leaving only cripples and a few stragglers behind. 



The biological impetus for mass fall migration is not understood. In 

 some birds, like adult drake Pintails, the movement is related to the recovery 

 from the flightless period of the eclipse ; by links with the molting schedule, 

 its timing may be traced directly to the breeding period. There is consider- 

 able variation among species in the time of departure (witness the Septem- 

 ber passage of Blue-winged Teal and the mid-October departure of Canvas- 

 back), but each kind shows a calendar regularity that differs little from year 

 to year. Species migrating in September or early October from the Canadian 

 prairies leave an abundance of food and a span of fair weather behind them. 



Though fall departure occurs at about the same time each year, weather 

 has a "trigger efEect" which influences the exact day when the mass migra- 

 tion will start. Regardless of the stage of the autumn — mild September or 

 cold November — mass fall migrations begin with the arrival of the clear 

 sky and fair weather of the anticyclone; but once started, migration may 

 overtake and continue through bad weather. 



The mass migrations of waterfowl have been poorly documented in the 

 literature, perhaps because the passages so often begin late in the day and 

 move so high that they escape notice. Mass migration is typical of many 

 other groups of birds, but here again, the record is not clear in the literature, 

 and I believe we underrate the importance of this type of traditional passage 

 in the migration of inexperienced young birds. 



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