BIOLOGICAL ASPECTS 319 



but I doubt if they were late enough to have permitted their parents 

 to rear an earlier brood. Simon Paneak has related to me that 

 occasionally they find late nestling redpolls which have perished before 

 cold weather because the social attraction of the preautumnal flocks 

 of redpolls distracted the inclination of the parents from further care 

 for their young. The Eskimos believe that seasonal change in parental 

 attitude rather than immediate cold weather dooms the late-hatched 

 birds to destruction. I agree with the philosophical conclusion based 

 on Eskimo experience, that in arctic life a family is unlikely to sur- 

 vive behavior at variance with the regular seasonal program of a 

 population. Pitelka (1954) remarked that near Barrow late broods 

 are doomed because premigrational molting terminates parental care 

 of young. In his study of the behavior of snow buntings in Green- 

 land, Tinbergen (1939) remarked that second broods were rare and 

 he did not allude to the possibility of their successful contribution to 

 the population. 



Migration and the Reproduction Rate 



Loons and some other species do not produce more than two eggs 

 from each pair. Since visible casualties destroy some eggs and young 

 birds, I would estimate that these migrant populations returning 

 from the Arctic in autimm do not include more than one young bird 

 for each pair of adults which had arrived in spring. These species 

 could not preserve stable numbers unless at least two-thirds of the de- 

 parting migrants returned in the next spring. 



Among the broods of sandpipers, plovers, and robins, I have ob- 

 served two is a common number to survive into late summer and ap- 

 proach adult size while the families are still distinguishable. 

 Considering that some families are wiped out and that some adults do 

 not reproduce at all, I doubt if the arctic populations of these species 

 are twice as numerous at the start of the southbound migration as 

 they were when they arrived in spring. During the eight or nine 

 months of their absence from the Arctic and in migratory flights over 

 thousands of miles an average mortality of less than 50 percent is 

 provided for by the new birds raised in the arctic summer. 



It might be possible for the species like robins, which nest over 

 extensive ranges, regularly or occasionally to recruit from the sections 

 of their populations raised in warm climates in order to maintain the 

 arctic sections. But there is no nesting reservoir in milder climates 

 from which to recruit bird populations nesting exclusively in arctic 

 regions, and yet there is no sign that they are less stable than the popu- 

 lations of species which nest over a wide range of latitudes. 



Estimates of the young birds reared are, of course, conjectural, 

 but a population of birds reared in the Arctic seems unlikely to start 



