226 MIGRATION 



with the south winds and rising temperatures which normally accom- 

 pany them." 



Each of the aspects is characterized by a special group of insects 

 and other invertebrates from which the migrants take enormous toll. 

 In addition, the summer residents, which breed in the community, 

 require untold numbers of insects and other organisms to feed their 

 young as well as themselves, and a heavy demand continues as the 

 juveniles are added to the local population. The return of birds from 

 the north is in part late estival, but is chiefly serotinal or autumnal. 

 The population is again greatly augumented during this period, with 

 a corresponding increase in the number of insects, seeds, and fruits 

 devoured. By contrast, the hiemal or winter aspect exhibits relatively 

 few species, and the pressure from coaction is consequently greatly 

 lessened. 



Orientation and Sense of Direction. Like the causes of migration, 

 opinions as to the choice of direction and route have run the entire 

 gamut of possibilities from the attitude of insoluble mystery to the 

 view that birds use the same faculties as other animals. The extreme 

 form of the first assumption is exemplified by Cathelin (1920), who 

 regards migrants as automatons driven by equinoctial electromagnetic 

 currents. The popular view is probably represented by Lucanus 

 (1922), who concludes that the bird requires no particular guidance, 

 but follows an instinct that determines direction automatically. In 

 spite of an open-minded and comprehensive treatment of this question 

 (1926), Thomson feels constrained to the conclusion that there is some 

 inherited memory of path and goal. On the other hand, Nicholson 

 (1929) is of the opinion that "a great part of migration is performed 

 simply by the travelers keeping to one appointed direction instead of 

 traveling round and round one little area." Grinnell (1931) is even 

 more definite in expressing the view that migration employs the 

 faculties of everyday in the statement, "No so-called sixth sense, or 

 sense of direction has to be invoked to account for birds finding their 

 way during long seasonal migratory flights any more than in their 

 courses of daily movement." 



To these two views the ecologist can make no dissent. Testimony as 

 to this can be drawn from the training of "homing" pigeons (Lincoln, 

 1927) , from the experiments of Watson and Lashley, and in particular 

 from those of the Peckhams (1887) on the homing faculties of wasps. 

 Fabre's studies had led him to the conclusion that wasps were guided 

 in their return by a special sense not to be explained (1879), a view 

 that gained much support, but was decisively disproved by experi- 

 ments of the Peckhams, who demonstrated conclusively that memory 



