286 RELATION OF TERRITORY TO MIGRATION 



the eleventh year, we have 59,048 Yellow 

 Buntings occupying 29,524^ acres or 46 square 

 miles. This, then, will be the result of the 

 operation of the impulse, providing that all the 

 individuals survive and that no complications 

 supervene. 



But of course complications are numerous, 

 some of which retard while others accelerate the 

 rate of expansion. These complications arise 

 from various sources — in the first place from 

 natural enemies which prey upon the birds or 

 upon their eggs ; in the next place from climate 

 which, if it happens to be unfavourable, may 

 mean that food is scarce and that only a small 

 percentage of the young survive; and lastly 

 from rivals — and by rivals I mean closely related 

 forms that require a similar station and similar 

 food — which, by occupying available ground, 

 may check expansion, or, by forcing a continua- 

 tion of the search, may widen it. 



Now when individuals fail as many do fail 

 in their initial attempt to secure territory, the 

 activity of the impulse still persists, and there is 

 no control over the direction in which the bird 

 continues to wander whilst in search of its end. 

 Some therefore seek in this direction, others in 

 that ; some wander inwards into inhabited areas 

 and fail to find accommodation, or, according to 

 the relative strength of their impulse, perhaps 

 succeed and so set free a new competitor, 

 others wander outwards into country unin- 

 habited by the species. These latter we will 

 call "pioneers." They may find accommoda-. 



