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[[circled]] 13 [[/circled]]
s occupying new areas.
But it probably is strongly disadvantageous, in many cases, for birds not occupying new areas. Thus, there will be strong selection against it everywhere except on frontiers.
Whether or not gregariousness develops in any particular population of a suitable species must depend upon the balance of selection pressures for and against it. Populations with frontiers on three sides tend to develop gregariousness. Populations with frontiers on only one side usually do not.
The extreme development of gregariousness in the Western Cordillera of Colombia can be explained on the frontier hypothesis. The temperate and sub-tropical areas of the cordillera are eventually a series of small islands. Any population in any one of these islands will include a relatively large number of individuals on the borders and a relatively small number of individuals "inside", away from the borders.
The [[underline]] crucial [[/underline]] area will be the Central Cordillera of Colombia.
If the mixed flocks of the Central Cordillera are less highly developed than those of the Western Cordillera, then it seems quite likely that the "frontier effect" may explain almost the whole in the variations in gregariousness throughout the Andes.
If the mixed flocks of the Central Cordillera are as highly developed as those of the Western (or, better yet, more highly developed), then it may be necessary to involve another (additional) phenomenon or process. Possibly the frequency of mixed flocks in any given area depends upon the frequency with which that area has been invaded by other species - [[underline]] invaded by species which have had to cross over [[/underline]]