MIXED SPECIES FLOCKS IN PATAGONIAN FORESTS WITH REMARKS ON INTERSPECIES FLOCK FORMATION

Whether or not an interspecies flock will form after the encounter of 2 spp. will depend not only on the high degree of gregariousness of one of the sp. but also on the presence of a minimum set of behavioral prerequisites possessed by the 2nd sp. In a way, one is dealing here with a behavioral preadaptation. Each sp. could be considered as a "behavioral structure" that evolved primarily independently of the other, i.e., in response to different sets of selection pressures. If the 2 structures meet, their harmonious fusion into essentially one behavioral complex (mixed flock) will occur only if a threshold is reached at the time when a selection force acts to make the complex a balanced functional unit. In this model, selection forces both before and after the threshold are mostly ecological. Once 2 spp. have formed a mixed flock, whatever behavioral and morphological changes may take place during the subsequent evolution of the flock will be primarily in response to selection pressures relevant to the flock as such.