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II It is possible that mixed flocks near 9000 ft, now, are smaller and less specialized than the flocks at slightly lower elevations now. (See, especially, this morning's observations of Sooty Thrushes.) But the 9000 ft birds certainly do (still) form flocks with appreciable frequency. It is too early for me to tell if their flocks are "depressed", or "reduced", or otherwise altered, by bad weather.

III It has just occurred to me that two of the tanagers which occur in mixed flocks here most frequently, i.e. the Blue & Orange and the GT, have rufous or chestnut vent regions. Is this an adaptation to induce following responses (as it seems to be in some related species)???

IV Most of the species observed today played the social roles that I expected. But I was surprised that PL's were not more conspicuous in flocks. And even more surprised to find Agriornis-types as leaders. (This may be another area - like the Eastern Cordillera of Colombia - where there are more natural followers than natural leaders.)

V GENERAL. I must try to get my ideas in order. Let us assume that mixed flocks are absent from the central Andean area, but present along both the northern and southern frontiers. And highly developed, also, in the Western and Central Cordilleras of Colombia.

I think we may also assume that the formation of mixed flocks is a "frontier effect." Advantageous to individuals living on or near a frontier (probably because frontiers tend to move back and forth, and gregariousness is advantageous to bird