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                                     January 15, 1958
Barro Colorado.

THE GREGARIOUSNESS OF THE PLAIN TANAGER, AND ITS POSSIBLE ROLE IN MIXED FLOCKS.

The Plain Tanager is the most remarkable tanager around here.

It can be distinguished from all its local relatives by several characters, i.e. its dull and simple color pattern, its extreme gregariousness, its extreme restlessness and mobility, and the extreme frequency and rapidity of its Call Notes.

  The color pattern is quite unlike that of most other tanagers and honeycreepers (even the most emberizine-like forms); but it is [[underlined]] not [[underlined]] very cryptic. (Female Green Honeycreepers and Dacnis are probably less conspicuous in the tree-tops than Plain Tanagers.)

  The restlessness is peculiar, insofar a as flock will often leave a desirable food source (e.g. Cecropia) before the food is exhausted, fly to another source (e.g. another Cecropia) which is essentially identical with the first, and then return to the first source again.  All this in a matter of minutes.  The Plain Tanagers often seem to move just because they "like" moving --- when it isn't really "necessary" from a practical point of view.

  The restlessness and the "exaggerated" Call Notes (and the bright wing-patch) all seem to be "designed" to induce, i.e. take advantage of, the "following response" of other birds.

   These characters must help to maintain the cohesion of a flock of Plain Tanagers; but they may also help to attract birds of other species, keeping them in the vicinity of the Plain Tanagers. (If it is advantageous for the Plain Tanagers to aggregate in groups --- as it obviously must be --- then it may well be advantageous for them to increase the effective size of their flock by attracting all the other birds of roughly similar feeding habits in the neighborhood.)

  Blue and Palm Tanagers (whose coloration is also comparatively dull and simple) may become "integral" members of a Plain Tanager flock.  Blue Honeycreepers and Ultramarine Dacnis may follow a flock. Other species, including the Green Honeycreeper, the Scarlet-thighed Dacnis, the Golden-maned Tanager, and at least one species of oriole, may hang around in the vicinity of a flock.

  It should be noted that almost all the birds more or less closely associated with the Plain Tanagers here are largely blue and/or green (usually with some black), and that many of them are brighter or more intricately patterned than the Plain Tanagers.  This might suggest that the peculiar coloration of the Plain Tanager has been evolved because it is more or less "neutral".  It can't look too "strange" to the other species; but it is still quite distinct.  It won't provoke escape; nor will it release sexual or aggressive responses.

(It is possible that the Blue and Palm Tanagers have evolved in the same direction as the Plain Tanager; but they seem to be less specialized.)