The coexistence of species: MacArthur's bird-watching

In the early to mid twentieth century, the competitive exclusion principle had been taken to mean that multiple species with similar requirements could not possibly exist in the same general area (companion paper, Grinnell 1922).  However, observations by nature enthusiasts and ecologists for years had shown that seemingly very similar species were doing just that.  Robert MacArthur's 1958 study of five warbler species, all of similar size, all insectivores, all closely related, all living in the same forests, showed that slight differences in feeding zones and feeding strategies allowed them to coexist.  The greatness of this study comes from its beautiful simplicity: one man standing for hours on end, watching birds and counting the seconds they spent in different areas of trees.
Grinnell, J. 1922. The Trend of Avian Populations in California. Science 56:671-676.

The resulting figures from MacArthur's hours of bird watching, showing the tree areas in which bird species spent their time foraging.  MacArthur 1958

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