Birds in the Bush

The late Robert MacArthur, one of the most distinguished ecologists of this century, had a lifelong interest in birds. One of his earliest scientific works was based on his observation that keen ornithologists seemed to have an "intuition" about which species of birds would be found in a given habitat. He reasoned that if a birder could predict that Northern Parulas, Ovenbirds, or Red-eyed Vireos would be found in a given woodlot, then he or she must have based the prediction on a visible feature or combination of features of the woodlot. MacArthur endeavored to discover what those features were and to see, for a start, if they could be used to forecast, not exactly which species of birds would be found in a woodlot, but how many different species would live there.

Using his own experience as a bird watcher, he started with the hypothesis that bird species diversity had something to do with the vertical structure of the vegetation -- whether most of the foliage was concentrated near the ground, up high, evenly distributed, or whatever. It was a reasonable starting point because, after all, some birds are associated with open fields and others with mature forests. One does not seek Worm-eating Warblers in prairies or meadowlarks on heavily wooded slopes. To test his hypothesis, MacArthur developed a quantitative index for the distribution of vegetation density from the ground to the tops of the trees (if any). He called that index "foliage height diversity."

To calculate the index, a white board is mounted at different heights on a pole, and the proportion of it obscured by leaves at each height is recorded. The proportions are combined into a single number that is high if roughly the same amount of vegetation is found at each height (grass, shrubs, and trees intermixed), and low if the foliage is concentrated at a single height -- as in a grassland or in a forest with no undergrowth.

MacArthur counted the bird species and calculated his index in a series of habitats. He found that the diversity of bird species was proportional to the index of foliage height diversity. He had made an important discovery. From an avian point of view, the physical structure ("physiognomy") of a plant community (how the foliage is distributed vertically) is often more important than the actual species of plants making up the vegetation.

Subsequent work by ecologists John Wiens and John Rotenberry has shown that, on a more local scale, floristic composition can hold the key to bird diversity. Association with particular floras appears to be largely a result of the differential ability of plant species to provide food for birds. The basis of the association is obvious for fruit, seed, and nectar feeding birds, but less so for insectivores. However, different kinds of plants can support quite dissimilar faunas of insect herbivores, and thus be more or less attractive to warblers, flycatchers, and the like.

MacArthur's discovery was the kind of principle that science seeks: a simple pattern underlying seemingly great complexity, and one that stimulates research that leads to further understanding of how nature works.

SEE: Habitat Selection; MacArthur's Warblers; Bird Communities and Competition.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.