The Old World warblers occupy an astonishing variety of habitats, from montane and riparian forests to arid scrublands to marshes and river floodplains to city parks and backyards. Sylviids are found in all extremes of water availability with the exception of open water and harsh deserts. The family is represented at a wide range of altitudes, from lowlands at or near sea level to montane forests and dwarf scrub as high as several thousand meters. Considerable habitat adaptability is

Blue-gray gnatcatcher (Polioptila caerulea) chicks in their nest. (Photo by R. & S. Day/VIREO. Reproduced by permission.)

demonstrated by the varied habitats occupied by Acrocephalus warblers that have colonized oceanic islands.

Habitat partitioning is widespread among sylviid warblers. An example of foraging height segregation is found in the Sylvietta crombecs. The long-billed crombec is restricted to undergrowth in areas of sympatry with red-faced and red-capped crombecs. In other areas, the long-billed crombec is found at higher levels in the forest, demonstrating that competition limits the ability of this species to fully exploit its potential niche. Habitat partitioning also occurs in syntopic species of Sylvia, but there is considerable overlap, leading to interspecific territorial interactions.

Exploitation of topological niches within a habitat has been suggested as an important step in ecological and morphological divergence of closely related species. Adam Richman and Trevor Price have shown, in a 1992 study, that such a scenario has apparently occurred among a group of sympatric Phylloscopus warblers in the Himalayas. A related phenomenon is replacement, the presence of closely related species in different habitat types, with little overlap. Replacement is essentially habitat partitioning on a larger scale. Many Afrotrop-ical genera contain closely-related species that occupy similar ecological niches in dissimilar habitats.

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