Niche Convergence in Empidonax Flycatchers

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Abstract
The willow flycatcher (E. traillii) and western flycatcher (E. difficilis) are sympatric during the breeding season in southeastern Washington [USA]. At Kamiak Butte they occupy different habitats (the willow flycatcher in ninebark brush; the western flycatcher in Douglas fir) while at Palouse River they occupy the same habitat (floodplain forest). These species were more alike in foraging ecology when in the same habitat, than when in different habitats. In floodplain forest they were very similar in their foraging ecologies, contrary to the predictions of competition theory. Food superabundance, interspecific territories, habitat differences, and limiting factors other than food are discussed as possible explanations for this situation. Interspecific competition has not, in this case, resulted in a division of the food supply.