Effect of Sympatry on Meadowlark Vocalizations

Abstract

When the ranges of two similar species partially overlap, differences between the two species are usually greater in the areas of overlap than where they are separate. This is character displacement as described by Brown and Wilson (1956). Lack (1971) cited many examples of character displacement in such features as morphology, habitat, and feeding behavior. Contrary to the theory of character displacement, Rohwer (1973) suggested that, under certain conditions, interspecific territoriality and convergence in display characters can be expected in an interacting species pair. In his study of the two species of meadowlark (Sturnella) he found convergence in characters used in posturing displays. He also noted a loss of information content in vocalizations when territorial males of one species were exposed to the primary song of the other species. This resulted in increased fighting as a compensation for the ineffectiveness of song for maintaining territorial possession. Cody (1969) stated that similarities are often less pronounced or even absent where two species are allopatric and that sympatry is necessary for retention of any similarities. He proposed that songs, specifically in the case of the two meadowlarks, show convergence in the zone of sympatry in response to selection for interspecific territoriality. Moynihan (1968) predicted that related species which are sympatric will usually be more similar in overall appearance than equally closely related species of similar habits which are allopatric. Frequent contacts between individuals of sympatric species result in closer resemblances between them to facilitate social reactions.