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Abstract
AssTmAcr.-I analyzed two patterns of bird distribution in subtropical South America. The first was the disjunct distribution of pairs of species and subspecies between the southern Yungas and the Paranense forests, which are separated by 700 km of xerophytic Chaco woodland. Forest birds penetrate up to 200 km into the Chaco along gallery forests, but do not cross the rest of the Chaco, which constitutes an effective barrier. The second pattern involved several zones of secondary contact located in the Chaco lowland, where several woodland and grassland species and subspecies interact. I conclude that both bird-distribution patterns were produced by forest expansions along the Bermejo and Pilcomayo rivers that connected the southern Yungas to the Paranense region and interrupted the arid vegetation in the center of the Chaco. Vegetational fluctuations probably occurred several times during the Quaternary and produced phenotypic differentiation of sister taxa, both in the nowdisjunct forests and in the currently continuous Chaco. Received 26 June 1990, accepted 10 January 1992.
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