Bird Speciation in Subtropical South America in Relation to Forest Expansion and Retraction

Abstract

I analyzed two patterns of bird distribution in subtropicalS outh America. The first was the disjunctd istributiono f pairs of speciesa nd subspeciebs etween the southern Yungas and the Paranense forests, which are separated by 700 .kin 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 woodlanda nd grasslands peciesa nd subspecieisn teract.I concludet hat both bird-distribution patternsw ere producedb y forest expansionsa long the Bermejoa nd Pilcomayor ivers 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 nowdisjunctf orestsa nd in the currently continuousC haco.R eceived2 6 June1 990, accepted1 0 January 1992.