The occurrence of Phrygilus fruticeti (Aves, Emberizidae) in Chilean Patagonia in 1988 but its absence in 1985-1987 suggests that this species exhibits fluctuations of population density at the southern limit of its range. Data on distribution and breeding of P. fruticeti in Magallanes in 1988 are presented, and the possibility that these records correspond to the ''species border'' is discussed. Other bird species that may show the ''species border'' phenomenon in Patagonia are mentioned. This important ecological and evolutionary phenomenon has been little studied in South American birds and deserves further research. Southern Patagonia appears suitable for such a study, which will be best carried out through several annual cycles by resident ornithologists.