PLEISTOCENE DISTRIBUTION OF PARAMO AND PUNA IN SOUTH AMERICA 
routes. The distributions of these 
birds altered as the vegetation types 
fluctuated during the glacial- 
interglacial cycles. In glacial times 
(left), paramo and puna vegetation 
were more extensive and reached 
lower altitudes than in interglacial 
times, such as the present (right). 
Some new bird species arose 
as a result of isolation of 
paramo and puna. 
immigration from lowland areas? 
These questions boil down to a clas- 
sic problem in biogeography; in this 
case, how to explain the history of 
about 180 species of birds. The pro- 
cedure is a bit like detective work: 
look at the evidence, omit no detail, 
and reconstruct the circumstances of 
the crime. The crime was committed 
in only one way, but the circumstances 
can almost always be variously recon- 
larly, although the fauna of a region 
evolved in only one way, different sci- 
entists working with the same biogeo- 
graphical evidence may well suggest 
dissimilar reconstructions of that evo- 
lution. 
We need two kinds of clues: his- 
torical ones — ideally, abundant fossil 
remains — and modern ones, revealed 
in the distribution patterns of living 
species. Fossils of high Andean birds 
52 
