Ecologists have long attempted to predict the success of species that
are introduced into foreign environments. Some have emphasized qualiti
es intrinsic to the species themselves, whereas others have argued tha
t extrinsic forces such as competition may be more important. We test
some of the predictions made by both the extrinsic and intrinsic hypot
heses using passeriform birds introduced onto the island of Saint Hele
na. We found direct evidence that extrinsic forces are more important
predictors of successful invasion. Species introduced when fewer other
species were present were more likely to be successful. In a direct t
est of the alternative hypothesis that intrinsic forces play a more pr
ominent role in success or failure, we found a tendency for species wh
ich successfully established on Saint Helena to be also successful whe
n introduced elsewhere. However, the vast majority of species unsucces
sful at establishing on Saint Helena had probabilities of success outs
ide Saint Helena of 50% or greater, making this result somewhat equivo
cal. Finally, we found no evidence to support the hypothesis that spec
ies that are successful early are those that are intrinsically superio
r invaders. These results are consistent with similar analyses of the
introduced avian communities on Oahu, Tahiti, and Bermuda.