Abstract
The Rock Shag (Phalacrocorax magellanicus) was found to be similar in molt patterns to other cormorants studied. It was previously thought that two predefinitive plumages occur in this species; this study shows there is only one predefinitive plumage, which is polymorphic and geographically variable. In addition, although the alternate and basic plumages were thought to be very distinct in aspect, I found that these plumages are similar, and specimens previously thought intermediate between alternate and basic plumage combine individual and geographical variation. Differences in frequencies of juvenal plumage phases and adult plumage variants between Chilean and Argentinian coastal populations suggest evolution in isolation by populations separated by a vicariance event such as the Llanquihue Glaciation (20,000 to 10,000 ybp).
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