Colony attendance pattern of Rock Shags (Phalacrocorax magellanicus): an evidence of the use of predictable and low seasonally food sources?

SAPOZNIKOW, A.; QUINTANA, F

ORNITOLOGIA NEOTROPICAL

Año: 2006 vol. 17 p. 95 - 103

1075-4377

The Rock Shag (Phalacrocorax magellanicus) is an endemic species of  southern Argentina and Chile. In Argentina, the population is distributed along the Patagonian coast in 143 small colonies (5–377 pairs). As part of a broader study of its foraging ecology, we studied the colony attendance pattern of Rock Shags at Punta Loma Reserve, Chubut, Argentina. From October 2001 to September 2003, we visited the colony weekly and counted the birds every three hours from sunrise to sunset. The abundance of adults was similar through the year, but the proportion of adults, juveniles and birds younger than one year, varied along the study period. The daily abundance showed the same pattern during spring, summer, fall and winter. In all cases, the highest abundance was found during the sunrise, decreased at midday, and increased during the sunset. However, the number of birds attending the colony at noon was higher during the spring than during the fall.  The colony attendance pattern indicates that cormorants remain at the colony site through the year and suggests that post-breeding migration of adults is rare. Our results support the hypothesis that cormorants exploit a predictable and low seasonally food source located at the proximity of the colony during the whole year. The use of  a patchy and/or restricted food source, and the consumption of low energy prey items could be some of the reasons explaining the abundance and distribution pattern described for Rock Shag’s colonies along the Patagonian coast.