Although there have been several studies of the foraging behavior of the Imperial Cormorant ( Phalacrocorax atriceps ) and the Rock Shag ( Phalacrocorax magellanicus ), none document the feeding performance and the use of feeding areas by these birds breeding in mixed or adjacent colonies. We studied birds nesting sympatrically in two colonies at Malaspina Inlet, Patagonia, Argentina. Both colonies were located on islands separated by 2.2 km: Vernacci Oeste, inside Malaspina Inlet and Vernacci Este at the mouth of the inlet. Rock Shags from both islands and Imperial Cormorants from Vernacci Oeste fed inside the inlet, and showed similar dive duration, surface interval and foraging range. Imperial Cormorants from Vernacci Este foraged outside the inlet and showed longer dive duration, surface interval and foraging range. Imperial Cormorants made longer foraging trips than Rock Shags (4.9 ± 1.9 vs. 1.9 ± 0.7 h) regardless of its colony. There was no overlap between the foraging areas used by Imperial Cormorants from the two colonies. Despite Rock Shags from both islands feeding inside the inlet, there was a little overlap in their foraging locations (3-22%). Rock Shags and Imperial Cormorants breeding in the same colony showed an overlap in their foraging areas, being lower at Vernacci Este than at Vernacci Oeste. Although both species are usually described as having different diving capacities, we found that their diving behavior was similar when feeding in areas of similar environmental conditions.

No abstract

The rock shag (Phalacrocorax magellanicus) is an endemic species from southern Argentina and Chile. In Argentina, the population is distributed along the Patagonian coast in 143 small colonies (5-377 pairs). We studied colony and nest site fidelity of rock shags at Punta Loma, Patagonia, Argentina. We banded 37 adults from December 2001 to June 2003, during two breeding and two non-breeding seasons. Nest site fidelity was extremely high during consecutive breeding and also non-breeding seasons. The high percentage of banded birds residing in the colony in consecutive seasons suggests that rock shags are highly faithful to breeding sites and supports the idea of low post-breeding dispersion.

The Common Tern ( Sterna hirundo ) breeds in North America and migrates to South America at the end of the northern summer. At Punta Rasa (Buenos Aires Province, Argentina) this species is present from November to April. Between December 1995 and March 1996, the peak numbers of 20,000 individuals were recorded in March. These numbers place Punta Rasa as one of the most important wintering sites in South America. The daily pattern of abundance was similar throughout the austral summer with a peak at dusk. Although tidal height did not alter this pattern, it determined the availability of roost sites, constraining the spatial distribution of terns.

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