Biology of the Heard Island Shag Phalacrocorax nivalis. 3. Foraging, Diet and Diving Behaviour

Summary

The Heard Island Shag Phalacrocorax nivalis feeds mainly on scale worms and fish. The high incidence of scale worms in the diet is unusual for shags; they were taken by all birds in the non-breeding season but in the breeding season those feeding young changed to a diet of fish. The fish were generally small nototheniids that are easy to transfer from adult to young. Diving data for breeding and non-breeding birds did not reveal any differences that could account for the difference in diet. It appears that scale worms are a preferred food item and occur in a similar water depth to that at which the fish were taken. Food selection by adults for their young may be determined by ease of transfer (hence small fish) and energy value for volume (scale worms contain a high proportion of indigestible spines that would fill the nestling's stomach without providing nutrition). Comparisons with results found elsewhere suggest less diving effort by Heard Island Shags, possibly as a result of high food availability in shallow water.