Food and Feeding Biology of the Black Skimmer in Argentina: Evidence Supporting Offshore Feeding in Nonbreeding Areas

Abstract

-We investigated the diet of the Black Skimmer (Rynchops niger) in three consecutive years (autumn 1998, autumn 1999 and summer 2000) by analyzing 642 pellets collected at Punta Rasa, Samborombon Bay, Buenos Aires, Argentina. Fish was the main prey in the diet (occurrence = 100%), while insects were present in only 2.7% of samples. Among the 14 fish species identified, Argentine Anchovies (Engraulis anchoita) (occurrence = 38.7%, N = 31.5%), Silversides (Odontesthes spp.) (occurrence = 38.3%, N = 26.2%) and White Croakers (Micropogoniasfurnieri) (occurrence = 11.6%, N = 20.5%) were the most important prey. Fish prey averaged 77 mm in length (?34 mm, range 15-169 mm, N = 123), and had an average mass of 6.9 g (?7.3 g, range 0.1-31.6 g, N = 123). Marked differences were observed in the frequency of occurrence and numeric importance of the fish prey species found in the three sampling seasons; while typical marine fishes, such as the Argentine Anchovy, constituted the bulk of the diet during 1998, estuarine juvenile fish prevailed in 1999 and 2000. The comparison of size and mass of prey between seasons also revealed large differences. Fishes caught in 1998 were significantly larger and heavier than those caught in other seasons. The diet of skimmers overlapped with that of South American Terns (Sterna hirundinacea), Royal Terns (S. maximus) and Sandwich Terns (S. sandvicensis) in the area during 1998, when prey was mostly marine fish. Received 17 April 2001, accepted 25June 2001.