Diving Behavior of Rock Shags at a Patagonian Colony of Argentina

Abstract

-The diving behavior of ten adult Rock Shags (Phalacrocorax magellanicus) foraging at Punta Loma, Patagonia, Argentina was studied, using radiotransmitters deployed during two chick-rearing periods. Shags dived continuously during foraging trips, spending 75% of their time at sea underwater. Mean dive duration was 47 ? 14 sec while mean subsequent surface interval was 14 + 6 sec (N = 11,212 dives). During both seasons, there was a weak linear relationship between dive duration and recovery time. Only four of the birds showed a positive and significant linear function between these diving parameters. Recovery time at the surface was independent of the previous dive time only during low tides. Dive efficiency (dive time/recovery time) was the highest reported for a foot-propelled diver (mean = 3.7 and 4.2 for 1996 and 1997, respectively). Food resources located in shallow waters at Punta Loma allowed the shags to optimize their searching period underwater by decreasing the energetic costs associated with diving. Received 11June 1999, accepted 24July 1999.