Oxygen stores and foraging behavior of two sympatric, planktivorous alcids

Seabird species with overlapping diets commonly coexist at breeding colonies. For example, ancient murrelets Synthliboramphus antiquus and Cassin's auklets Ptychoramphus aleuticus are similar-sized small alcids that feed on krill and small fish. Little is known regarding their partitioning of aquatic resources, so we assessed the O-2 stores and foraging behavior of sympatrically breeding populations of these species. The attachment of recorders (1 to 3% of body mass) caused substantial nest desertion, but we reduced these effects by only equipping experienced birds early in the day. Auklets and murrelets had 18 to 24% higher mass-specific O-2 stores than slightly larger non-diving kittiwakes Rissa tridactyla. When compared to published values, blood hemoglobin content was higher, muscle pH buffering capacity was similar and muscle myoglobin content was lower for small divers than for larger non-phocid divers. The slightly higher O-2 stores of Cassin's auklets was reflected in their aquatic behavior, as auklets dived longer than murrelets at any given dive depth. Moreover, chick-rearing auklets spent 31% more time underwater than incubating auklets and 50% more time underwater than incubating murrelets. In total, 45% of dives by chick-rearing auklets, 36% of dives by incubating auklets and 13% of dives by incubating murrelets exceeded their estimated aerobic dive limits. Murrelets primarily used V-shaped dives while auklets generally exhibited W-shaped dive profiles with a protracted bottom phase. Thus the O-2 stores and foraging behavior of the 2 sympatric seabirds differed.