Energy Expenditure and Food Requirement of Cassin's Auklets Provisioning Nestlings

We used the doubly-labeled water technique to measure the field metabolic rate (FMR) of free-ranging adult Cassin's Auklets (Ptychoramphus aleuticus) that were provisioning half-grown nestlings. FMR averaged 3.68 ± 0.38 mL CO2 g-1 hr-1 (n = 9), which is equivalent to a daily energy expenditure of 413 ± 59 kJ. Although Cassin's Auklet FMR is high for nonpasserines in general, it is 85% of the value predicted allometrically for a cold-water seabird that uses flapping flight. Based on stomach samples, crustaceans comprised 99.2% of the birds' diet, with euphausiids comprising 83.4% of total food items. Cassin's Auklets need to consume 67% of their body mass in euphausiids per day in order to meet their energy expenditure during chick rearing. Water flux rate averaged 872 ± 114 mL H2O kg-1 day-1, which is about five times that predicted allometrically. High FMR reflects the high costs of flapping flight and pursuit diving in this pelagic feeding alcid. High water flux rates may be attributed to diet and foraging mode.