Estimating the critical body mass of king penguins

King penguins (Aptenodytes patagonicus) can fast for over a month. However, they return to sea to forage before their body mass reaches a critical value (cMb), beyond which there is an increase in rate of mass loss and in protein catabolism, termed phase III of fasting. Thus when studying king penguins onshore, accurate estimation of their cMb and, in turn, the date at which that body mass would be reached, will be informative to behavioural and physiological data being collected. For penguins being studied during fasts in captivity, knowing cMb is particularly important because of the need to release the birds back into their colony while they are still in good nutritional condition. The present study investigates the validity of using measures of beak, flipper and foot length together to estimate cMb in king penguins and provides a simple and effective prediction equation for researchers. The three morphometric measurements, along with body mass just prior to going to sea after the moult fast ( taken to represent cMb), were obtained for nine king penguins in a colony at the Crozet Archipelago. A multiple linear regression of the three morphometric measurements against cMb provided an R-2 of 71.2%. Mean absolute percentage error of the estimate of cMb over the nine birds was 8.82 +/- 1.20%. The described technique could probably be employed for estimating cMb in other long-fasting seabirds.