Habitat selection by foraging macaroni penguins correlates with hematocrit, an index of aerobic condition

Reproductive investment during the chick-rearing period is an important contributor to lifetime fitness. Key to chick-rearing is the success of parental foraging, as food deliveries affect chick growth and survival up until fledging. For seabirds, oceanographic conditions including factors such as sea surface temperature are known to influence foraging decisions, but few studies have examined the physiological variables that might affect those decisions. We used global positioning systems (GPS), time-depth recorders (TDR), and physiological sampling techniques to explore links between ocean temperature, diving behavior, and foraging success in chick-rearing female macaroni penguins Eudyptes chrysolophus. We then explored correlations between these foraging variables and measures of individual physiological condition, specifically aerobic capa city (hematocrit) and metabolic state (corticosterone). In GPS-tracked penguins, 2 principal for aging tactics were observed: penguins made deep dives in cool, near-shore areas surrounding the breeding colony at Bird Island, South Georgia, or they traveled farther to dive shallower in warmer shelf-break areas. TDR-equipped penguins showed similar patterns. Blood sampling of TDR penguins at the onset of trips revealed strong positive correlations between hematocrit and the mean duration of foraging trips, the ocean temperature experienced during these trips, and the relative efficiency of foraging activity in terms of the number of foraging behaviors recorded per dive. These results suggest that aerobic capacity might be an important determinant of for aging trip range, as well as workload. Corticosterone was unrelated to diving behavior, which counters previous studies examining the effects of experimental increases of this hormone on foraging behavior, and we discuss reasons for this disparity.