Animals must make “decisions” (e.g., when or whether to breed, the effort to put into a breeding episode) by integrating physiological, environmental and social inputs. This integration can be studied only in a field context. In Adélie penguins (Pygoscelis adeliae) reproduction is constrained by foraging ecology, mode of transport, and the extreme latitude at which they live. The decision whether to breed in a given year is influenced by body conditions. Adélie penguins must fast for several weeks during the early reproductive stages and use stored fat for metabolic energy. Females that return to the colony, but do not breed, are 10–12% lighter than females that do breed. Birds that are relatively low in body mass tend to have lower reproductive success than heavier birds, and an individual's reproductive success is positively correlated with the body fat stores it had on arrival. After eggs are laid, parents alternate in attending the nest. Nest failure occurs if one parent does not make a timely return and its fasting partner must eventually leave. During normal-length fasts plasma corticosterone and glucose levels do not change. Blood β-hydroxybutyrate levels gradually increase during the fast while uric acid levels remain low, but in birds with the longest fasts (>∼50 days), ketone levels may fall and uric acid levels increase, indicative of a switch from using fat to using body proteins for metabolism. In incubating males, hematocrit and hemoglobin concentrations also increase, suggesting dehydration can accompany energy stress during the breeding fast.