Data from: Variation in growth drives the duration of parental care: a test of Ydenberg's model for seabirds

The duration of parental care in animals varies widely. Such variation may represent a trade-off between growth and safety. Auk species show wide variation in the age of departure. We tested the idea that offspring energy gain is low at the nest but that safety is high (Ydenberg's model). To directly test the model assumptions, we attached time-depth recorders to murre parents (fathers, which do all parental care at sea, and mothers; N = 14 of each). Except for the initial mortality experienced by chicks departing from the colony, the mortality rate at sea was similar to the mortality rate at the colony. However, energy gained by the chick per day was ~2.1 times as high at sea compared to at the colony because the father directed more food to the offspring. Compared to the mother, the father spent ~2.6 times as much time diving per day and dived in lower quality foraging patches. We provide a simple model for optimal departure date based only on (1) the difference in growth rate at sea relative to the colony and (2) the assumption that transition mortality, from one life history stage to the other, is size-dependent. Apparently, large variation in the duration of parental care can arise simply due to variation in energy gain without any trade-off with safety.