The avian family Alcidae is unique among birds in having species with widel
y divergent developmental modes. In all alcids, the juvenile phase is compo
sed of a period spent in the nest and a subsequent period at sea completing
growth; the age at transition varies greatly between species Previously, s
emi-precocial (species completing more than half of growth in the nest befo
re departure to the sea) and intermediate (one-quarter to one-third of grow
th) alcid species have been modelled. Here a model is developed to investig
ate selective factors favouring the evolution of precocial nest departure i
n the Ancient Murrelet Synthliboramphus antiquus. The fitness-maximizing ag
e to make the transition from nest to sea (nest departure) is calculated un
der various assumptions, for both parents and offspring. The model shows th
at the potential for growth at sea following nest departure is the stronges
t factor influencing the age at departure. A second important factor is the
danger posed by predators to provisioning parents while the two-egg clutch
of murrelets (most other alcids lay a single egg) is less important. There
is a small region of the parameter space (with high ocean growth and dange
rous provisioning) within which precocial nest departure is favoured from t
he point of view of both parents and offspring. The zone of conflict betwee
n parents and offspring is also narrow. These results are evaluated with re
spect to the precocity hypothesis of Gaston.