Precocial nest departure in the Alcidae

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.