Timing of nest departure in the Thick-billed Murre and Razorbill: Tests ofYdenberg's model

Marine birds of the family Alcidae exhibit extreme variation in the mass an d age at which offspring leave the nest. A previously proposed, theoretical model of intraspecific variation in optimal timing of nest departure in al cids predicts that: (1) variation in growth rate among chicks will result i n negative correlations between departure mass and departure age, and (2) i n the absence of seasonal declines in chick growth rates (such declines are consistent features of alcid biology), variation in timing of hatching amo ng chicks will cause both mass and age at departure to decline with hatchin g date, We tested these predictions for Thick-billed Murres and Razorbills, two alcids that employ a unique development strategy. To control for the c onfounding effects of seasonal variation in growth rates, caused at least i n part by variation in the characteristics of adults breeding at different times, we switched eggs at random among breeding pairs in each of two years for both study species. Contrary to prediction 1, there were no significant negative correlations b etween departure mass and departure age, and in fact, there was one signifi cant positive relationship. Contrary to prediction 2, departure mass did no t decline with hatching date; in agreement with prediction 2, departure age often declined with date. Our results suggest that, for Thick-billed Murre s and Razorbills: (1) variation in growth rate probably does not influence timing of nest departure in the manner outlined in the model, and (2) it is unlikely that variation in hatching date affects the departure decision in the manner predicted by the model. Based on our experimental results, and previously reported observations of Thick billed Murres, we develop alterna tive hypotheses, emphasizing the roles of parental quality and wing growth (rather than increase in mass), to explain the close correspondence between the model's predictions and patterns in the timing of nest departure withi n the Alcidae that are reported frequently.