Flexible Growth Rates in Fork-Tailed Storm-Petrels: A Response to Environmental Variability

Abstract

We examined the degree that growth in Fork-tailed Storm-Petrel (Oceanodroma furcata) chicks varies among individuals and years. Data on wing chord length and body mass were collected on 10 or more chicks per year on the Barren Islands, Alaska, during seven years over two decades. In contrast to the apparently uniform growth rates in other storm-petrels (e.g. Leach's Storm-Petrel (Oceanodroma leucorhoa) and British Storm-Petrel (Hydrobates pelagi- cus)), Fork-tailed Storm-Petrel chicks on the Barren Islands displayed a two-fold variation in both wing growth and mass gain. Variation in growth rate was apparent both within and among years. Correlations between wing growth and mass gain were significant in only four of seven years, a finding we interpret as indicative of the importance of changes in food quality and quantity on growth. The decadal changes in growth rate of this species between the 1980s and 1990s are consistent with the regulating role that environmental variation appears to play in the growth and survival of storm-petrels. We suggest that the wide range of observed growth rates among individuals, years, and between decades is a response to environmental variability. Received 7 November 1996, accepted 9 June 1997.