GROWTH AND ALLOCATION IN CAPTIVE COMMON MURRE (URIA AALGE) CHICKS

Auk, The, Jul 2006 by Benowitz-Fredericks, Z Morgan, Kitaysky, Alexander S, Thompson, Christopher W

ABSTRACT.-

In birds, relative growth rates of morphological characters change in response to restricted food intake during development. Differential allocation of limited resources is hypothesized to reflect functional priorities for developing chicks. Body mass, wing, and flight feathers have been identified as potential priorities for seabird chicks. We used allometry to examine allocation in captive Common Murre chicks fed within a range of natural provisioning. During days 10-45 post-hatch, chicks were fed one of four diets that varied in biomass, energy content, and composition. Energy intake had a more profound effect on growth and development than diet composition; it significantly reduced absolute growth of body mass, manus, and tarsus. Between day 15 and day 20, allocation changed in all treatments: growth of manus was maintained at the expense of body mass. Chicks in more restricted treatments shifted allocation to manus at a lower body mass than those in less restricted groups, but subsequently allocated similarly. Wing loading was higher for chicks than for adult alcids, but scaled similarly. Growth of primary feathers was the most sensitive to small differences in diet composition. Our data also suggest that some changes in allocation may be ontogenetically determined rather than part of an adaptive response to reduced food intake.

Received 25 June 2004, accepted 23 September 2005.

Key words: alcid, allometry, Common Murre, development, food intake, Uria aalge, wing loading.

Croissance et Allocation chez des Oisillons Uria aalge

RÉSUMÉ.-Chez les oiseaux, le taux de croissance relatif aux traits morphologiques change en réponse à une consommation de nourriture limitée au cours du développement. L'hypothèse d'une allocation différentielle des ressources, lorsqu'elles sont limitées, est avancée afin de refléter les priorités fonctionnelles des oisillons en développement. La masse corporelle, les ailes et les plumes de vol ont été identifiées comme des priorités potentielles chez les oisillons d'oiseaux marins. Nous avons utilisé l'allométrie pour examiner l'allocation chez des Uria aalge captifs nourris avec une gamme d'aliments naturels. Au cours des jours 10-45 suite à l'éclosion, les oisillons ont été nourris avec une des quatre diètes qui variaient en termes de biomasse, de contenu énergétique et de composition. La consommation énergétique a eu un effet plus important sur la croissance et le développement que la composition diététique, ce qui a significativement réduit la croissance absolue de la masse corporelle, de l'aile et du tarse. Entre le jour 15 et le jour 20, l'allocation a changé dans tous les traitements : la croissance de l'aile a été maintenue au dépend de la masse corporelle. Les oisillons soumis a des traitements plus restrictifs ont modifié l'allocation destinée à l'aile lorsque leur masse corporelle devenait plus faible que celles des oisillons dans les groupes moins restreints. L'allocation devenait similaire par la suite. La charge de l'aile était plus grande pour les oisillons que pour les adultes, mais dans un même ordre de grandeur. La croissance des plumes primaires était la plus sensible aux petites différences dans la composition diététique. Nos données suggèrent que certains changements dans l'allocation pourraient être déterminés de manière onthogénétique plutôt que d'être induits par une réponse adaptative à une consommation de nourriture réduite.

FOOD AVAILABILITY FOR developing birds is often highly variable. Chicks' developmental responses to food shortages vary among species. In some species, chicks are able to reduce metabolic rates and slow or nearly arrest growth until food becomes available again (Boersma 1986, Kitaysky 1999). In other species, chicks do not suppress growth of all body parts equally; some elements may continue to grow at nearnormal rates, whereas others are substantially retarded (Boag 1987, Congdon 1990, Hario 2001, Ashton and Armstrong 2002, Dahdul and Horn 2003). Mechanistically, physiological "internal" constraints dictate patterns of relative growth (Ricklefs et al. 1998). However, the "adaptive growth" hypothesis predicts that selection should drive chicks to allocate limited resources to maintain growth of those elements that serve the most immediate needs, thereby maximizing their survival (O'Connor 1977, Ricklefs et al. 1998). This hypothesis has been used to infer that the morphological elements that are least sensitive to food restriction are the most critical to chick survival (e.g., O'Connor 1977, Øyan and Anker-Nilssen 1996). However, this hypothesis is difficult to test directly. Furthermore, there is a need for more information about the developmental responses of chicks with different juvenile life-histories to controlled variation in quality and quantity of food (but see Boag 1987, Johnston 1993, Øyan and Anker-Nilssen 1996, Schew and Ricklefs 1998, Kitaysky 1999, Dahdul and Horn 2003). Experiments quantifying how chick development is affected by levels of food restriction experienced in the wild are critical for addressing questions of developmental flexibility and constraints, and their implications for fitness.