This study addresses the question: are seabirds digestive opportunists or specialists? The extent of specialization in seabird digestive processes to different diets and foraging methods, is investigated experimentally. Gut passage rates of different food types of tested in vitro digestibility may reflect dietary specialization, with favoured prey types excreted more rapidly than less frequently encountered prey. Mean retention times of solid digesta are significantly correlated with foraging trip duration, and with gut length. Gut length and volume in tum scale with body mass. Assimilation efficiencies of various dietary components are not predictable purely on the basis of food composition, and show a high degree of inter- and intraspecific variability. Energy assimilation efficiency does not reflect dietary specialization, and may be maintained at approximately 75% regardless of diet. Assimilation efficiency is, however, temporarily elevated in energetically-stressed birds, such as penguins that have recently completed moult. Digestive specializations are reflected in seabirds' abilities to assimilate substances specific to certain prey organisms. Unlike most terrestrial vertebrates, seabirds are able to digest wax esters, compounds important in marine food webs. Procellariiforms exhibit unique gastric adaptations facilitating extended foraging trips and efficient transport of food to their young, both important advantages for predators exploiting patchy and unpredictable food resources. Seabirds which naturally feed on crustaceans secrete the specific enzyme chitinase from their gastric mucosae, permitting digestion of the chitinous exoskeleton of the prey. The ability to secrete this enzyme is probably a retained ancestral trait rather than a newly evolved one, and may have been lost by seabirds that do not prey on crustaceans. Differences between penguins and procellarids reflect unique adaptations to the different foraging techniques employed by these two families. The synthesis of the thesis explores the adaptive significance of gut passage rate and allometry of the gut in relation to the two predominant foraging techniques employed by seabirds: long- distance aerial soaring and subsurface swimming. Scaling of seabird gut size may play an important role in the interplay between metabolic rate, the energy demands of foraging, and digestive physiology. The allometric approach taken here is potentially useful for studies of seabird digestion, and has application in studies linking the evolution of avian body size, and foraging ecology.

Bibliography: p.222-233. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Zoology, 1995?

Achievement of endothernic capacity is vital for independence from ambient temperature changes, sustained activity, optimal biochemical reactions and optimization of parental care. During early avian development, the core tenets of transition from ectothermy to endothermy are development of metabolic capacity (oxygen consumption, mitochondrial bioenergetics), enhanced cardiovascular function (heart rate and cardiac output), pulmonary ventilation and thermogenic capacity. Thyroid hormones, particularly T3, are key metabolic regulators of basal metabolism, thermogenesis, pulmonary ventilation and mitochondrial respiration. Thyroid hormone fluctuation patterns during both precocial and altricial avian endothermic transition suggest a prominent role in maturation of endothermy, cardiovascular, respiratory and skeletal muscle physiology. This body of work explores effects of T3 manipulations in two avian species: the precocial Pekin duck and the altricial Red-winged Blackbird. Increased plasma T3 during late incubation resulted in increased cardiac mass, elevated resting and intrinsic heart rate, intrinsic mean arterial pressure, increased cholinergic tone and blunted alpha-adrenergic tone in the precocial Pekin duck. In both Pekin duck and Red-winged blackbird, plasma T3 levels correlated with changes in the trajectory of endothermic ontogeny, systemic oxygen consumption, thermogenesis, maturation of pulmonary ventilatory function, altered growth and effects on skeletal and cardiac mitochondrial bioenergetics. These observations support the role of thyroid hormones as metabolic and developmental regulators at the time of attainment of endothermy during the perinatal period in precocial and altricial avian species. Insights into the role of thyroid hormone as a metabolic and development regulator at the time of avian endothermic attainment provide a more thorough understanding of metabolic and physical transitions a hatchling bird must undergo to reach the adult endothermic phenotype. Such insights also deepen understanding of the complex role thyroid hormones play in homeostasis and offer implications about the evolutionary history of endothermic capacity.