S37.Summary: Digestion in avian ecology
Franz Bairlein1 & William H. Karasov2
1Institut für Vogelforschung ‘Vogelwarte Helgoland’, An der Vogelwarte 21, D-26386 Wilhelmshaven, Germany, e-mail t-bairlein@rz-fs-2.rz.fh-wilhelmshaven.de; 2Department of Wildlife Ecology, University of Wisconsin, 226 Russell Labs, 1630 Linden Drive, Madison WI 53706-1598, USA, e-mail wkarasov@facstaff.wisc.edu
Bairlein, F. & Karasov, W.H. 1998. Digestion in avian ecology. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2182-2183. Johannesburg: BirdLife South Africa.
Digestion is of interest to avian ecologists for at least four reasons (Karasov, W.H. 1996. In: Carey, C. (ed.) Avian Energetics and Nutritional Ecology, Chapman & Hall, Inc., New York: pp. 61-84.). (1) Features of digestive physiology such as enzyme reaction rates, nutrient absorption rates, and retention time of food in the gut determine an animal's digestive efficiency - that fraction of the ingested energy or nutrient that is absorbed. Measures of this efficiency are very important for a predictive ecology that requires estimates of animal feeding rates and hence foraging distance and time. (2) In some cases digestion rates are rate-limiting steps in the flow of energy from the environment to the bird. Hence, digestion can play a role in determining rates of growth, reproduction, and perhaps even geographic distribution limits. (3) For birds with the behavioural flexibility to take a number of kinds of food types, the digestive system's flexibility in processing different food types can limit diet breadth. A subtle digestive characteristic can constrain dietary diversity. (4) Absorption pathways for nutrients may also be pathways for antinutrients such as naturally occurring toxins in plant and insect food.
Bairlein (Bairlein, F. 1999. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2221-2246. Johannesburg: BirdLife South Africa) provides an up-to-date review of knowledge about efficiencies of birds at extracting energy and specific nutrients from foods, based on a review of more than 1000 feeding trials with 184 different species of 60 different families. Energy assimilation efficiency (1 - [kJ d-1 in excreta/kJ d-1 in food]) varies considerably, but most variation is due to type of food and its physical and chemical properties. However, when controlling for food type there is some evidence for differences according to avian digestive physiology, as reflected by significant variation in efficiency according to phylogeny, age, photoperiod (season), and temperature.
Hilton et al. (Hilton, G.M., Houston, D.C., Barton, N.W.H & Furness, R.W. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2184-2197. Johannesburg: BirdLife South Africa) provide a perfect example of such an apparent difference, the mechanistic basis underlying it, and its possible evolutionary origin. Within meat- and fish-eating birds there is significant variation in assimilation efficiency. The interspecific variation in efficiency is correlated with an interspecific trade-off between gut retention time and digestive efficiency. Shorter retention time is associated with shorter small intestine or a stomach of lower mass. Hilton et al. (1999) suggest that in species that capture prey by means of high speed aerial pursuit there has been selection for a slightly smaller gut, even at the cost of slightly lower assimilation efficiency.
McWilliams (McWilliams, S.R. 1999. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2198-2207. Johannesburg: BirdLife South Africa) provides other examples of differences in digestive efficiency among species eating a single food type that relate to differences in physiological features. He discusses how body size puts limits on the quality of foods that can be eaten by herbivores and influences the processes by which the food is digested. Recent work on avian herbivores demonstrates that selective retention of digesta and elevated nutrient uptake rates may help minimise these allometric constraints. He presents exciting new evidence on the capability of the avian caeca to breakdown and absorb complex nutrients such as protein.
The specific mechanistic bases underlying differences in assimilation efficiency are sometimes unclear. Models of digestion have been useful in studying interactions because they show how the features of digestive physiology (listed above) relate to each other and to the rate and efficiency of nutrient extraction from food, two parameters of whole-animal and hence ecological importance. Current-generation digestive optimisation models are based on the assumption that net energy gain from digestion is maximised. They imply that digestive efficiency within an individual eating a type of food is not fixed but is instead conditional on the costs of acquiring the food or on its richness. If true, this might mean that laboratory measures of assimilation efficiency as reviewed by Bairlein (1999) or analysed by Hilton et al. (1999) might not apply under different ecological conditions. Karasov (Karasov, W.H. 1999.In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2247-2258. Johannesburg: BirdLife South Africa) reviewed five tests of current digestive optimisation models and found that major predictions have been rejected over and over again. The models’ poor fit to the data is likely due to invalid assumptions and/or a misguided framework. Nonetheless, several important empirical findings resulted, new lines of research were indicated, and the findings indicate that laboratory estimates of digestive efficiency are likely robust.
A major gap in our knowledge of birds’ utilisation of foods is how naturally occurring toxins in the foods affect utilisation efficiency. Levey and Cipollini (Levey, D.J. & Cipollini, M.L. 1999 In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2208-2220. Johannesburg: BirdLife South Africa) provide the best summary to date of research on this topic in birds. They summarise what is known about avian nutritional ecology and plant secondary metabolites in seeds, foliage, and fruits, identify the most promising directions for future research as well as the pitfalls, and underscore some practical implications of such work.
Taken together, the five papers in this symposium touch on all the reasons listed at the outset why digestion is of interest to avian ecologists. They summarise current knowledge and highlight interesting new directions for the next generation of studies on the physiological ecology of digestion.