Ebook Diet, Fruit Choice And Variation In Body Condition Of Frugivorous Warblers In Mediterranean Scrubland
Most dispersal mutualisms between animals and plants depend on the inclusion of plant propagules (seeds, pollen) as a part of the animal food, and result from the animal feeding on the plant (Janzen 1985). A major constraint on the evolution of a tight mutuatistic relationship between animals that gather food and plants that provide this food is that no single plant part produces an adequately balanced set of nutrients for the consumer and, as a consequence, the animal must build up a diverse diet in terms of both plant species and plant parts. The consequences of these nutritional constraints for the animals involved have been repeatedly documented (e.g. Belowski 1978, Westoby 1978, Bergeron & Jodoin 1984, Karasov 1985), and involve, as a generalization, the need to build up a diverse diet including 'minor' components that might be difficult to obtain. This has consequences for the patterns of habitat use, time budget, digestive physiology or body condition.
Frugivorous birds feeding on fleshy fruits experience nutritional constraints quite similar to those of herbivores feeding on vegetative plant parts (Foster 1978, Moermond & Denslow 1985). The fruit pulp is extremely scarce in protein and, if the amount present is adequate, it is usually imbalanced with other fractions, such as lipids or carbohydrates, resulting an inadequate food even when ingestion rate is not limiting (Foster 1978, Robbins 1983). The need to obtain minor nutrients (minerals, vitamins) and the presence of secondary plant metabolites, acting as digestion inhibitors or poisons, might impose deviations from the predictions derived from energetic models by creating partial preferences (Westoby 1978). Shortage of specific nutrients may result in individual preferences for particular meals, i.e., combinations of food items that better supply the nutritional needs (Rozin & Kalat 1971, Clark 1980). In addition, the seed(s) acts as an indigestible ballast decreasing the profitability of the nutrients within the fruit (Herrera 1981, Levey 1986). Therefore, a short retention time of the ingesta is the most commonly reported characteristic of frugivores. It can be seen as an adaptation to deal with digestive bottlenecks, arising when the caloric demand is fulfilled but the nitrogen or mineral need is not supplemented because food processing is limiting (Sibly 1981).
In this paper I examine in detail the patterns of diet composition in two warbler species, the Blackcap Sylvia atricapilla and the Garden Warbler S. borin. Both species are insectivorous during the nesting period but can feed the nestlings sporadically with fleshy fruits (Berthold 1976b, Simms 1985). Both warbler species show annual periodicities in food preferences, with spontaneous changes in the amounts of animal and vegetable food ingested. For the Blackcap, an endogenous rhythm of preference for fruit develops in connection with the migratory restlessness, preceding migration, but the fruit diet must be supplemented, as determined for captive birds, with animal food (Berthold 1976a, p. 141). The extreme protein deficiencv of the fruit pulpseems lo be the major determinant of this nutritional insufficiency but the generalized presence of secondary compounds in the pulp of ripe fruits may also affect the patterns of fruit choice and the degree of dependence on fruit food that can be developed (Herrera 1981). Previous studies of these species in Mediterranean shrublands (Herrera 1984a, 1985a. Jordano 1981, 1987, Jordano & Herrera 1981. Rodríguez eí al. 1986, Gardiazábal 1986) indicate that they are strongly frugivorous during the migration and wintering periods. Here, I relate variations in fruit choice at the population level to seasonal and annual variations in the fruit supply and examine the relation between dietary patterns and body condition of the birds. I document intra population variation in fruit use in the populations studied, the consumption of particular combinations of fruit species (meals) differing in nutrional reward to the birds, and the correlates of these trends for the body condition of the individuals, specially body mass variations and fat accumulation.
CONTENTS
1. Introduction
2. Methods
- 2.1. Study site
2.2. The fruit supply
2.3. Diet analyses
3. Results
- 3.1. Fruit supply and composition of the diet
3.2. Correlates of consumption for individual fruit species
3.3. Combinations of fruit species in the diet
3.4. Body condition of the birds and associated variation in the frugivorous diet
4. Discussion
- 4.1. Fruit consumption: correlates of availability and characteristics of the fruits
4.2. Frugivory in relation to body condition
4.3. Consequences for the dispersal of the seeds
5. Acknowledgements
6. Summary
7. References
8. Samenvatting
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