Illinois Natural History Survey - University of Illinois

Fruit Complementarity in Feeding Birds


As any home owner trying to raise raspberries or blackberries for personal consumption can attest, many bird species are also avid consumers of fruit. Careful observation of the foraging or feeding behavior of a particular bird often reveals that diet selection of fruiting birds tends to be quite diverse, even within a particular day. There are theoretical reasons to be surprised by such observations. Consider the following facts:

* Many plant species produce huge standing crops of fruits that often ripen more or less synchronously, resulting in a large and concentrated food resource. In addition, fruits of many species are conspicuously colored, making their discovery by potential consumers relatively easy.

* Based on gross nutritional reward (chemical composition of the pulp), we know that fruits of different species differ in such characteristics as energy reward per unit fruit mass dry weight. This in turn allows an a priori ranking of suitability as an energy source, in decreasing order from greatest to least energy per unit fruit mass.

* In temperate regions, like Illinois, many plant species seem to ripen their fruit to coincide with the mass movement of autumnal migrants. Migration is obviously an energetically costly activity, when energy reward from food would seemingly be at a premium.

* Standard theoretical models of foraging predict that inclusion of an item in the diet is dependent only on the abundance of the highest ranked item currently available. If it is above a certain threshold of abundance, it should be selected exclusively.

Taken together, the above facts suggest that specialization, not generalization, in fruit selection should be the norm and not the exception. So why is generalization so pervasive?

Before tackling that question, what actually is the scientific evidence that specialization on single fruit species diets is in fact uncommon? Studies reporting on diet selection of fruit-consuming "frugivorous" birds fall into a number of categories. One method consists of examining the gut contents of birds. This can be done in any one of three ways: 1) by sacrificing the animal, removing the gastrointestinal tract, and removing and examining the digesta; 2) by using an emetic--a solution that induces the recipient to vomit--and collecting the regurgitated items; 3) by collecting and examining fecal matter. A second method consists of following an individual bird and recording its behaviors and the foods it consumes, usually in serial order. A third method of studying diet selection consists of experimentally offering either captive or free-ranging birds a variety of fruits from which to select and recording the results. Evidence from all three methods of analysis suggests that diet generalization is the norm in birds.

The issue first struck me when analyzing results of field experiments conducted as a postdoctoral associate of Mary F. Willson, then in the Department of Ecology, Ethology, and Evolution at the University of Illinois at Urbana-Champaign. In our experiments we offered free-ranging, autumnal migratory songbirds natural fruits on artificial fruit displays. These displays (bamboo skewers with double-sided sticky tape or wood doweling with metal hooks to which fruit were attached) allowed us to control various aspects about how birds would naturally encounter fruits. In one experiment, we offered birds fruits of plant species that we knew differed in gross nutritional rewards. We expected that the migratory birds eating these fruits would preferentially select those high in lipids (fats) because of their greater "bang for the buck." Instead, we detected clear preferences for some fruiting species over others, but not in a way easily explained by gross nutritional reward. One clue to such complex foraging patterns may lie in the nutritional relationship that fruits of different plant species represent to bird consumers. Specifically, if fruits of different plant species represent "complementary" resources (resources for which joint consumption results in greater reward than consumption of an equal amount of either resource singly), we would predict that fruit specialization would be rare. This result, in turn, would suggest that such resource complementarity needs to be accounted for in theoretical models of diet selection.

To test this hypothesis, I and several colleagues conducted field experiments in 1993 and 1994 at The Morton Arboretum in Lisle, Illinois. The arboretum is an ideal site for such a test because of the large number and variety of fruiting species that are grown throughout its 1,600 acres. In the experiments, we compared the consumption of fruits from artificial displays that occurred in each of two foraging backgrounds, which consisted of fruiting plants of the species being compared. In each background, the "home" fruit is superabundant, while the "foreign" fruit is rare. Given this foraging scenario, if the species of fruit under comparison are complementary, each should be relatively more preferred when "foreign" than when "home." This is precisely what we found in 10 out of 12 comparisons, a result not expected by chance. Although our results do not address the underlying cause of complementarity, there is growing evidence to suggest it may result from toxins that occur in the pulp of many plant species.

What is the significance of such findings? For one, such resource complementarity suggests that the extent of feeding on the fruit of a given plant species may be at least partially dependent on what other fruits occur in its general vicinity. This in turn will affect patterns of movement of bird consumers, and consequently, patterns of seed deposition. In this way, complemen-tarity could play a role in plant dispersal, succession, and even invasion by exotic species, like the bush honeysuckles, whose fruit are eaten by birds. More practically, the results may suggest ways to increase fruit consumption in projects like ecological restorations, or to decrease fruit consumption in agricultural settings.

Christopher J. Whelan, Center for Biodiversity

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