Information use and species interactions in a hummingbird guild

Abstract

How might an individual's ability to learn environmental information affect competitive interactions among species? Learned behavior can influence foraging decisions. Competition for food resources can influence patterns of species coexistence via habitat selection. I wondered if both learned behavior and competition might act together to influence interactions among species. I used Rosenzweig's shared-preference isoleg theory to predict four possible ways that differences in environmental information could affect density-dependent habitat selection. To test these predictions, I conducted field experiments to examine the foraging behavior of free-living hummingbirds. I studied black-chinned (Archilochus alexandri), blue-throated (Lampornis clemenciae), and magnificent (Eugenes fulgens) hummingbirds where they coexist in the Chiricahua Mts. of Southeastern AZ. I gave hummingbirds two types of habitats (rich and poor feeders) and let them learn to associate colors with feeder quality. I confirmed that learned color associations can increase hummingbird foraging efficiency. All birds shared a preference for the rich feeders. However, they will shift their preference toward poor feeders when competitor densities are high (Pimm et al. 1985). I quantified hummingbirds' preferences for the rich feeders when both competitor densities and information (via learned color cues) varied. The data support my fourth prediction---that birds foraging with complete information enjoy reduced negative effects from competition. Without complete information, the two subordinate species (black-chinned and magnificent) shifted their preference away from rich feeders in response to high densities of the dominant species (blue-throated). Each subordinate shifted in a unique way; the black-chinned reduced its foraging efficiency, while the magnificent reduced its total foraging time. Birds foraging with complete information remained highly selective on rich feeders even with high competitor densities. Thus, learned information affected competitive interactions (for rich feeders) among these species. My results require us to consider the information-gathering (e.g., learning) abilities of individual decision-makers when we evaluate density-dependent habitat selection. These results should help us better explain patterns of species diversity and distribution, especially for cases in which species learn environmental cues. This study provides the first demonstration, either theoretically or empirically, of a link between learned behavior and its cascading effects within a guild of coexisting species.