Contact Information:
Elizabeth A. Sandlin
520-621-5202
[email protected]
Cued in: Hummingbird's ability to learn affects competitors, too Hummingbirds' ability to learn from their environment saves them from suffering the costs of severe competition and may -- just may -- increase the number of species which potentially might coexist in a given habitat, an ecologist from The University of Arizona in Tucson has discovered.
Her study is possibly the first to show the effects of the learned behavior of individuals of a species on the interactions between its related species competing for shared resources. Also, her study is the first of its kind done in the "real" world rather than in a laboratory.
Researchers in the UA department of ecology and evolutionary biology have for decades used one of the most species-rich areas in the world -- southeastern Arizona -- as a natural laboratory in which to study questions of species diversity, habitat selection, population dynamics and evolutionary ecology.
Elizabeth A. Sandlin, an ecologist interested in foraging behavior, has for the past five years been studying hummingbird foraging at the American Museum of Natural History's Southwest Research Station in the Chiricahua Mountains near Portal, Ariz. Her experiments with three species of hummingbirds show that a hummingbird's ability to learn and act on information from the environment not only makes a huge difference in its own foraging success, but affects the success of hummingbird competitors as well.
"Hummingbirds are ideal subjects for scientists trying to understand how different species coexist and compete with each other for food resources and habitat," said Sandlin, who recently completed her UA dissertation based on this work. "They are aggressive competitors with enormous energy demands. Basically, a hummingbird is just a support system for the pectoralis muscle that powers its wings. Literally, if a hummingbird doesn't get a meal within 20 or 30 minutes, it can no longer fly."
Earlier research by another doctoral student in the UA department had shown how hummingbirds use color cues to choose food resources. Impressed by this study on how animals decide what to eat, Sandlin theorized that learned information will not only enhance a species' foraging success, it will affect hummingbird competitors' foraging success as well. Because the birds all eat the same food, Sandlin said, "how efficiently an individual forages is going to affect how much food is left for everyone else."
Of the three species Sandlin studies -- blue-throated, black-chinned and magnificent hummingbirds -- the blue-throated hummers are by far the meanest of the neighborhood, the dominant species. They aggressively hold the best riparian habitat and commandeer the choicest food. The magnificents, which are about as big as the blue-throats but not as chunky, are far less aggressive. Black-chinned hummers are only about a third the size of the other two birds, light enough to be mailed with one first-class postage stamp. The tiny black-chinned birds have few physiological energy reserves. They just don't store energy as well as the other two species do.
Sandlin provided a limitless source of food to hummingbirds in her study using 10 feeders. Five feeders held a richer, stickier syrup solution and five held a weaker sugar solution. Bee guards on the feeders were painted either yellow or blue, colors which birds quickly learned to associate with superior rich syrup or the poorer quality stuff. Feeders that sported these "reliable" color cues significantly increased hummingbirds' foraging efficiency.
Even in what Sandlin called a "high competitor density" situation, where great numbers of hummingbirds sought food, the birds did not suffer effects of intense competition because they still had access to rich feeders. Given reliable cues to rich feeders, the birds weren't significantly bothered by strong competition.
Outfitting the feeders with "ambiguous" cues -- bee guards painted both yellow and blue -- produced dramatically different results. A hummingbird's chances of finding rich food were now no better than random. Fierce competition took a toll on the three species in different ways.
Just as theory earlier developed by UA Professor Michael Rosenzweig predicted,some animals had to quit being choosy about their food and settle for whatever nourishment they could get. This was the lot of the black-chinned hummingbirds. They could not afford to be picky. They sipped weaker sugar solution when competition made it too costly to search for rich feeders.
The blue-throated hummingbirds sampled and rejected poor feeders until they hit one that supplied the rich sugar water. Then they stayed put. "The blue-throateds still have the highest foraging efficiency of any species I studied in this situation," Sandlin said. "That, I think, tells us something about the benefits of being dominant. They still control the resources. They still get the best stuff. Relatively speaking, they are the ones that suffer the least. I think's that the payoff for being the bad guys." The cost to the blue-throated birds is energy invested in battle, or at least very vocal aggression.
The magnificent hummingbirds responded to ambiguous cues and keen competition in another, and intriguing, way. Unlike the black-chinned hummingbirds, the magnificents did not settle for poor food if that was immediately available. And unlike the blue-throats, the magnificents did not diligently search and defend rich food.
"The magnificents just stop foraging," Sandlin said. "They just quit. They fly away from the foraging area." Sandlin added that she doesn't know how far they fly. "They could stay in the area or fly to another canyon within the Chiricahuas."
Flight is an energetically expensive activity for hummingbirds. Where do the magnificents go? Sandlin's marked birds were turning up in backyards in Portal, 3 miles or more from the research station.
"There are a variety of possibilities about where they go. My pet hypothesis is that these birds just go somewhere nearby and eat arthropods. All hummingbirds eat insects and spiders for the protein. I think the magnificents just decide to stop wasting time searching for rich feeder food and go eat bugs."
This research shows that animals that can learn about their environment divide up resources more efficiently, Sandlin says. "There's a possibility -- note I emphasize 'possibility' -- that if individuals in a suite of species competing for the same resources are highly efficient foragers, others of their species or even another species might successfully move into the area. This has implications for species diversity. The ability to learn might be something that allows animals to live in more places and in more different kinds of places."
******
RSS