Hummingbirds are the only vertebrates capable of sustained hovering, one of the most energetically expensive forms of locomotion. Yet they are abundant and diverse even at high elevations, where the thin air poses severe challenges for flight physiology and mechanics. Earthwatch teams working with Dr. Douglas Altshuler (now at the University of California at Berkeley) investigated the flight performance in Peruvian hummingbirds at a range of elevations to help unravel their adaptation to mountain environments.
Altshuler, principal investigator for Earthwatch's Flight of the Hummingbird project, presented his findings at the recent Society for Integrative and Comparative Biology meeting in Anaheim, California. In a lecture titled "Burst power and competitive ability of hummingbirds along elevational gradients," Altshuler examined the relationship between hummingbird flight performance, elevation, and competition.
"One of the more significant findings of this project was that maximum the power required for hummingbird hovering did not change across elevations or among species of different body weights," said Altshuler. " In other words there are many combinations of morphology and physiology that can produce a hovering hummingbird. This was despite the fact that hummingbird body size varied by an order of magnitude among the hummingbird species in the study and the study sites spanned a range of elevations from 400 to 3,900 meters."
As Altshuler explains, hummingbird species can differ in a variety of ways including body mass, wing size, wingbeat frequency, wing stroke amplitude, and elevation of occurrence. This provides for a huge number of possible configurations for the flight machinery of hummingbirds, allowing them to compensate for the challenges of increased elevation.
Earthwatch volunteers helped Altshuler and renowned flight physiologist Dr. Robert Dudley (University of Texas) collect birds along the slopes of the Andes, where the highest diversity of hummingbirds occurs. Here they used mist nets to capture over 500 individual hummingbirds from 43 species across a range of elevations and habitats.
"The work of Earthwatch teams was critical to the success of this project," said Altshuler. "Volunteers provided the people-power for us to have access to large numbers of birds that would otherwise not be possible." Working in a portable field laboratory, the scientists measured the flight morphology of the captured hummingbirds and filmed them hovering to estimate the power requirements of hovering flight. Maximum flight performance was also assessed through load lifting, in which the hummingbirds lifted weighted threads.
While maximum power production also did not vary across elevations, Altshuler reported that hummingbirds at higher elevations had a reduced reserve capacity for burst flight activity. In another study, he examined the relationship between reserve burst capacity and competitive ability in two hummingbird species in the Colorado Rockies. Altshuler found that the hummingbird with the higher power reserve for burst capacity was competitively dominant at higher elevations. The hummingbird with higher costs of hovering flight, but lower power reserves, was dominant at lower elevations.
"Reserve capacity for burst performance of hovering hummingbirds affects species interactions and dominance relationships across elevations," said Altshuler. The Colorado study was important because it allowed the measurement of competitive behaviors between two species in a relatively simple system. Assessing dominance relationships in the hummingbirds of more complex tropical ecosystems, where the factors affecting their behavior are more multifarious, will require more exacting study of hummingbird behavior in the field.
Altshuler and Dudley are continuing their research with Earthwatch volunteers, in collaboration with ecologist Gary Stiles of the Univerisidad Nacional de Columbia, at diverse sites in Venezuela and the Chiricahua Mountains, Arizona. Their pioneering research stands to help scientists interpret the astounding flight performance of hummingbirds in ecological terms.
Earthwatch Institute is an international nonprofit organization which supports scientific field research worldwide by offering members of the public unique opportunities to work alongside leading field scientists and researchers. The Institute's mission is to engage people worldwide in scientific field research and education to promote the understanding and action necessary for a sustainable environment.