Climate change leads to modifications in the behavior of hummingbirds

Research shows that rises in temperature reduce the metabolic rate of hummingbirds, which fly less as a result.

By Elton Alisson

Agência FAPESP – Climate change can cause a reduction in the flight activities of hummingbirds and, consequently, in plant pollination by this non-passerine family (Trochilidae).

This is one of the findings of a study by researchers at the University of Taubaté (UNITAU), in São Paulo State, Brazil, who are collaborating with colleagues at the National Space Research Institute (INPE), the University of São Paulo’s Lorena School of Engineering (EEL-USP) and the University of Toronto Scarborough in Canada for the Thematic Project “Assessment of impacts and vulnerability to climate change in Brazil and strategies for adaptation”, supported by FAPESP as part of an agreement with the National Scientific & Technological Development Council (CNPq).

“We observed that rising temperatures lead to a fall in hummingbirds’ metabolic rates or weight-specific oxygen consumption in relation to energy expenditure. As a result, their wing beat frequency falls, they fly less, and they forage less for nectar in flowers,” said Maria Cecília Barbosa de Toledo, a professor at UNITAU’s Biology Department and principal investigator for the study.

The researchers focused on eight hummingbird species found at varying altitudes in the Paraíba Valley region of São Paulo State: two low-altitude species – the saw-billed hermit (Ramphodon naevius) and festive coquette (Lophornis chalybeus); two high-altitude species –the green-crowned plovercrest (Stephanoxis lalandi) and white-throated hummingbird (Leucochloris albicollis); three species that occur throughout the elevational gradient of Paraíba Valley – the violet-capped wood nymph (Thalurania glaucopis), Brazilian ruby (Clytolaema rubricauda) and glittering-throated emerald (Amazilia fimbriata); and one migratory species – the black jacobin (Florisuga fusca).

The researchers chose this bird family because of its high metabolic rate, which is largely a reflection of environmental factors such as temperature and altitude.

“Our starting hypothesis was that climate change might have a significant impact on hummingbirds, which could therefore be useful bioindicators,” Toledo told Agência FAPESP.

In simulating the effects of variations in climate on these hummingbirds, the researchers used the altitude-climate gradient of the Paraíba Valley region, ranging from three meters (e.g., Ubatuba and Caraguatatuba) to 1,800 meters (e.g., Campos do Jordão), as a yardstick.

For these different altitudes, where mean temperatures range from 10 to 30 degrees Celsius, they estimated the field metabolic rates for C. rubricauda, one of three hummingbird species found along the entire altitude gradient in the Paraíba Valley region.

Hummingbirds were captured and placed in a chamber with a feeder in the upper portion. The feeder comprised a plastic tube containing a 20% sucrose solution. A perch in the lower portion was coupled to a set of scales to weigh the birds.

To feed on the sucrose solution, the birds had to hover nearby and insert their heads into the feeder’s plastic tube, which functioned as a respirometry mask with an airflow of 2,500 milliliters per minute.

While the birds hovered with their heads in the respirometry mask, the researchers measured temperature, oxygen consumption and carbon dioxide production.

These variables were used to estimate metabolic rates for birds with habitats along the altitude-temperature gradient in the Paraíba Valley.

“The system enables us to estimate the field metabolic rates of hummingbirds during activity, which is key to measuring the effects of climate change on their metabolism,” Toledo said.

Less flight

One of the findings from the experiments was that a rise in ambient temperature lowered the metabolic rate of C. rubricauda, which was higher at temperatures in the range of 20.1-25 degrees Celsius than at 25.1-30 degrees.

In the higher temperature range, according to Toledo, hummingbirds seek to maintain their metabolic rate and reduce energy expenditure by beating their wings less frequently, resting in the shade and flying less.

“Behavioral changes of this type may cause a decline in plant pollination by hummingbirds,” she said. “They visit wild flowers in search of nectar less frequently and thus transport less pollen from one flower to another.”

Some hummingbird species have climate preferences, the study showed.

C. rubricauda, for example, occurs mostly in parts of the Paraíba Valley where temperatures range from 20 to 25 degrees and is not seen at low altitudes during the summer, when the mean temperature reaches 28 degrees, according to Toledo.

“If the overall temperatures rises enough to generate a higher mean temperature at higher altitudes, hummingbirds will probably attempt to accompany this change,” she said.

“During the study, we observed altitude-related morphometric changes; for example, the mass, wing length and area, beak length and total length all varied. However, we don’t yet know whether these birds will be capable of effecting such morphometric adaptations fast enough to keep up with climate change.”

Temperatures in this higher range do not pose a threat to the survival of hummingbirds, which are relatively resilient to heat.

According to Toledo, their normal body temperature is approximately 40 degrees, so they can comfortably survive in habitats with ambient temperatures of approximately 38 degrees, considered hot for humans.

“However,” she noted, “hummingbirds can’t withstand this level of heat stress for reasonably long periods unless they have a sufficient supply of energy from flower nectar. This is evidently a major concern.”

A study by researchers at EEL-USP, also as part of the project, found that the amount of energy available to hummingbirds from plants in the Paraíba Valley varies according to altitude.

Some plant species visited by hummingbirds at higher altitudes such as the Campos do Jordão area offer nectar with more sucrose, which is the form of sugar preferred by hummingbirds, than do plants in areas near sea level such as Ubatuba, the study showed.

“The key question is whether the plants visited by hummingbirds will be able to adjust their nectar concentration in time to accompany climate change and continue supplying the energy these birds need,” Toledo said.