Silence Of The Hums – Hummingbirds ‘Could Be Wiped Out By Global Warming’
Hummingbirds could be wiped out by global warming, warns a new study.
They will have to move north to seek cooler climes – or disappear, say scientists.
Hummingbirds perform the most exhausting type of flight in the animal world.
Their habit of hovering demands far more energy and oxygen than conventional flight.
Yet thin air poses no barrier to these birds. They thrive atop high mountain ranges from Alaska to South America.
A study found the challenges of relocating may be too much for the agile little aeronauts.
Lead author Austin Spence, a PhD student at the University of Connecticut, said: “‘Overall, these results suggest low air pressure and oxygen availability may reduce hovering performance in hummingbirds when exposed to the acute challenge of high-elevation conditions.”
Thinner and colder air is particularly problematic for creatures struggling to keep warm when less oxygen is available.
The study focused on Anna’s hummingbirds (Calypte anna) which are comfortable up to elevations of around 2,800 meters (9,000 ft).
They lured them into net traps from sites in California – ranging from 10m (30 ft) above sea level in Sacramento and up to 2,400m (7,800 ft) in Mammoth Lakes.
Then the researchers transported them to an aviary in western California at 1,215m (3,900 ft).
Once the birds had spent a few days in their new home, a tiny funnel was set up into which they could insert their heads as they hovered while sipping tasty syrup.
Metabolic rate was measured overnight as the tiny creatures allowed their metabolism to tumble when they rest.
The form of mini-hibernation conserves energy during sleep. Hummingbirds can cool their bodies to less than 4°C (40°F) at night – the lowest temperature recorded in any bird.
They have wings that beat more than ten times a second and during daylight they use their hovering ability to suck nectar from thousands of flowers.
To keep up, their tiny hearts beat around 1,000 times a minute, but this drops to just 50 during rest.
Spence and colleagues relocated the birds to a nearby research station 3,800m (12,000 ft) up near the peak of Mount Barcroft.
The air is thinner with roughly 39 percent less oxygen and about 5°C colder. After four days at the new altitude, metabolic rates were assessed again.
The hovering hummingbirds should have been working harder to remain aloft in the thin air 1000m (3,200 ft) above their natural range.
But they actually experienced a 37 percent drop in metabolic rate. When the team compared the energy used by birds that originated close to sea level and from the higher end of their range, they all struggled equally on the mountain top.
What is more, the birds resorted to dropping their metabolic rate for lengthier periods at night. They spent more than 87.5 percent of the chilly high-altitude night in torpor.
Spence said: “It means that even if they are from a warm or cool spot, they use torpor when its super-cold, which is cool.”
The team also checked the size of the animals’ lungs to find out whether they got bigger in those that originated from higher altitudes to compensate for meager oxygen supply.
They did not.
But the birds did have larger hearts to circulate oxygen around the body.
The findings in the Journal of Experimental Biology have implications for their future as they seek more comfortable conditions due to climate change.
Spence added: “‘Our results suggest lower oxygen availability and low air pressure may be difficult challenges to overcome for hummingbirds.”