Findings: Big-drinking small birds

March 22, 2002

Hummingbirds are high-maintenance creatures, writes Caroline Davis.

They are one of the smallest warm-blooded vertebrates and their heart rate is about 1,260 beats a minute, their wings can beat as fast as 200 times a minute during courtship and they fly thousands of miles during migration.

An American researcher believes that highly specialised kidneys are the secret of their success. To keep energy levels high, they can take in up to five times their body mass in nectar every 12 hours, but this means they are drinking several times their body mass in water a day. Humans succumb to polydipsia - water poisoning - before they get anywhere near to drinking their body mass.

Todd McWhorter, a doctoral ecologist at Arizona University, said:

"Hummingbirds have a water turnover rate higher than aquatic amphibia and nearly as high as freshwater fish. How do they deal with all that water?"

Earlier theories assumed that the birds absorbed only a fraction of the water and let the rest pass through.

But Mr McWhorter, working with Carlos Martinez del Rio of Wyoming University, carried out field trials by injecting birds with a harmless radioactive marker. They then measured the birds' nectar intake, marker decline and body mass. They discovered the birds absorbed 80 per cent of the dietary water they took in, implying all this liquid must be processed by the kidneys.

But they also found that powerful as these kidneys must be at excreting large volumes of dilute urine, they have their limits.

If hummingbirds consume too much water, for example in cooler climates, they have trouble absorbing the calories they need to stay warm and maintain their weight.

Mr McWhorter said the work would help towards understanding diabetes as nectar is high in sucrose. "Hummingbirds have very high blood sugar. But these birds have none of the eye, liver, kidney or other problems associated with diabetes."

Mr McWhorter's research is published in the Journal of Experimental Biology .

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