Keeping fit and healthy on a low-fat, fibre-free diet isn't easy, but despite the nutritional disadvantages of life on a liquid lunch, hummingbirds flourish by supplementing their nectar intake with tiny arthropods. But the beneficial snacks come at a high metabolic price; flies don't sit still, so hummingbirds work hard chasing their protein. Just how much nitrogen a hummingbird extracts from the protein in its diet, or the amount of effort needed to gather it, wasn't clear, so Francisco Bozinovic and his colleagues in Chile, began tempting the tiny birds with nitrogen laced nectar and found that although their protein requirements were relatively meagre, the tiny creatures' metabolic demands were colossal: 43 kJ day-1(p. 3349)!

`Hummingbirds are great to work with' says Bozinovic, `they respond quickly, or they die', so he knew that he would soon see how the birds fared as he toyed with their diet. Working with Victoria López-Calleja and Maria Jose Fernández, he trapped almost 40 green-backed firecrowns in central Chile, before transporting them to an aviary in Santiago ready to test out their metabolism.

Back in the lab, the team prepared nectar solutions with different concentrations of amino acids to see how much protein the birds needed to maintain a stable body weight. By filming the birds as they sipped from feeders the team could measure the amount of energy and nitrogen that the birds consumed. But to calculate the bird's nitrogen uptake, Fernández and López-Calleja also needed to know how much waste nitrogen the birds lost. Setting up an around-the-clock watch, they collected all of the birds'faeces, making sure that none dried out, and measure the nitrogen content.

Not surprisingly, the birds that were fed small amounts of protein began losing weight quickly, even though they were able to sip as much high-energy nectar as they wanted. However, the birds that were fed 1.82% nitrogen or more, held their weight. Bozinovic calculated that the tiny aeronauts need at least 10 mg nitrogen per day to maintain a stable body weight, or else they waste away.

But what does that translate to in terms of flies? Fernández and López Calleja provided the birds with 500 fruit flies to snack on while offering them either an unlimited nectar supply, a restricted nectar intake,or no nectar at all. After five days of access to flies and nectar, the birds were fit and healthy, catching around 150 flies a day, sufficient to supply them with 5% nitrogen. The birds that had a reduced nectar supply also maintained a stable weight, although they went into torpor overnight to conserve energy. But the birds that were fed flies alone were in trouble. Their weight began dropping, not matter how hard they worked to feed themselves.

Fernández says that she's surprised that `flies are not a complete food source for hummingbirds'. She suspects that although the flies should supply all of the hummingbirds needs, the birds simply have to work too hard to catch flies to rely on them as their soul food source.

López-Calleja, M. V., Fernández, M. J. and Bozinovic, F. (
). The integration of energy and nitrogen balance in the hummingbird Sephanoides sephaniodes.
J. Exp. Biol.