Abstract

Floral nectars are rich in energy but contain only traces of amino acids, and it has been hypothesized that arthropods may be an important source of energy and amino acids for hummingbirds. We studied the nitrogen requirements of hummingbirds as well as how they use small arthropods to satisfy their nitrogen and energy requirements and how organ mass varies with nitrogen intake. Non-reproductive green-backed firecrowns Sephanoides sephaniodes were maintained for 10 days on diets containing 0%, 0.96%, 1.82%, 4.11% and 11.1% nitrogen (dry-matter basis). A second group of individuals were fed with varying amounts of nitrogen-free nectar supplemented with fruit flies. Finally, non-reproductive hummingbirds were captured as a control group for analysis of organ mass and size as well as fat content. The maintenance nitrogen requirement of green-backed firecrowns determined by regression was 1.42 mg N day-1, yet they required nearly 10 mg N day-1 to maintain body mass. When arthropods were available, we observed that hummingbirds required approximately 150 fruit flies to maintain body mass, which corresponds to a 5% nitrogen diet. Interestingly, when nectar was restricted (to 4 ml day-1), or was absent, arthropods alone were not able to satisfy the body mass balance requirements of hummingbirds, suggesting that arthropods are not adequate as an energy source. In the group offered an 11.1% nitrogen diet, the size and surface of the small intestine, and liver and kidney mass increased in comparison with the control group (non-reproductive field hummingbirds) or the nitrogen-free group, suggesting a nitrogen overload. Our results are in agreement with other studies showing low nitrogen requirements by nectarivores. An important point to stress is that nitrogen digestibility declined in the 11.1% nitrogen diet, which strongly supports our nitrogen absorption saturation hypothesis.