The production of dilute nectars by hummingbird and honeyeater flowers

A survey of data from tropical and temperate regions confirms that nectars of hummingbird and honeyeater flowers are dilute, especially relative to nectars of bee flowers. We use these data, along with theoretical considerations, to examine three recently proposed hypotheses to explain low concentration of hummingbird nectars. None of the quantitative or qualitative predictions of these three hypotheses appears to be upheld. We discuss possible weaknesses of each hypothesis and then present a general framework which may be useful in generating new hypotheses to explain the evolution of nectar concentration.