A NEW DIMENSION TO HUMMINGBIRD - FLOWER RELATIONSHIPS

The close correspondence between the bills of hummingbirds and the len gths of the flowers they feed from has been interpreted as an example of coadaptation. Observations of birds feeding at flowers longer and s horter than their bills, however, and the lack of experimental mental evidence for any feeding advantage to short bills, seem to contradict this interpretation. I address this problem by considering a little-st udied dimension of floral morphology: corolla diameter. In laboratory experiments on female ruby-throated hummingbirds (Archilochus colubris ), probing abilities (maximum extraction depths) increased with increa sing corolla diameter. Handling times increased with decreasing coroll a diameter, resulting in ''handling time equivalents'', i.e., flowers having the same handling times but different lengths and diameters. Lo nger-billed birds had greater maximum extraction depths and shorter ha ndling times than shorter-billed birds at all corolla diameters greate r than the width of the bill. In contrast, shorter-billed birds made f ewer errors inserting their bills into narrow flowers. Hence, differen ces in bill lengths apparently are associated with trade-offs in forag ing abilities, whereby longer-billed birds are able to feed at long fl owers and may do so more quickly, whereas shorter-billed birds are abl e to feed more successfully at narrow flowers.