Topic: Hummingbirds, sunbirds and honeyeaters
One of the most well known examples of convergence among birds is between hummingbirds, sunbirds and honeyeaters, all of which are small, dominantly nectar-feeding birds.
One of the most well known examples of convergent evolution among the birds is the convergence between Neotropical hummingbirds, African-Asian sunbirds and Australasian honeyeaters, all of which are small, dominantly nectar-feeding birds.
Origins and distributions
Hummingbirds, sunbirds and honeyeaters all originated from independent ancestors, although sunbirds and honeyeaters are both classed as ‘passerine’ birds and are more closely related to each other than to hummingbirds. Hummingbirds are ‘apodiform’ birds of the family Trochilidae, native to neotropical parts of the Americas, from Argentina through Mexico and the south western United States. Interestingly, they apparently originated in the Old World, (the oldest known fossils are from the early Oligocene of Europe) and evolved by a transition from tree-dwelling to aerial foraging forms. Sunbirds comprise the family Nectariniidae and can be found throughout Africa (especially diverse in Southern Africa) and Southern Asia. Honeyeaters belong to the family Meliphagidae, and are distributed throughout Australia, New Guinea and to a lesser extent New Zealand and various Pacific Islands. In addition, it has been discovered that a very recently extinct group of ‘honeyeaters’ from Hawaii are in fact a new family of nectar-feeding birds, termed the Mohoidae, comprised of two genera: Moho and Chaetoptila. The ancestors of the Mohoidae were American songbirds rather than honeyeaters, but their morphology, behaviour and ecology were resoundingly similar to certain Australo-Pacific honeyeater species, thus representing an exciting new case of convergence.
Hummingbirds, sunbirds, honeyeaters and mohoids are small, light birds, with beaks that may be highly elongated and either straight or recurved, depending on what type of flower they typically probe for nectar. Their tongues are extensible, tipped with brush-like filaments and are either tubular or grooved (‘semi-tubular’) in order to generate capillary action for drawing nectar. All these birds are critical pollinators for a number of flowers (e.g. Phaethornis ‘hermit’ hummingbirds pollinate Heliconia), and as an adaptation to the large amount of pollen they are exposed to their nostrils (technically termed ‘nares’) have a cover or ‘operculum’.
Long tarsal bones and strong feet are shared adaptations for effectively reaching into flowers when perched, and hummingbirds and most sunbirds are also able to hover when feeding. Hummingbirds are famous for their ability to hover with incredible precision, and can even fly backwards, the aerodynamics of which has been studied in the Rufous hummingbird Selasphorus rufus. Sunbirds such as the Crimson sunbird Aethopygon siparaja and the Malachite sunbird Nectarinia famosa of Southern Africa have short hummingbird-like wings, fast, direct flight, and are also capable of hovering, but they attain somewhat less precision in the air and more often feed from a perched position.
A diet of nectar is very sugar and water-rich, but low in proteins, which are essential for growth, basic body functioning and repair. Hummingbirds, sunbirds and honeyeaters all augment their diet with essential proteins by sporadically eating small arthropods such as insects and spiders. Closely related to the sunbirds are the flowerpeckers (family Dicaeceeae) of the genera Prionochilus and Dicaeum, which are small, stout birds that do not have elongated sunbird-like beaks, but they do share beak curvature for reaching into floral nectaries and possess a specialised tubular tongue for sucking nectar up to the mouth. They augment their diet with berries (especially mistletoe) and insects.
Hummingbirds and hawk-moths
While nectar-feeding hummingbirds, sunbirds and honeyeaters are convergent, an even more striking and evolutionary distant convergence is that between hummingbirds and nectar-feeding or ‘hummingbirdoid’ hawk-moths. This bird-insect convergence is striking in many respects: Not only does it include general body shape and hovering in a precise spot, but it also extends to metabolic, physiological and energetic convergences. Both have effectively arrived at an identical solution. Like other birds hummingbirds are warm-blooded, but so independently are hawk-moths, which like a number of insects have evolved thermoregulation – in both cases elevated body temperatures are essential for the intense levels of metabolic activity. For an idea of hummingbird metabolism, it has been found that they beat their wings at 12-90 times a second, have the highest metabolism of any animals other than insects, and have an active heart rate of up to 1,260 beats per minute (e.g. in Lampornis clemenciae, the Blue-throated hummingbird)!
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Map of Life - "Hummingbirds, sunbirds and honeyeaters"
July 21, 2018