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Hummingbirds, like all other forms of life, have undergone various changes to the overall morphologies in order to meet the requirements of the environment. The bird’s habitat is exclusively in the Western hemisphere making it diverse, ranging from the southern parts of Alaska to Chile. The evolutionary changes in the dimorphism have caused various hummingbird species to have distinct morphologies in respect to the shape and size of bills. The birds are vital to the environment because of the active participation in the very diverse and ecologically vital notion of pollination. The long, tapered bills allows for the retrieval of nectar from tubular flowers and in doing so, the bird collects pollen from flowers that are solely dependent on hummingbirds for the spreading of pollen.

The variation in the form of bills is hypothesized, in part, to be due to differences in feeding habits, mating rituals, and the environment. These variations can influence the structure of the bill due to the role that the bill plays in selection. For example, if the bill is better suited for the flowers in the area, the birds are more likely to be able to feed and thus are more likely to reproduce (Temeles and Roberts, 1992). Moreover, if there is selection based on how well the organism can feed then post-reproductive selection can influence the morphology of the bill. For example, if a female’s bill is compatible with more flowers then the female is required to provide for her offspring, the more food sources available will influence the different shapes of the bill. Finally, in the case of the environment various factors such as genetic drift, the fluctuations in the allele frequencies due to chance events, as well as predation on the hummingbird’s food source may cause some hummingbirds that have a bill specific for the targeted flower will not be as successful. However, on islands one cannot ignore the influence of the founder effect and how the population bottleneck may have altered the gene frequencies and produced a population that was carrying traits for over or under exaggerated traits.

Reproductive Influence There seems to be a strong correlation in the evolution of the dimorphism of the bills due to the reproductive strategies of the hummingbird in respect to the mating rituals carried out by males (Bleiweiss, 1999). The intersexual selection or selection genders apply to the other may cause, for example, males to gain the attention of females by securing the most flowers which directly correlates to the success in acquiring a mate. Since the H. caricaea has more flowers per plant, the males may have developed bills more specialized to the straight flowers of this plant since reproductive success is dependent upon the number of flowers the bird can acquire (Scheinbach, 2000). Moreover, when the species first came to the island the males may have out-competed the females for the more fruitful H. caricaea due to the larger size. This would have forced the females to adapt and find a different source of food, H. bihai. Overtime, those that had bill curvature were more suited for the curved flowers and would become more prominent, potentially leading to the overall change in bill shape. Another potential reason why females have the curved bill and therefore the ability to feed on more diverse flowers may have arisen due to the need for females to be able to feed in order to reproduce (Hainsworth, 1977). Since the female requires more energy in reproduction than males due to the size of the egg and the energy expended in providing for the zygote, the female bird needs to be more diverse and able to feed off of whatever available food options are present. If the female lacked this novel ability to feed on more flowers then offspring may be lacking and overall the population may suffer. Coupled with the idea that females are attracted to mates with a higher floral density in the male’s territory, it can be hypothesized that through natural selection these females have evolved to have these selective behaviors.

Resource Partitioning The female is the sole caretaker of the eggs, creating the need to have more food available (Hainsworth, 1977). This emphasizes why male bills have been selected not based on the ability to be diverse with to be able to use more potential food sources. Unlike the males, the females have the need to be able to readily gather food from any area, regardless of the food sources around. If the male fails to feed then another male will take his place but if the female lacks the ability to provide for her young then the population suffers. Having a curved bill allows them to feed from both straight flowers and curved flowers. Moreover, since the males generally have little cost in providing gametes as well as in providing for offspring, it is no surprise that the bills are not as applicable to the various flowers since selection would not be as heavily dependent upon this notion (Hainsworth, 1977). Furthermore, by allowing the genders to feed on specialized food sources also known as resource partitioning, the birds can more efficiently use energy and, in the case of females, have more time to care for the nest and young or to look for potential mates. By creating separate food sources for the genders, the hummingbirds have increased the likelihood for both genders to be able to feed and thus strengthens both genders while also increasing the likelihood of a prosperous population.

References

Bleiweiss, Robert. 1999. Joint effects of feeding and breeding behavior on tropic dimorphism in hummingbirds. The Royal Society: 2491-2496.

Hainsworth, Reed F. 1977. Foraging Efficiency and Parental Care in Colibri coruscans. The Condor, 79:69-75

Scheinbach, Saul. 2000. A diet That Fits the Bill. Science Watch. http://www.hras.org/sw/science.html

Temeles, Ethan J. and Roberts, W. Mark. 1992. Effect of sexual dimorphism in bill length on foraging behavior: an experimental analysis of hummingbirds. Ocologia, 94: 87-94.

How has hummingbird bill size evolution impacted their ecological niche?

5 primary sources

Nathan Muchhala The American Naturalist Vol. 169, No. 4 (April 2007), pp. 494-504 Published by: The University of Chicago Press Article Stable URL: http://www.jstor.org/stable/10.1086/512047

Effect of Sexual Dimorphism in Bill Length on Foraging Behavior: An Experimental Analysis of Hummingbirds Ethan J. Temeles and W. Mark Roberts Oecologia Vol. 94, No. 1 (1993), pp. 87-94 Published by: Springer Article Stable URL: http://www.jstor.org/stable/4220321

Carlos Rengifo, Luis Cornejo and Iván Akirov Journal of Tropical Ecology Vol. 22, No. 6 (Nov., 2006), pp. 613-619 Published by: Cambridge University Press Article Stable URL: http://www.jstor.org/stable/4092122

Ethan J. Temeles, Yan B. Linhart, Michael Masonjones and Heather D. Masonjones Biotropica Vol. 34, No. 1 (Mar., 2002), pp. 68-80 Published by: The Association for Tropical Biology and Conservation Article Stable URL: http://www.jstor.org/stable/4132958

Ethan J. Temeles, Jill S. Miller and Joanna L. Rifkin Philosophical Transactions: Biological Sciences Vol. 365, No. 1543, Darwin's Galápagos finches in modern evolutionary biology (12 April 2010), pp. 1053-1063 Published by: The Royal Society Article Stable URL: http://www.jstor.org/stable/20721443