In Season 1, Episode 2 of Planet Earth II, Sir David Attenborough describes how one special hummingbird has gone through great lengths to avoid competition. In tropical jungles, nectar is first come first serve, so competition is furious among the hundreds of hummingbird species. However, one particular hummingbird species, the sword-billed hummingbird (Ensifera ensiferas) has developed certain adaptations to completely avoid this. One of these adaptations is the development of an extraordinarily long beak, which can range anywhere from 8.1 to 12 cm in length [1]. The sword-billed hummingbird is the only bird species to have a bill longer than its body, which allows it to reach deeper into flowers and drink nectar. This specialist bird mutualistically evolved to drink nectar from the unique flower Passiflora mixta [2]. This flower, located primarily in Central and South America, have floral tubes that can range in depth from a few centimeters to 14 centimeters [3]. Mutualism happens when mutually beneficial relations drive reciprocal species evolution. In this case, the benefits that the long-beaked hummingbird receives from interacting with P. mixta have promoted the development of a longer beak. As no other hummingbird has developed this long-beak adaptation, the long-beaked hummingbird is the only pollinator that is able to reach the pollen grains and drink the nectar from the plant [3].
With this said, there are some trade-offs. Even though sword-billed hummingbirds have the advantage of a secure food source, they lose the ability to preen traditionally. It is simply impossible for these hummingbirds to use their bills to maintain their feathers and remove harmful ectoparasites. Research has shown that even with their long, unwieldy beaks hummingbirds can effectively groom themselves using their feet to scratch out louse eggs and parasites [4]. In this case, evolution has selected for an adaptive grooming behaviour to compensate for the bill morphology.
Some would think that the sword-billed hummingbirds developed their long beaks by trying to adapt. It is a common misconception that natural selection involves organisms trying to adapt. However, natural selection does not involve effort, trying, or wanting. According to Charles Darwin’s postulates, natural selection occurs when variants in a population produce more offspring than others due to an advantageous heritable trait. For sword-billed hummingbirds, their beaks became longer over time because they provide an advantage for surviving and producing offspring. In this case, the evolutionary advantage is their ability to reach the nectar of long petaled flowers, which is an unattainable food source to others.
By Cheng Jiao, Sarah Nguyen, Jerry Yue
References
- Soteras F, Moré M, Ibañez AC, Iglesias MR, & AA Cocucci. 2018. Range overlap between the sword-billed hummingbird and its guild of long-flowered species: An approach to the study of a coevolutionary mosaic. PLoS ONE. 13(12); e0209742
- Lindberg AB, Olesen JM. 2001. The fragility of extreme specialization: Passiflora mixta and its pollinating hummingbird Ensifera ensifera. Journal of Tropical Ecology 17: 323-329
- Abrahamczyk S, Souto-Vilaros, D, & Renner, SS. 2014. Escape from extreme specialization: passion flowers, bats and the sword-billed hummingbird. Proceedings of the Royal Society Biological Sciences 281: 1471–1478
- Clayton D & Cotgreave P. 1994. Relationship of bill morphology to grooming behaviour in birds. Animal Behaviour 47: 195-201
The Evolution of Planet Earth 