Editing Parallel evolution (section)

== Parallel coevolution of traits between hummingbirds and sunbirds contributing to ecological guilds ==

Hummingbirds and sunbirds, two [[Nectarivore |nectarivorous]] bird lineages in the New and Old Worlds have parallelly evolved a suite of specialized [[Behavioral ecology |behavioral]] and [[Anatomy |anatomical]] traits. These traits (bill shape, digestive enzymes, and [[Convergent evolution#Flight |flight]]) allow the birds to optimally fit the flower-feeding-and-pollination [[ecological niche]] they occupy, which is shaped by the birds' suites of parallel traits. Thus, a parallel coevolved behavioral syndrome within the birds creates an [[Emergence#Living, biological systems |emergent]] [[Guild (ecology) |guild]] of highly specialized birds and highly adapted plants, each exploiting the other's involvement in the flowers' pollination in the Old World and New World alike.<ref>{{Cite journal |last1=Janeček |first1=Štěpán |last2=Chmel |first2=Kryštof |last3=Uceda Gómez |first3=Guillermo |last4=Janečková |first4=Petra |last5=Chmelová |first5=Eliška |last6=Sejfová |first6=Zuzana |last7=Luma Ewome |first7=Francis |date=February 2020 |title=Ecological fitting is a sufficient driver of tight interactions between sunbirds and ornithophilous plants |url=|journal=Ecology and Evolution |volume=10 |issue=4 |pages=1784–1793 |doi=10.1002/ece3.5942 |issn=2045-7758 |pmc=7042734 |pmid=32128116}}</ref>

The [[Beak |bill]] shape of nectarivores, being long and needle-like, allows them to reach down a flower's [[Flower#Reproductive |pistil/stamen]] and get at the [[nectar]] within. Nectarivores may also use their specialized bills to engage in [[nectar robbing]], a practice seen in both hummingbirds and sunbirds in which the bird gets nectar by making a hole in the base of the flower's [[corolla tube]] instead of inserting its bill through the tube as is standard, thus "robbing" the flower of nectar since it is not pollinated it in return.<ref>Juan Francisco Ornelas. Serrate Tomia: An Adaptation for Nectar Robbing in Hummingbirds?. ''The Auk'', Volume 111, Issue 3, January 1994, pp. 703-710.</ref>

Nectarivores and [[Glossary of plant morphology |ornithophilous]] flowers often exist in mutualistic guild relationships facilitated by the bird's bill shape, food source, and digestive ability acting in concert with the flower's tube shape and adaptation to pollination by hovering or perching birds. The birds eat nectar using their long, thin bills and, in so doing, collect pollen on their bills; this pollen is then transferred to the next flower they feed on. This mutualism coevolved in parallel between the Old World and New World birds and their respective flowers.<ref name=":0">{{Cite journal |last1=Janeček |first1=Štěpán |last2=Bartoš |first2=Michael |last3=Njabo |first3=Kevin Yana |date=2015-01-22 |title=Convergent evolution of sunbird pollination systems of ''Impatiens'' species in tropical Africa and hummingbird systems of the New World |url=|journal=Biological Journal of the Linnean Society |volume=115 |issue=1 |pages=127–133 |doi=10.1111/bij.12475 |issn=0024-4066 |doi-access=free }}</ref> Moreover, the digestive enzyme activity in nectarivores matching the nectar composition in their respective flowers appears to have coevolved in parallel between plants and pollinators across continents, as the nectarivorous lineages independently evolved the ability to digest the nectar specific to their flowers, resulting in distinct guilds.<ref name=":0" /><ref name=":1" />

The capacity of nectarivores to digest [[sucrose]] is far greater than that of other avian [[taxa]]. This difference is due to an analogous high concentration of '''sucrase-isomaltase''', an [[enzyme]] that [[Hydrolysis |hydrolyzes]] sucrose. Sucrase activity per unit intestinal surface area appears to be higher in nectarivores than in other birds, meaning these nectarivorous avians can digest more sucrose more rapidly than other taxa.<ref name=":1">{{Cite journal |last1=McWhorter |first1=Todd J. |last2=Rader |first2=Jonathan A. |last3=Schondube |first3=Jorge E. |last4=Nicolson |first4=Susan W. |last5=Pinshow |first5=Berry |last6=Fleming |first6=Patricia A. |last7=Gutiérrez-Guerrero |first7=Yocelyn T. |last8=Martínez del Rio |first8=Carlos |date=July 2021 |title=Sucrose digestion capacity in birds shows convergent coevolution with nectar composition across continents |url=|journal=iScience |volume=24 |issue=7 |pages=102717 |doi=10.1016/j.isci.2021.102717 |issn=2589-0042 |pmc=8246590 |pmid=34235412}}</ref> Moreover, the Adaptive Modulation Hypothesis does not apply for nectarivores and sugar-digesting enzymes, meaning that two lineages of nectarivores should not necessarily both have high sucrase-isomaltase concentrations even though they both eat nectar. Thus, parallel acquisition of analogous sucrose digestive capability is a reasonable conclusion because there is no apparent cause for the two lineages to share this high enzyme concentration.<ref>{{Cite journal |last=Karasov |first=W. H. |date=1992-09-01 |title=Tests of the adaptive modulation hypothesis for dietary control of intestinal nutrient transport |url=https://www.physiology.org/doi/10.1152/ajpregu.1992.263.3.R496 |journal=American Journal of Physiology. Regulatory, Integrative and Comparative Physiology |language=en |volume=263 |issue=3 |pages=R496–R502 |doi=10.1152/ajpregu.1992.263.3.R496 |pmid=1415633 |issn=0363-6119|url-access=subscription }}</ref>