| Green hylia | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Passeriformes |
| Family: | Hyliidae |
| Genus: | Hylia Cassin, 1859 |
| Species: | H. prasina |
| Binomial name | |
| Hylia prasina (Cassin, 1855) | |
 | |
Thegreen hylia (Hylia prasina) is amonotypicgenus widespread in tropical Africa, where it mostly inhabits theunderstory and mid-stratum ofmoist forest. It is a canopyinsectivore which had been tentatively placed within the family ofCettiidae warblers, but in 2019 its assignment to a new family, theHyliidae, was strongly supported.[2][3]
Hylia prasina is amonotypicsongbird that has proven difficult to place within thephylogenetic tree.[4][5] Connections betweenHylia and othergenera have been attempted by comparingDNA sequences, physical similarities and even behavioural characteristics. None of these links have been sufficiently proven to allow definite classification of the green hylia.[4]
The green hylia belongs to the orderPasseriformes and thesuperfamilySylvioidea, however the classification at family level is under dispute. The species is often attributed to the familyCettiidae (bush warblers) along withScotocerca,Erythrocercus,Tesia,Cettia andAbroscopus. Investigation intoindels does not support a strong relationship betweenHylia and the Cettiidae.[4]
Anatomically, the green hylia has similarities tosunbirds andwarblers. Characteristics shared with sunbirds include a long hyloid with flattened epibranchial horns, a brush-tipped tongue andmembrane-covered nostrils. Similarities to some warblers include very comparable colouring.[5] The relationship between the generaHylia andPhylloscopus (leaf warblers) has been examined but seems to have low empirical support.[6] A sister relationship betweenHylia andAegithalidae (long tailed tits) was similarly poorly supported.[4] Close relationships withNectariniidae (sunbirds),Estrildidae (finches) andMeliphagidae (honeyeaters) have also been rejected.[5]
The sister relationship betweenHylia andPholidornis (tit hylia) is strongly supported, based onmitochondrial data and physiological similarities.[4][5] It has been suggested that both genera should be placed in a new family 'Hyliidae', pending further confirmation. It is likely that the green hylia, along with other closely related species, have an uncertain position in the phylogenetic tree as a result of rapid radiation of the families within the Sylvioidea.[4]
The green hylia has dark brown irises, a black bill and olive green feet.[7] This species is small, weighing approximately 14 g.[5] There is no noticeable difference in colour between the sexes, however males are generally larger.[8]
There are twosubspecies,H. p. prasina andH. p. poensis, which differ in the colouring under the throat.H. p. prasina has an olive green chin and throat and olive-grey underbelly whileH. p. poensis has a white-grey throat and underbelly.[8][9]
The green hylia uses several different calls, the most common being clear whistles (kee kee) or dry scolding rattles (trrit trrrit).[5] It is known for its recognisable, pure, two-note song. Alarm calls used to warn of nearbypredators are short as this makes it difficult for predators to locate exactly where the call is coming from.[10][11]
When played imitation recordings, males reacted more strongly to longer notes.[11] Analyses of green hylia songs indicated that a build-up of reverberations at the samefrequency will lead to longer and louder note tails. This means that dense vegetation can alter birdsong transmissions to allow for a longer and louder signal, using the same amount of energy for vocalisation. The strengthening of the green hylia's song could be beneficial to the species for defending territory or attracting mates.[10] The elongation of signals with narrow bandwidth has been found to be a beneficial consequence of reverberations on birdsong transmission.[11]
Song characteristics will vary depending on habitat structure andambient noise. Since the species uses narrow bandwidth songs they must adjust their frequency relative to influences such as insect vocalisations in order to be heard. Green hylias were found to sing at a lower frequencies when at higher elevations, cooler temperatures and lower tree coverage. When insect sounds are at a low frequency, the songs are found to be even lower. This behaviour has been interpreted asbehavioural plasticity (auditory feedback in each specific environment to determine the clearest frequency), determination bygenetic components, or a combination of both.[12]
The green hylia occurs in a wide range throughout most of tropical Africa. The subspeciesH. p. poensis can only be found on islands in theGulf of Guinea.H. p. prasina is much more widely distributed throughout the Guineo-Congolian forest and is found from westernGambia to westernKenya.[9]
The green hylia is a common forest generalist, occurring at forest edges and gaps between vegetation as well as forest interiors.[13][14] Its preferred habitat is mostly intact, semideciduous moist forest.[15] Green hylia habitat includes plant species such asElaeis guinensis,Celtis zenkeri,Cola gigantea,Acalypha ornata,Markhamia platycalyx,Coffea canephora,Albizia sp.,Fagara macrophylla,Funtumia elastica,Pycnanthes angolensis,Musanga cecropioides andXylopia aethiopica.[14][16]Liana andScleria species provide suitable habitat as they tangle amongst other vegetation, providing nesting materials and a supply of food.[16] Analysis of carbon sources in green hylia food, indicating the origin of the consumed insects, showed that carbon present derived exclusively fromC3 plants, which are mostly forest plants, and notC4 plants as might be found in farmland crops, even when these areas border forest habitats.[17]
The green hylia is afoliage-gleaning insectivore of the canopy and forest understory, usually foraging at a height of about 10 metres.[13][14][16] Its diet includes insects, ants or butterflies which are accessible in its feeding area, on the underside of leaves or amongstforest litter.[5] The species is considered a forest generalist as it is capable of surviving in a wide range of environmental conditions.[18]
The green hylia is usually observed alone or in a mating pair.[8] Mating pairs roost together in their nest. The eggs are white and otherwise not very distinctive.[5] The nest is built a few feet from the ground in a suitable location, such as the vertical forks of young palm trees. The nest is oval and domed in shape with a 20 mm circular opening at the side of the top. Nests are crumbly andfriable, the bulk being made up of loose tufts of plant matter which are not fragmented, interwoven or compacted. The outside is covered by a fine layer of fibrous strips topped with the occasional leaf skeleton. The nest measures 150 mm from top to bottom and 90 mm side to side and front to back.[8] It differs from that of other sunbirds in that it is positioned in forks of vegetation and not suspended in the air.[5]
The green hylia is currently classified asLeast Concern by theIUCN based on large population numbers and wide distribution.[1] The species is however likely under pressure fromhabitat destruction. While the green hylia as a forest generalist is not as sensitive to habitat disruptions as other species, understory and leaf litter clearance does affect its opportunities for foraging and nest building. A significant decline in green hylia population has been observed inplantation areas (as opposed to logged areas) where these habitat components are absent.[18]
Thescarp forest habitat of the green hylia is increasingly being cleared forcharcoal production and to make way foragriculture. The entire forest understory in certain areas is being cleared throughslash and burn techniques to make way for the farming of crops such as bananas, maize or beans. These crops do not support species such asHylia prasina as they do not possess sufficient understory environments. It is currently unknown how quickly the destruction of these habitats is occurring. While the green hylia has so far not been observed to be affected by this type of habitat clearing, many species with shared habitat requirements are in decline.[19]
Parasites in African habitats may affect the species.Plasmodium parahexamerium was identified as infecting the green hylia in 2009, with as yet unclear ramifications. This parasite was thought to only inhabit New World hosts, thus its appearance in an Old World bird is both interesting and concerning. Despite this, parasites are not known to be a significant threat to the species as a whole.[20]
