| Japanese White-eye | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Passeriformes |
| Family: | Zosteropidae |
| Genus: | Zosterops |
| Species: | Z. japonicus |
| Binomial name | |
| Zosterops japonicus (Temminck and Schlegel, 1847) |
|
The Japanese White-eye (Zosterops japonicus), also known as the mejiro (メジロ, 目白), is a small passerine bird in the white-eye family. The specific epithet is occasionally written japonica, but this is incorrect due to the gender of the genus. Its native range includes much of east Asia, including Japan, China, Vietnam, Taiwan, and the Philippines. It has been intentionally introduced to other parts of the world as a pet and as pest control, with mixed results. As one of the native species of the Japanese islands, it has been depicted in Japanese art on numerous occasions, and historically was kept as a cage bird.
Introduced to Hawaii in 1929 as a means of insect control, it has since become a common bird on the Hawaiian Islands, and has become a vector for avian parasites that are now known to adversely affect populations of native birds such as Hawaiian honeycreepers, as well as spreading invasive plant species through discarded seeds [1].
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The Japanese White-eye is olive green on its back, from anterior to posterior, and is pale green on its underside. Its feet, legs, and bill range from black to brown.[1] It has a green forehead and a yellow throat. The White-eye has rounded wings and a long, slender bill – both of which indicate this bird to be very acrobatic. Its wings are dark brown, but outlined in green. Like other white-eyes, this species exhibits the distinctive white eyering that gives it its name (mejiro also meaning "white eye" in Japanese). Adults range from 4 to 4.5 inches in length, and weigh between 9.75 and 12.75 grams.[1]
This bird species is rarely found on the ground. It is a very sociable species that may form flocks with other species, in which the birds form groups to forage during flight; White-eyes only flock with birds of other species outside of the breeding season.[1] Allopreening - the art of cleaning, grooming, and maintaining parts of the body – is extremely common. Interspecific allopreening (between different species) has been observed in captivity.[1] While sociable, however, the White-eye typically forms monogamous relationships with mates – it has only one mate at any one time.
Social hierarchy in a flock is established through physical displays. Some of these displays are not sex dependent, such as wing flicks exposing the underwing, wing flutters and vibrations, as well as open beak displays and beak snaps (rapid shutting of the beak to make a snapping noise).[1] During breeding seasons, however, males establish territories via the sex-specific activity of singing loudly.[1] Males will fend off intruders of the same species, yet will allow other species of birds to nest inside of their territory.
Pairs of individuals, generally monogamous, choose a location for the nest between 1 and 30 meters above ground level.[1] Construction of the nest lasts 7-10 days on average, and a variety of nesting material may be used (living and non-living); spider webs, moss, lichens, and mammal hair are all examples of building media that the birds employ. When building nests, they often steal material from the nests of other birds.[2] Nests tend to be cup shaped, with a diameter of 56.2 mm and a depth of 41.7 mm.[1] The majority of nests are only used once, but some may be used up to three times in any given season.[1]
The species is omnivorous, living on a diet of fruit from several species of flowering plants, various types of insects, and nectar at all levels of foliage. It feeds on insects by searching the leaves of flowers and scouring tree bark for larvae. [3] Consequences of its diet include regulation of local insect populations and dispersal of seeds; however, the White-eye’s seed-dispersal ability does not seem to be significant in Hawaii.[4]
The Japanese White-eye is naturally found in Japan, Taiwan, eastern China, and the northern Philippines. Migratory populations of the bird spend winters in Burma, Hainan Island, and Vietnam. The White-eye is widespread and common in Japan, considered one of the more dominant bird species.[5]
The Japanese White-eye, originally introduced in O’ahu in 1929, has rapidly expanded its population and can now be found on every island of Hawaii; the climates of these islands range from tropical rain forests to deciduous forests.[6] After subsequent releases and natural range expansion (enlargement of the area it occupies), the White-eye was determined to be the most abundant land bird on the Hawaiian Islands as early as 1987.[7]
Organisms known to prey upon native Hawaiian avian species include small mammals, like the Polynesian, black, and Norway rats, and the mongoose.[8] There are no documented predators of the Japanese white-eye. It can only be inferred that organisms that prey on small mammals and birds in the same area as the Japanese White-eye also prey on the White-eye itself.
In its natural habitat, the Japanese White-eye competes with other native passerine bird species, including those of the same genus, such as the Bonin White-eye (Apalopteron familiare).[9] In Hawaii, the Japanese White-eye competes with native passerines such as the Common 'Amakihi, for food (such as nectar and fruit), as well as for space.[10][11] In Hawaii, the Japanese White-eye has been observed visiting endemic (native) floral species thought to have coevolved with endemic nectarivorous avian species (those that eat nectar).[7] This means that, over time, changes in native flowers have triggered changes in native birds that feed on the nectar of these flowers; the latter change then triggers another change in the flowers, and the whole process continues to repeat itself. The visitation of the White-eye, along with the disappearance of those endemic nectarivorous passerine bird species, suggests that the White-eye is out-competing those endemic species for the floral resource (nectar).
The Japanese White-eye’s successful invasion of the Hawaiian Islands can be partially attributed to the lack of coevolution between endemic species and the White-eye.[10] The occurrence of coevolution is driven by species interactions that directly impact physical development. In many cases coevolution is derived from competition in which both species vie for an edge or advantage to maximize their dietary or “resource” acquirement. Because the White-eye did not coevolve with avian species native to Hawaii, the White-eye has certain advantageous characteristics, such as the resistance to avian malaria, that the native species do not possess.[12] The native species never had a chance to change in response to evolutionary changes in the White-eye.
White-eye range expansion has also been cited as a negative effect on native bird species.[7] White-eye expansion is arguably characteristic of what E.O Wilson called a “Taxon Cycle”.[13] The cycle attempts to model the interactions between endemic species and newly immigrated non-native populations through the use of “Stages” –each with defined characteristics with respect to overall population behavior.[13] A newly immigrated species (Stage 1) is expected to experience rapid growth through the new habitat largely due to the lack of effective endemic (Stage 3) persistence – the ability of the community to repel outside forces that may cause changes in its species composition. This prediction matches much of the current data indicating that the White-eye has become highly common on most if not all of the Hawaiian Islands.[10] The force of natural selection has promoted the dietary or “resource” specialization of many of the later stage endemic species – a prime example being the honeycreeper.[10] This particular species has become especially affected by the White-eye presence due to honeycreeper dependence on nectar as a primary resource.[14] In contrast the White-eye maintains a highly diverse selection of dietary options (including nectar)[14] and is able to take full advantage of numerous habitats on the islands. The differences in resource limitations between the two species has resulted in the drastic decline of the honeycreeper population, as they are out competed by the invasive White-eye.[10][14] This occurrence is best explained by the Competitive Exclusion Principle, which dictates that two complete competitors cannot co-exist.
Increases in the Japanese White-eye population in Hawaii have negative effects on the growth and survival of native birds in the community. In one study, the bill length, tarsus length, and mass of native Hawaiian passerine birds were measured during 1987-2006 using the technique of mark and recapture. In 2000, juveniles (not-yet-adults) of every native species showed lower mass and shorter bills than before.[14] These changes led to decreased juvenile survival and decreased survival of second year individuals/older adults, respectively. These birds also showed shorter tarsi, the group of bones in the hind feet of some vertebrates; although this change was less drastic than that seen with the bills. Birds with original bill lengths closest to that of the White-eye suffered the most, undergoing changes that lowered their foraging efficiency. For example, the endangered Hawaiian akepa was viable during 1987-1999 but not during 2000-2006, associated with an abrupt increase in White-eyes.[14] These facts document strong community-wide exploitative competition for food between the Japanese White-eye and passerine birds native to Hawaii, meaning that the White-eye depletes the availability of food for other bird species. They also compete for space; the White-eye has expanded its range into remote areas within the last two decades: 1980-2000.[11] The distribution of the Japanese White-eye has been shown to negatively correlate with the distributions of native birds, meaning that as the white-eye becomes more highly distributed, native birds become less distributed.[11] Many Hawaiian birds are endangered or already extinct; this occurrence is believed to be related to the invasion of the White-eye.[7]
Because many of the Hawaiian birds that have gone extinct acted as pollinators, there is a concern about the survival of certain plant species, such as the Hawaiian Lobelioideae. The Japanese White-eye has been pinned as a possible replacement pollinator.[7] Lobelioids that were pollinated by native birds that are now extinct may benefit from the presence of the White-eye, but more detailed studies are needed to determine the degree of pollen transfer affected by White-eyes. Also, this replacement could lead to differing levels of outcrossing compared to those resulting from visits by the native species of birds. Outcrossing is the introduction of unrelated genetic material into a breeding line. Therefore, White-eye pollination may prove to be beneficial, as outcrossing increases genetic diversity, thus reducing the probability of all individuals being subject to disease; increasing genetic diversity also reduces genetic abnormalities. Again, a more detailed study is needed to determine the potential benefits of White-eye pollination.
A similar trend to that of Hawaii’s was seen when the bird was introduced to the Bonin Islands of Japan; while native to the country, the White-eye was not originally present on these islands. As in Hawaii, the study conducted in the Bonin Islands suggests that the Japanese White-eye may be taking over the actions of native birds in relation to plants present on the islands. Unlike Hawaii (in which this action was pollination), the action taken over by the White-eye in the Bonin Islands is seed-dispersal; this dispersal was previously carried out by native birds – whether still present or already extinct.[15] The White-eye has not been cited as taking over seed-dispersal in Hawaii because they rarely ingest the seeds of Hawaiian plants, such as the Firetree.[4] For this reason, they cannot disperse the seeds over great distances, making them poor seed dispersers.[4] While the actions of the White-eye on the Hawaiian and Bonin Islands are indeed different, they are both significant. This is because islands themselves are particularly vulnerable to the drastic effects of introduced competitors, predators, and diseases, because these communities have evolved in their absence.[15]
Native species need normal juvenile mass and bill length to recover/persist, but in order for this to happen, food must be restored to former levels. There is support that an introduced bird, such as the Japanese White-eye, is responsible for the food shortage. Control of the White-eye is therefore essential for the recovery of native Hawaiian birds.[14] The determination of the status of native birds is essential; those found to be endangered could possibly benefit from the designation of critical habitat.[11] In 1980, a program to eradicate (eliminate) the Indian White-eye in California involved mist-netting and shooting the birds, and this proved to be the most successful of the various capture methods explored.[16] Whether eradication is feasible and applicable to other instances of invasive exotic birds is yet to be determined, but could be considered a possibility for the eradication of the Japanese White-eye in the Hawaiian Islands. However, because the White-eye’s current ecological role is not fully understood (i.e. seed disperser, pollinator, etc.) further studies are necessary before any drastic measures are taken.
"Zosterops japonicus". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=179912. Retrieved 4 February 2006.