Bridled White-eye

Zosterops conspicillata conspicillata

Description

The Zosterops conspicillata conspicillata (Bridled White-eye) has light green upper parts, which become slightly lighter on the rump. It has a broad and white orbital ring. Auriculars are grayish green. The chin and throat are yellowish-white and the breast and abdomen are a dingy yellow. The wing and tail feathers are dark brown with greenish-yellow edges. The bird's upper mandible is yellow and darker than the lower mandible. Legs and feet are dark olive-grey and the iris is light amber in color. Adult females have lighter underparts, but this difference is not discernable in the field. Immatures have underparts paler yellow and the upper mandible is a light yellowish-brown.

Another name applied to this species is Zosterops conspicillatus conspicillatus. The indigenous people of Guam, the Chamorros, call this bird the "Nossa".

Behavior

This species is non-territorial, even when nesting, and it has been suggested that the white-eye breeds year-round on Guam. Nests are known from February and March; a survey reported nesting from May to July; Division of Aquatic and Wildlife Resources field notes reported a nest in June; and an observer reported a bird with enlarged gonads in August. Adults have been seen feeding fledglings in October and December. One nest contained two light blue-green eggs, and nests were found with two or three clutches. No information is available on incubation, nesting, or fledgling periods.

The bridled white-eye, like all passerines, is highly altricial, that is, helpless and naked when hatched. It is not known whether both parents participate in nest building, incubating, brooding or feeding of young. No information is available on incubation, nesting or fledgling periods. The bridled white-eye does not migrate. Based on a 1978-79 study of the bridled white-eye on Guam, it was concluded that it feeds primarily on insects, apparently taking little interest in fruit or nectar. Other members of this genus have been seen taking berries and other small fruit. In addition, it was reported that Z. c. saypani from Saipan regularly fed on an artificial "nectar-protein" solution, small meal worms, and various types of fruit.

During a study on Guam, the species was observed in flocks of three to eight individuals feeding frequently in large Ficus spp. and Guettarda sp., two of the larger trees found in the mature limestone forest at Ritidian Point in Northern Guam on Andersen Air Force Base. Flocks of birds occasionally fly high above the forest canopy, which suggests a widely separated feeding circuit. The non-territorial bird is primarily a canopy feeder that forages "warbler-like" by creeping along branches and occasionally leaf sprays searching for insects.

Habitat

The bridled white-eye has been found in the past in most available habitats on Guam including mature, pristine limestone forest, scrubby second growth, grasslands and foothills of southern and central Guam, beach strand, wetlands of Agana swamp and mixed woodlands and second growth of the northern plateau. This bird was last found on Guam in mature limestone forest.

Distribution

Predation by the introduced, arboreal, nocturnal brown tree snake, Boiga irregularis, is thought to be responsible for the present decline of the bridled white-eye on Guam. Other factors presently or formally limiting the white-eye are unknown. Because the endemic the bridled white-eye may presently be extirpated on Guam, its recovery potential is low.

Endemic to Guam, the Guam subspecies of the bridled white-eye formerly occurred island-wide. The species has closely related subspecies on Saipan, Tinian, and Aguigan (Z. c. saypani ) and on Rota (Z. c. rotensis ). A survey done by the U.S. Fish and Wildlife Service and the Guam Division of Aquatic and Wildlife Resources in 1981 estimated that 2,200 individuals remained in only 2% of their known historical range in northern Guam. By 1982 the birds were restricted to Pahon Basin at Ritidian Point (U.S. Naval Communications Area) in extreme northern Guam and a survey done in the spring of 1983 indicated that there were less than 50 remaining. Individuals were last observed on Guam in June 1983.

In addition to Department of Defense lands, the bridled white-eye may possibly occur on Federal Aviation Administration land in the Finagayen, Dededo area. This species could also occur on Government of Guam land at Anao Point and Falcona Beach, and on private lands at Urunao and Jinapsen in northern Guam.

Threats

This species was a common, endemic insectivore sought by bird watchers. It was also important as a pollinator and seed dispersal vector of native plants. Predation of eggs, young and adults by the introduced nocturnal brown tree snake, Boiga irregularis, is thought to be responsible for the present decline of the Bridled white-eye and other forest birds of Guam.

Recently, introduced diseases were considered as possible causes for both the range reductions and extinctions noted for most native forest birds on Guam, but to date, no infectious organisms have been isolated on Guam that could account for the decline. Feral dogs, cats and rats, which are found on Guam, are thought to be a major factor in the decline of the white-eye.

There have been major changes in Guam's vegetation before, during, and after the Second World War. However, substantial native habitat still remains on Guam and habitat degradation is not thought to be a major factor at present. Pesticides may have been a factor for decline in the past, however, an intensive pesticide survey conducted in 1981 on Guam concluded that pesticides were not a significant factor at present. While agonistic interactions between The bridled white-eye and black drongo, Dicrurus macrocerus, have been noted, Maben found competition to be an unlikely factor in the white-eye's decline on Guam.

Conservation and Recovery

One of the first bird species to disappear from Guam was the bridled white-eye. Because hope remains that a few specimens may yet survive, the species is included in the Endangered Native Forest Birds of Guam and Rota of Mariana Islands Recovery Plan. The recent, drastic decline of forest bird populations on Guam has been one of the most alarming and challenging endangered species problems of modern times. Current evidence points to an efficient predator of small birds, the brown tree snake, as the culprit. One of the Plan's primary objectives is to locate any specimens left in the wild and to ascertain if captive breeding is possible as a means to prevent extinction. In addition, it is imperative to develop methods of reducing predation by brown tree snakes; this action would then allow reintroduction of captive birds back into their historic range.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Reference

U. S. Fish and Wildlife Service. 18 Jan. 2000. "Guam Bridled White-Eye / Zosterops conspicillatus conspicillatus / Nosa." Threatened and Endangered Animals in the Pacific Islands. http://pacific.fws.gov/pacific/wesa/whteyeindex.html