Diet and Foraging

Microhabitat for Foraging

On Guam, the earliest description of foraging microhabitat (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ) was of birds using roadside bushes. The favored feeding locations were thought to be small Triphasia trifolia bushes that grew by the roadside, as well as waste places on the island. On shrubs, they hopped about on the branches, first on one side and then on the other . By the 1940s (1 Baker, R. H. (1951). The avifauna of Micronesia. Its origin, evolution, and distribution. University of Kansas Publications, Museum of Natural History 3(1):1–359. Close ), observers found birds to be restricted to certain areas on Guam, where they were found in low trees, including those of high, coastal cliffs. Other localities were in the central part of the island in upland low trees. Stophlet (7 Stophlet, J. J. (1946). Birds of Guam. Auk 63(4):534–540. Close ) found them in grasslands on foothills. In 1960, Hartin (54 Hartin, M.H. (1961). Birds of Guam. Elepaio 22:34−38. Close ) found the species to climb on bushes and trees and inhabiting tangantangan (Leucaena leucocephala) and Casuarina equisitifolia. During 1978–1979 (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ), birds fed frequently in large Ficus and Guettarda, two of the larger trees found in the mature native limestone forest at Urano and Ritidian Point. They were once common in the Agana Swamp and were observed once in coastal strand near Pati Point Beach. They appeared to be primarily a canopy feeder. Foraging occurred mostly among twigs and small branchlets, but birds also occasionally foraged among leaf sprays . In 1981, Engbring and Ramsey (55 Engbring, J., and F. L. Ramsey (1984). Distribution and Abundance of the Forest Birds of Guam: Results of a 1981 Survey. United States Fish and Wildlife Service, Washington, D.C. and Honululu, HI. Close ) observed them to forage actively in the upper canopy.

In 1931, Coultas (in 1 Baker, R. H. (1951). The avifauna of Micronesia. Its origin, evolution, and distribution. University of Kansas Publications, Museum of Natural History 3(1):1–359. Close ) described the Bridled White-eye as frequenting gardens and shrubs in villages on Saipan and Tinian, where it climbed over potted plants on the window ledges of dwellings. He thought it no longer a bird of the forest, as there was none to go to. In the 1940s on Tinian (56 Downs, T. (1946). Birds on Tinian in the Marianas. Transactions of the Kansas Academy of Science 49: 87–106. Close ), it occupied low brush or trees and occurred at edges of open fields. Also in the 1940s, Marshall (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ) described foraging habitat as small twigs in all kinds of habitat, but there was a marked preference for trees and bushes that had small leaves or leaflets. On Saipan (43 Stott, K. (1947). Notes on Saipan birds. Auk 64(4):523–527. Close ), it was observed frequenting Casuarina equisitifolia stands, semi-wooded hillsides, and sugarcane fields. More recently in the Northern Mariana Islands, Engbring et al. (36 Engbring, J., Ramsey, F.L. and Wildman, V.J. (1986). Micronesian Forest Bird Survey, 1982: Saipan, Tinian, Agiguan, and Rota. US Fish & Wildlife Service Report, Honolulu, Hawaii. Close ) reported foraging microhabitat as the upper canopy of mature native forest or scrubby second growth, although urban areas and even dense herbaceous vegetation along fence rows and in fields were occupied.

In native limestone forest on Saipan in 1988 (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ), the species was found to forage mostly in the top outer portions of trees, where the foliage was most dense. The only other tree zone used extensively was the middle outer zone. Forest edge and forest interior observations did not significantly differ. This same pattern of outer canopy preference also held for observations made in introduced tangantangan thickets, where results also did not significantly differ for edge and interior data. Only 8 of 146 (5.5%) observations in limestone forest and 24 of 214 (11.2%) observations in tangantangan thickets came from understory trees. In addition to foraging in trees, birds fed in herbaceous weeds and gleaned from Miscanthus grass leaves while perched on their stems. On occasion, foraging birds used roadsides and lawns.

In both limestone forest and tangantangan thickets (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ), foraging birds appeared to prefer sunlit areas. In the morning on the west-facing Marpi Cliffs, most foraging birds were at the top of the cliffs—the area first illuminated by sun. Similarly, in the early morning, birds fed actively in the sunlit Leucaena thickets at the base of these cliffs. However, few birds used Leucaena by late morning, when daytime temperatures neared their peak. Numerous thicket-foraging birds were again present in the afternoon, but when thickets became shaded toward evening, there were few birds. On the Marpi Cliffs native forest illuminated by late afternoon sun, birds foraged to nearly sunset.

In limestone forest and tangantangan thickets (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close , R.J. Craig personal observation), the species foraged mostly among leaves. It searched buds, fruits, trunks, dead leaves, and rolled leaves only infrequently. Birds fed from either upper or lower surfaces of leaves. While foraging among leaves, they reached above and below and also dangled beneath perches. When foraging in trees with large leaves, they sometimes stood on leaf surfaces. This latter behavior was possible because birds averaged only 8.2 g in mass. There were no significant differences in use of foraging surfaces between forest edge and forest interior. Perches 0.25−0.5 cm in diameter were preferred in both limestone forest and tangantangan thickets, with <0.25 and 0.5−<1.0 cm perches used secondarily. Although birds could use larger perches, they did so infrequently. There was no significant difference in perch use between edge and interior observations or an interaction between habitat and edge and interior observations, but a significant difference between habitats in perch use, with birds more frequently choosing larger perches in limestone forest. Birds foraged in the following limestone forest taxa: Guamia [=Meiogyne] mariannae, Pisonia grandis, Cynometra ramiflora, Ficus spp., Premna obtusifolia, Melanolepsis multiglandulosa, Ochrosia mariannensis, Erythrina variegata, Aidia cochinchinensis, Morinda citrifolia, and Artocarpus spp. At high elevations on Mt. Tapotchau, they foraged in Pisonia umbellifera, Pandanus spp., and Claoxylon marianum. Birds also foraged in native and introduced tree species in such environments as beach strand, native mangrove (Bruguiera gymnorrhiza) swamps, strand woodlands dominated by Barringtonia asiatica, Hibiscus tiliaceus and Hernandia sonora, and formerly cultivated areas vegetated by such introduced tree species as Acacia confusa, Samanea saman, Ceiba pentandra, Cocos nucifea, and Persea americana.

In 1988−1989 (57 Craig, R. J. (1990). Foraging behavior and microhabitat use of two species of white-eyes (Zosteropidae) on Saipan. Auk 107(3):500–505. Close ), the Bridled White-eye and the Golden White-eye (Cleptornis marchei) were observed to forage similarly in native limestone forest and introduced tangantangan thickets. These similarities imply that foraging strategies are not greatly altered in order to use the different habitats. The Golden White-eye foraged predominantly in the top outer portion of trees in limestone forest and tangantangan thickets, although the proportion was significantly less than for the Bridled White-eye. The Golden White-eye foraged for less time on live leaves and longer on dead leaves, fruits, branches, and trunks in both limestone forest and tangantangan thickets than did the Bridled White-eye. In limestone forest, the Bridled White-eye chose perches (1.1 ± 2.1 cm, n = 64) about the same size as the Golden White-eye (0.9 ± 1.1 cm, n = 135), whereas in tangantangan thickets the Bridled White-eye used smaller perches (0.5 ± 0.5 cm, n = 167) than the Golden White-eye (1.6 ± 2.0 cm, n = 91) .

In 1988−1991 comparisons of microhabitat use in limestone forest (58 Craig, R. J., and K. G. Beal (2001). Microhabitat partitioning among small passerines in a Pacific island bird community. Wilson Bulletin 113(3):317–326. Close ), the species diverged significantly in use of forest zones, with the Bridled White-eye specializing in canopy foraging and the Golden White-eye using canopy and mid-forest strata. The Bridled White-eye chose to forage in taller trees, i.e., more canopy trees, and specialized more on live leaf and flower foraging surfaces. Moreover, the Bridled White-eye selected smaller perches and gleaned more but used other foraging methods less than the Golden White-eye, which in both cases was more of a generalist. Use of woody vegetation types as foraging perches differed significantly between the white-eye species, although the tree most frequently used by both species was Cynometra ramiflora. Overall, the Bridled White-eye (n = 333) specialized more on C. ramiflora, whereas the Golden White-eye (n = 234) was more generalized in use of vegetation. Both species showed a negative relationship between preference and availability, with more abundant woody species such as Pisonia grandis, Intsia bijuga, and Premna obtusifolia used less, and less abundant species such as Psychotria mariana, Aidia cochinchinensis, and Maytenus thompsonii used more than their availability. However, the second most common forest tree, Cynometra ramiflora, showed use by both Bridled White-eye and Golden White-eye to be slightly greater than availability. Foraging between years also differed significantly. Principal differences noted were that the Bridled White-eye foraged less from live leaves, more from other foraging surfaces, and more from smaller perches in 1990−1991 compared with 1988−1989.

On Aguiguan (53 Craig, R. J., R. Kaipat, B. A. Lussier and H. Sabino. (1993). Foraging differences between small passerines on Aguiguan and Saipan. Proceedings, Marianas Research Symposium 1:16−22. Close ), the Bridled White-eye most frequently used the forest canopy and foraged mostly by gleaning from live leaves on 0.25−0.5 cm perches. Aguiguan data differed significantly from 1991 Saipan data only in use of perches, with Aguig­uan bir­ds feeding more from larger perches. As on Saipan, the species significantly differed from the Golden White-eye in use of forest zones, with the Bridled White-eye forag­ing more in the forest canopy. Aguiguan birds tended to feed more from live leaves and small perches than the Golden White-eye, which also mirrored find­ings from Saipan.

Food Capture and Consumption

Early descriptions of behavior from Guam (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ) stated that in foraging and flight the Bridled White-eye behaved like a goldfinch (Spinus spp.). On Saipan, it was likened to a Bushtit (Psaltriparus minimus) due to its infiltration manner of moving through the vegetation (43 Stott, K. (1947). Notes on Saipan birds. Auk 64(4):523–527. Close ). Food capture techniques were described by Marshall (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ) as involving prey being taken as birds moved rapidly through foliage while occasionally hawking or hovering in front of a leaf spray. A common attitude in foraging was to use a horizontal body position, with bill, body and tail in line, with birds flicking wings constantly. The species also was described (1 Baker, R. H. (1951). The avifauna of Micronesia. Its origin, evolution, and distribution. University of Kansas Publications, Museum of Natural History 3(1):1–359. Close ) as being very active, always moving rapidly through vegetation or flying across open areas to disappear into scrub foliage. During 1978-1979 on Guam (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ), birds were reported to forage in a warbler-like manner as they creeped along branches searching for prey. In 1982 on Tinian (36 Engbring, J., Ramsey, F.L. and Wildman, V.J. (1986). Micronesian Forest Bird Survey, 1982: Saipan, Tinian, Agiguan, and Rota. US Fish & Wildlife Service Report, Honolulu, Hawaii. Close ), a flock was observed to hawk insects repeatedly 3–10 m above the forest canopy, although more typically birds engaged in foliage gleaning and feeding on fruits. On Tinian, they fed upon the fruits of Passiflora foetida and on Aguiguan they fed upon nectar in flowering Erythrina variegata trees.

The Bridled White-eye is extremely versatile in its foraging behavior. In 1988 (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ), the principal foraging method used on Saipan in both native limestone forest and introduced tangantangan thickets was gleaning. Birds hovered and sallied only when chasing invertebrates. They probed flowers, apparently for nectar (n = 8); bark (n = 1); dead leaves (n = 3); rolled leaves (n = 1); and passionfruits (Passiflora foetida) (n = 5). When foraging, they searched methodically, inspecting numerous surfaces before seizing prey. There was no significant difference in use of methods between forest edge and forest interior or between habitats. Flocks (52 Craig, R. J. (2002). Aspects of flocking behavior in an endemic Pacific island white-eye. Journal of Field Ornithology 73(1):70–73. Close ) were encountered at heavily flowering (particularly Erythrina variegata and Cynometra ramiflora) and fruiting trees (particularly Ficus spp. and Premna obtusifolia).

In a 1988−1989 comparison of foraging methods of the Bridled White-eye and Golden White-eye (57 Craig, R. J. (1990). Foraging behavior and microhabitat use of two species of white-eyes (Zosteropidae) on Saipan. Auk 107(3):500–505. Close ), both species principally employed foliage gleaning in both limestone forest and tangantangan thickets. Probing was a more important method for the Golden White-eye than for the Bridled White-eye in limestone forests and tangantangan thickets . In a 1988−1991 comparison of foraging methods between these species (58 Craig, R. J., and K. G. Beal (2001). Microhabitat partitioning among small passerines in a Pacific island bird community. Wilson Bulletin 113(3):317–326. Close ), the Bridled White-eye gleaned more but used probing and aerial foraging less than the Golden White-eye, which in both cases was more of a generalist. On one occasion (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ), a Bridled White-eye was observed to follow a Micronesian Rufous Fantail (Rhipidura versicolor) while foraging .

Captive birds of subspecies saypani were observed to engage in the following feeding behaviors (59 Smith, O., and T. Wassmer (2016). An ethogram of commonly observed behaviors of the Endangered Bridled White-eye (Zosterops conspicillatus) in a zoo setting. Wilson Journal of Ornithology 128(3):647–653. Close ):

Birds either searched for food on the ground or gleaned from a perch. In ground foraging, birds walked, hopped, ran, or stood on the ground to find prey. They moved rapidly while perch-gleaning, with short hops or flights. In food preparation, waxworms (Galleria mellonella) were held in the beak and beaten on branches with lateral head movements. Birds ran waxworms through the beak sideways to kill and soften them. Sometimes prey were abandoned after preparation. When eating, food items were grasped in the beak, the head lifted, and the item consumed. Fruits were consumed while perched rather than while hovering. On occasion, birds hung upside down to eat fruits.

Major Food Items

On Guam, insects were thought to form the principal part of the diet of the Bridled White-eye (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ). However, the principal food also was thought to be berries with hard pits 4.5 mm in diameter (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). Seeds, fruit, caterpillars, ants, and other small insects have been reported, with grasshopper parts and small snails sometimes found in stomachs (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). On Saipan and Tinian, it was believed to be a seed eater (Coultas, in 1 Baker, R. H. (1951). The avifauna of Micronesia. Its origin, evolution, and distribution. University of Kansas Publications, Museum of Natural History 3(1):1–359. Close ). On Tinian, a bird ate a large, green fuzzy caterpillar (56 Downs, T. (1946). Birds on Tinian in the Marianas. Transactions of the Kansas Academy of Science 49: 87–106. Close ). Guam birds were later described as feeding primarily on insects that were gleaned from twigs or foliage. If fruits and seeds were taken, they were considered to comprise only a small portion of the diet (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ).

On Saipan in 1988 (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ), the Bridled White-eye fed on foliage invertebrates, flying insects, nectar, fruits and seeds, with invertebrates (~2–22 mm in length) being the most frequently taken food. Birds fed on seeds of herbaceous weeds and took nectar from flowers. However, they ate less fruit than the Golden White-eye (58 Craig, R. J., and K. G. Beal (2001). Microhabitat partitioning among small passerines in a Pacific island bird community. Wilson Bulletin 113(3):317–326. Close ). The species has been recorded as feeding from the following native and introduced plants (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ): taking seeds from Momordica charantia and Bidens pilosa, fruit from Momordica charantia, Passiflora foetida, Jasminum marianum, Premna obtusifolia, Ficus spp., Melanolepsis multiglandulosa, Artocarpus spp., Pipturus argenteus, Lantana camara, Carica papaya, and Muntingia calabura; eating flowers of Mikania scandens, Jasminum marianum, Pisonia grandis, Cynometra ramiflora, and Leucaena leucocephala; and taking nectar from Operculina vetricosa, Pisonia grandis, Cynometra ramiflora, Premna obtusifolia, Erythrina variegata, Psychotria mariana, Morinda citrifolia, Hibiscus tiliaceus, and Albizia lebbeck . On Aguiguan in 1992 (60 Craig, R. J., and R. Chandran (1993). Wildlife species recorded during the Aguiguan expedition: 20–25 May 1992. Proceedings, Marianas Research Symposium 1:1–7. Close ), it frequently fed in Lantana camara, and mist-netted birds had black, watery droppings indicative of feeding upon Lantana fruits. Individuals also were observed taking nectar from Lantana flowers.

No information available.

No information available.

No information available.

Drinking

Captive birds lower their beak into water or nectar and tilt their heads upwards to swallow, as is typical for many passerines. They also may drink rain water drops off of objects (59 Smith, O., and T. Wassmer (2016). An ethogram of commonly observed behaviors of the Endangered Bridled White-eye (Zosterops conspicillatus) in a zoo setting. Wilson Journal of Ornithology 128(3):647–653. Close ).

Defecation

When feces are passed, the birds spread their vent feathers and undertail coverts and lean forward (59 Smith, O., and T. Wassmer (2016). An ethogram of commonly observed behaviors of the Endangered Bridled White-eye (Zosterops conspicillatus) in a zoo setting. Wilson Journal of Ornithology 128(3):647–653. Close ).