Behavior

On Guam, the Bridled White-eye was first described as an active flocking species whose behavior was like that of goldfinches (Spinus spp.) (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ), titmice (Baeolophus spp.) (62 Safford, W. E. (1902). Birds of the Marianne Islands and their vernacular names. Osprey 6:39-42, 65−70. Close ), and chickadees (Poecile spp.) (54 Hartin, M.H. (1961). Birds of Guam. Elepaio 22:34−38. Close ). It was reported to hop about on branches, first on one side and then on the other (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ). On Saipan, it was thought to resemble a Bushtit (Psaltriparus minimus) in its infiltration manner of moving through the vegetation (43 Stott, K. (1947). Notes on Saipan birds. Auk 64(4):523–527. Close ). It was similarly described as being very active and always moving rapidly through vegetation or flying across open areas to disappear into scrub foliage (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 ). Mees (18 Mees, G. F. (1969). A systematic review of the Indo-Australian Zosteropidae (Part III). Zoologische Verhandelingen 102:1–390. Close ) thought the species’ movements were typical for Zosterops. In 1981, Guam birds were found to be inconspicuous and difficult to observe (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 ). It was characterized as being the only native passerine of the Mariana Islands that was nonterritorial even when nesting (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ), although the Micronesian Starling (Aplonis opaca) also does not appear to defend all-purpose territories (48 Craig, R. J. (2021). The structure and dynamics of endangered forest bird communities of the Mariana Islands. Pacific Science 75:543−559. Close ).

Captive individuals moved from one place to another by hopping and propelling themselves with their feet. When they flew, they used short and rapid wing beats to move from one place to another (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 ).

Preening, Head-Scratching, Stretching, Sunbathing, Bathing, Anting, etc.

In captivity, birds use their beak to smooth and clean the wings, throat, vent, and area around the legs. The neck and abdomen are groomed by bending the neck backwards and approaching from above. The breast is groomed by bending the head downwards. The top of the wing is smoothed and cleaned by stretching the wing out, bending the head sideways, and approaching from above. The underwing is groomed by lifting the wing and approaching from underneath. Bathing birds flutter on leaf petioles during rain. When sun bathing, birds sit near a heat lamp, puff, and preen themselves. Birds use indirect scratching where one wing droops and the foot on the same side is brought over the shoulder to scratch the head. When stretching, a bird fans its tail and extends one leg and wing contralaterally and then extends the opposite wing and leg after retracting the first. In another variation, the bird pulls its wings together along the back. To smooth plumage, feathers are erected from the head downwards towards the vent and then smoothed down rapidly (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 ).

Sleeping and Roosting

Captive birds sit together for ~5 min to rest after bathing or preening. They tuck their heads, close their eyes, and perch on one foot. To perch, birds rest on a branch with breast feathers puffed, but they remain alert with open eyes. When panting, birds breathe with an open beak while perching (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 ).

Physical and Communicative Interactions

On Saipan in 1988, Bridled White-eye individuals were observed to chase and supplant each other on perches, suggesting that dominance relationships existed in flocks. Chases involved bill clattering and whining calls. Wing fluttering appeared to precede attacks. Such observations were typical of behaviors observed year-round. Aggression was detected virtually anywhere flocks of birds were present. Aggression could not be clearly related to territorial encounters except within several meters of the nest; instead, it most frequently appeared to involve disputes over access to food (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close , 47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close , 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 ).

During intraspecific aggression in captivity, a defending bird faces the intruder, stretches out straight with smoothed head feathers, holds the wings down while flitting them, and vocalizes with cheeps. Only males were observed to give threat displays. A bird flies directly towards another bird in a rapid, straight flight causing the other bird to fly away without a fight. Both sexes displace other birds. Birds make physical contact by bumping breasts and biting each other. One bird may be pinned against a branch. Both sexes engaged in fights. Birds retreat when attacked by conspecifics (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 ).

Courtship, Copulation, and Pair Bond

Evidence of the social nature of individuals on Saipan included observation of one bird preening another, which was probably its mate (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ). Allopreening was indeed frequent. Moreover, pairs of birds were observed together foraging, gathering nesting material, and responding in pairs to playback of calls (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 ).

In captive birds, the courtship display involved a male presenting a piece of nesting material with its beak to a female and fluttering its wings quickly. The male also allopreens the female, focusing efforts around the neck, head, contour feathers, hind-flanks, and tail. Copulation occurs after a male preens the female. He rapidly mounts her back, balancing himself with his wings and dismounts to her side (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 ).

Degree of Sociability

Flocking was first described on Guam as involving groups of 10−20 individuals (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ). Stophlet (7 Stophlet, J. J. (1946). Birds of Guam. Auk 63(4):534–540. Close ) reported flocks of only 6−7, Baker (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 ) reported a group of 12, Tubb (45 Tubb, J. A. (1966.) Notes on birds of Guam. Natural History Bulletin of the Siam Society 21:135-138. Close ) found flocks of 6−10, and Hartin (54 Hartin, M.H. (1961). Birds of Guam. Elepaio 22:34−38. Close ) found flocks of 2−10. In 1976, small flocks of up to 15 birds were found at the northern parts of the island, but by 1978 the Bridled White-eye was absent from these same areas. Only two flocks of 4−5 birds were present at Ritidian Point, and none were found elsewhere (Pratt et al. 1979). Flocks of 3−8, with most commonly groups of 3−5, were found by Jenkins (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). By 1981, flocks of 3−6 were present (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 ).

On Tinian, observations immediately after World War II revealed the Bridled White-eye to be common but in flocks of only 2−7 (56 Downs, T. (1946). Birds on Tinian in the Marianas. Transactions of the Kansas Academy of Science 49: 87–106. Close ). On Saipan during the same years, it was reported to be in groups of 3−20 (43 Stott, K. (1947). Notes on Saipan birds. Auk 64(4):523–527. Close ). In the Mariana Islands in general, flocks of 12−50 were reported (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). In 1982, flock size in the Northern Marianas was 6−12 with a maximum of 50 (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 ). By the late 1980s on Tinian, flocks of up to 100 individuals were found (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ). In a 1992−1993 study of flocking behavior on Saipan, flocks larger than family groups (~3−5 individuals) were typically 10−40 (mean = 17.6, n = 25), although larger flocks of at least 50 were encountered. Male-female pairs also fed away from 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 ). An analysis of 1990−1993 Saipan social group encounters revealed that, in native forest, groups of 8.2 ± 4.3 (n = 180) were present in the wet season, whereas 8.4 ± 4.0 (n = 212) were present during the dry season. In contrast, 10.1 ± 3.7 (n = 58) were observed in disturbed habitats in the dry season (48 Craig, R. J. (2021). The structure and dynamics of endangered forest bird communities of the Mariana Islands. Pacific Science 75:543−559. Close ).

Flock size appears to show some relationship to population density, although estimates of flock size vary widely. Typically, small flocks occur when populations are low, and large flocks occur when populations are large (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ). As on Guam, when the Rota White-eye (Zosterops rotensis) was undergoing a precipitous population decline, maximum flock size dropped from 23 in 1988 to ~10 in 1991 (63 Craig, R. J., and E. Taisacan (1994). Notes on the ecology and population decline of the Rota Bridled White-eye. Wilson Bulletin 106(1):165–169. Close ).

Flocks occasionally were observed to fly high above the forest canopy, which indicated a widely separated foraging circuit, although when flying across open spaces from one forage tree to the next, birds were within 1–2 m of the ground. Each flock made rounds of a fixed forage circuit with individuals flicking their wings constantly (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). In a study of flocking behavior on Saipan in 1992−1993, 97 birds were color banded. Plotted data from 16 recaptures and 135 resightings, including 10 birds resighted ≥5 times, traced roughly elliptical areas, of which six were ~200 m maximum diameter, three were ~250 m in diameter, and one was 100 m in diameter. Six of the 10 were in roughly overlapping areas, whereas two were in areas encompassing but greater than those of the six. The remaining two were located in portions of areas used by these other birds (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 ).

Flocks on Saipan in 1992−1993 showed cohesiveness; i.e., members continuously communicated through contact calls and flew between trees (foraging sites) in groups. On several occasions, perched birds communicated with flock members by tilting the head at 45°, moving the head from side to side, and flicking the wings while giving whining calls. Flock departure and arrival occurred over a period of seconds rather than simultaneously, however, as individuals completed foraging at one site before moving to the next. In two instances where larger (~15 birds) foraging flocks were followed, groups remained cohesive for 100−200 m, but then fragmented into groups of 3−5 birds, which headed in differing directions. In one instance, an individual flew for ~110 m. Flocking was a conspicuous behavior throughout the year and showed little clear change in character. Observations of year-round nesting and inability to find evidence for a pronounced breeding season helps to explain this uniformity in behavior. Moreover, the intensive phase of this investigation lasted from the end of the dry season into the wet season, yet no behavioral alteration was apparent (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close , 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 , R.J. Craig, personal observation).

Observations on movements, flock size, and social interactions led to the conclusion that flocks showed characteristics intermediate between permanent membership and site-dependent flocks. Small groups of at least three birds were clearly family groups and may be considered permanent at least to the extent that family groups remain cohesive over time. However, observations of larger groups of ~50 birds foraging together and then dispersing into smaller flocks is most consistent with the concept of site dependency, where smaller groups opportunistically converge on dense food resources. Resightings of banded birds also were consistent with a pattern of independent, overlapping home ranges of individuals or small groups. Individuals within such home ranges have the opportunity to converge on attractive food resources and thus form temporary large 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 ).

Nonpredatory Interspecific Interactions

The Bridled White-eye appears to be socially subordinate to other small forest passerines. It was not observed to chase or supplant other passerines. Of the four small passerines that inhabit the forests of Saipan, the Golden White-eye (Cleptornis marchei) and Bridled White-eye were the only species to show regular interspecific aggression (n = 9 observations). Two Golden White-eye supplanted Bridled White-eye from their perches, chased them, and flew through flocks of foraging Bridled White-eye, thereby dispersing them; however, the two species were also observed to forage within 2 m of each other. The Micronesian Rufous Fantail (Rhipidura versicolor) (n = 2) and Micronesian Myzomela (Myzomela rubratra) (n = 1) also chased the Bridled 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 ). Another interspecific social interaction on Saipan involved a single observation of a Bridled White-eye following a foraging Micronesian Rufous Fantail (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ). In another instance, a Micronesian Rufous Fantail sought out and followed a group of foraging Bridled White-eye (R.J. Craig, personal observation). The Bridled White-eye also was reported to steal nesting material from nests of the Micronesian Rufous Fantail and Tinian Monarch (Monarcha takatsukasae) (64 Swift, K., F. Moore, and B. Gardiner (2024). White-eyed, red-handed: novel observations of nest material kleptoparasitism by Bridled White-eye (Zosterops conspicillatus) on Tinian Island, Northern Mariana Islands. Wilson Journal of Ornithology 136:356-359. Close ).

Kinds of Predators

On Guam, the Guam Kingfisher (Todiramphus cinnamominus) was reported to attack flocks of Bridled White-eye, which set up a confusion chorus at each appearance of a kingfisher. However, no actual capture of a white-eye was observed. On Saipan and Tinian, the Mariana Kingfisher (Todiramphus albicilla) was described as an unrelenting assailant of the Bridled White-eye (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). Also on Saipan, the Mariana Kingfisher was observed to take a probable fledgling from a branch while being vigorously scolded by a pair of white-eyes (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ).

Response to Predators

On Tinian, Bridled White-eye called aggressively in response to the presence of a flying Mariana Kingfisher, although the birds remained stationary (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). Birds on Guam also mobbed the Micronesian Starling (Aplonis opaca) (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). On Saipan, birds mobbed the Mariana Kingfisher (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close ). In addition, birds scolded and chased after a Yellow Bittern (Botaurus sinensis) when it flew (R.J. Craig, personal observation).