Demography and Populations

A 2004 study showed that the Bridled White-eye had a daily nest survival of 0.904 in the incubation stage and 0.928 in the nestling stage. Daily and overall nest survival rates did not differ between February−March and April−May. Increased nest concealment, including side and canopy cover, appeared to decrease nest survival, with side cover appearing to have the greatest impact (68 Sachtleben, T. (2005). Predation and nest success of forest birds in native and nonnative habitat on Saipan, Mariana Islands. M.S. thesis, Colorado State University, Fort Collins, CO, USA. Close ).

Information needed.

Disease

The Bridled White-eye on Saipan and Tinian were almost universally infected with Haemoproteus, a red blood cell parasite, and had microfilariae in an incidence of about 15%. Neither parasite was found in Guam birds (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). Moreover, of 63 Saipan birds collected, two were infected with Plasmodium (avian malaria), and 46 were infected with Haemoproteus. One bird developed pox-like lesions, but the lesions resolved. A potential threat from West Nile virus also exists, as other Zosterops have proven susceptible to this disease, although to date this virus has not been detected in the Mariana Islands (71 U.S. Fish and Wildlife Service. (2007). Recovery plan for the Pacific Coast population of the Western Snowy Plover (Charadrius alexandrinus nivosus). U.S. Fish and Wildlife Service, Sacramento, CA, USA. Close ). Fecal examinations have revealed cestodes (tapeworms) and Coccidia (70 MAC Working Group (2014). Marianas Avifauna Conservation (MAC) plan: long-term conservation plan for the native forest birds of the Northern Mariana Islands. CNMI Division of Fish and Wildlife, Saipan, and U.S. Fish and Wildlife Service, Honolulu, HI. Close ).

Depredation

After its accidental introduction to Guam from the north Australia-New Guinea-Solomon Islands region, the predatory brown tree snake (Boiga irregularis) decimated native bird populations within several decades. It had a particularly rapid effect on the numbers of the Bridled White-eye, despite the bird’s previous abundance (72 Savidge, J. A. (1987). Extinction of an island forest avifauna by an introduced snake. Ecology 68(3):660–668. Close , 41 Wiles, G. J., J. Bart, R. E. Beck, and C. F. Aguon (2003). Impacts of the brown tree snake: patterns of decline and species persistence in Guam’s avifauna. Conservation Biology 17(5):1350–1360. Close ).

Nest predators in the Mariana Islands include the native Mariana Kingfisher (Todiramphus albicilla), the introduced green tree skink (Lamprolepis smaragdina), and rats (Rattus spp.) (40 BirdLife International 2024. Species factsheet: Saipan White-eye Zosterops saypani. https://datazone.birdlife.org/species/factsheet/saipan-white-eye-zosterops-saypani/text. Close ). However, predators such as rats, monitor lizards (Varanus indicus), and cats (Felis cattus) are not known to be major predators of tree nesting birds in the Marianas, although the native Micronesian Starling (Aplonis opaca) has been documented to prey upon nests of other forest birds (68 Sachtleben, T. (2005). Predation and nest success of forest birds in native and nonnative habitat on Saipan, Mariana Islands. M.S. thesis, Colorado State University, Fort Collins, CO, USA. Close ), and white-eyes were observed scolding a Yellow Bittern (Botaurus sinensis), which suggests that this species may act as a predator as well (R.J. Craig, personal observation).

Differences in the rat species occupying Saipan vs. Aguiguan may be related to differing population densities on these islands (73 Amidon, F., R. J. Camp, A. P. Marshall, T. K. Pratt, L. L. Williams, P. Radley, and J. B. Cruz (2014). Terrestrial bird population trends on Aguiguan (Goat Island), Mariana Islands. Bird Conservation International 24(4):505–517. Close ), although no empirical evidence supports this hypothesis. Predation has been suggested to be the causal factor in the decline of several bird species on Tinian (74 Camp, R. J., F. A. Amidon, A. P. Marshall, and T. K. Pratt (2012). Bird populations on the Island of Tinian: persistence despite wholesale loss of native forests. Pacific Science 66:283–298. Close ), although evidence for this is conjectural. Moreover, there is no evidence for a decline of the Bridled White-eye on Tinian.

Exposure

Variable circular-plot surveys on Rota conducted in 1982−2003 and U.S. Fish and Wildlife Service roadside surveys showed that typhoon frequency or severity might affect the abundance of some bird species (75 Ha, J. C., J. R. Buckley, and R. R. Ha (2012). The potential for typhoon impact on bird populations on the island of Rota, Northern Mariana Islands. Micronesica 43:214−224. Close ). Typhoon-related nest failures have been documented for white-eye species (76 Amidon, F. A. (2000). Habitat relationships and life history of the Rota Bridled White-eye (Zosterops rotensis). M.S. thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA. Close ). However, U.S. Fish and Wildlife Service roadside surveys conducted between 1991 and 2010 on Saipan showed no significant relationships between typhoon activity and bird counts (77 Ha, J., J. B. Cruz, S. Kremer, V. A. Camacho, and P. Radley (2018). Trends in avian roadside surveys over a 20-year period on Saipan, Commonwealth of the Northern Mariana Islands. Pacific Science 72:81–93. Close ). Now that a Bridled White-eye population is established on Sarigan (39 Radley, P. M. (2012). Marianas Avifauna Conservation (MAC) project: a second and first translocation of Golden White-eyes (Cleptornis marchei) and Mariana Fruit Doves (Ptilinopus roseicapilla), respectively, from Saipan to Sarigan, and an assessment of Bridled White-eyes (Zosterops conspicillatus) on Sarigan, 1-8 May 2012. Division of Fish and Wildlife, Saipan, CNMI. Close ), volcanic activity is also a potential cause of mortality, as the nearby island of Anatahan and a nearby undersea volcano are both volcanically active (78 Brainard, R. E. (2012). Coral reef ecosystem monitoring report of the Mariana Archipelago: 2003–2007. NOAA Pacific Islands Fisheries Science Center Special Publication, SP-12-01. Close ).

Data from nests located in 2003−2004 indicated that Bridled White-eye nest densities ranged from 0−115/km² in non-native forest and 0−11/km² in native/mixed forest. Statistical analyses indeed suggested that nest densities were greater in non-native than native forest, with 160 of 198 nests located in non-native forest (68 Sachtleben, T. (2005). Predation and nest success of forest birds in native and nonnative habitat on Saipan, Mariana Islands. M.S. thesis, Colorado State University, Fort Collins, CO, USA. Close ). However, this finding is likely to be an artifact of the comparative difficulty of finding nests in the ~15 m tall native vs. the typically 6 m tall non-native habitats, because the species occurs at far greater densities in native forest and principally occupies the forest canopy there (49 Craig, R. J. (1989). Observations on the foraging ecology and social behavior of the Bridled White-eye. Condor 91(1):187–192. Close , 48 Craig, R. J. (2021). The structure and dynamics of endangered forest bird communities of the Mariana Islands. Pacific Science 75:543−559. Close , 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 ).

Numbers and Trends

Early reports from Guam described the Bridled White-eye as common (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close , 17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ). In the 1930s, it was still found to be common along roadways (79 Bryan, E.H., Jr. (1936). Birds of Guam. Guam Recorder, volume 13. 2:14-15, 24-25; 3:10; 4:14, 30; 5:15-16, 30-31; 6:14-15, 34; 7:18-20, 34-35. Close ). However, by the 1940s birds were restricted to certain areas, although Arvey (field notes) saw them at Mt. Tenjo in July, 1946 (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 ); they were found on the foothills of the Mt. Tenjo area in 1945 and on the edge of forest near Ritidian Point (7 Stophlet, J. J. (1946). Birds of Guam. Auk 63(4):534–540. Close ). However, they were missed in roadside counts in 1945 (80 Baker, R.H. (1947). Size of bird populations at Guam, Mariana Islands. Condor. 49(3): 124-125. Close ). Hartin (54 Hartin, M.H. (1961). Birds of Guam. Elepaio 22:34−38. Close ) saw white-eyes frequently in 1960, but found them less common than the Micronesian Myzomela (Myzomela rubratra), and King (46 King, B. (1962). Guam field notes. Elepaio 23:29−31. Close ) observed them near Tarague Beach. Guam Aquatic and Wildlife Resource notes indicate that the species was found in central Guam in the early 1960's and apparently was common in the Agana Swamp. By the 1970s, it was observed regularly only at the extreme northwestern cliff line near Uruno and Ritidian points. It was rarely seen even in the most undisturbed woods of Northwest Field, only once was present near Pati Point Beach, and was by then among the rarest of native birds. It was uncommon along the northernmost cliffs and rare in the most northern areas of Northwest Field. A small group of white-eyes was seen once in Marbo Annex (Andersen Air Force Base-South), the only plateau record outside Northwest Field. It had one of the most restricted ranges of any native bird and was thought to be near extinction (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). It was reported as last observed in summer, 1983 (72 Savidge, J. A. (1987). Extinction of an island forest avifauna by an introduced snake. Ecology 68(3):660–668. Close ), although the last confirmed sighting was in 1984. The Pajon Basin was the last area on Guam to support the full ensemble of native forest birds at historical densities, with count results high and relatively consistent for the nine species of forest birds present in 1981 and 1982. Declines were first detected in May 1983, when Bridled White-eye abundance fell sharply. They were well underway by May 1984, when four species had been extirpated and two others were in rapid decline. Average counts of white-eyes per survey were 54.0 in 1981, 49.0 in 1982, 0.8 in 1983, and zero thereafter (41 Wiles, G. J., J. Bart, R. E. Beck, and C. F. Aguon (2003). Impacts of the brown tree snake: patterns of decline and species persistence in Guam’s avifauna. Conservation Biology 17(5):1350–1360. Close ).

In 1981, the population estimate for Guam was 2,220, and the density estimate for Pajon Basin was 1,027/km²—the highest density estimate for any species. Before the Bridled White-eye’s decline, it was likely among the most abundant species. Although still common in certain regions, it exhibited a distribution more restricted than any other native forest bird. Over 97% of the population resided in less than 2% of the land area on Guam, and there was evidence that the range had diminished from the previous year. Guam Aquatic and Wildlife Resource personnel recorded the species commonly at Pati Point in 1980, but none were found in this vicinity in 1981 (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 ).

In 1931, Coultas (field notes) found the species to be common on Saipan and Tinian (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 ). In 1945, the population on Tinian was estimated at 500+ (81 Gleize, D.A. (1945). Birds of Tinian. Bulletin of the Massachusetts Audubon Society 29:220. Close ). Moran (82 Moran, J. V. (1946). Birds of Saipan and Tinian. Bulletin of the Massachusetts Audubon Society 30:261-262. Close ) found it common on Saipan and Tinian and Downs (56 Downs, T. (1946). Birds on Tinian in the Marianas. Transactions of the Kansas Academy of Science 49: 87–106. Close ) found it to be abundant on Tinian. In 1945−1946, it was described as common on hillsides east of Lake Susupe, Saipan (43 Stott, K. (1947). Notes on Saipan birds. Auk 64(4):523–527. Close ). Pratt et al. (83 Pratt, H. D., P. L. Bruner, and D. G. Berrett (1979). America’s unknown avifauna: the birds of the Mariana Islands. American Birds 33(3):227–235. Close ) found it to be the most abundant bird on Saipan and Tinian. In 1977, a density estimate for Saipan based on 7.74 km of survey route was 1,360 birds/km² (84 Ralph, C. J., and H. F. Sakai (1979). Forest bird and fruit bat populations and their conservation in Micronesia: notes on a survey. ’Elepaio 40(2):20–26. Close ).

In 1982, the first variable circular plot population survey of all natural habitats on Saipan, Tinian, and Aguiguan revealed that the Bridled White-eye was by far the most abundant bird species on these islands. Computed densities were greatest on Tinian, with 2,931 birds/km2 and a total population of 241,352. There were 2,221 birds/km² and a total population of 229,138 on Saipan and 1,930 birds/km² and a total population of 10,763 on Aguiguan. In each case, the distribution was island-wide (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 ).

Using updated analytic methodology to examine data from several island-wide surveys, estimates of birds/km² for Saipan were 4,710 ± 332 for 1982 (over twice that of the 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 computation), 5,344 ± 407 for 1997, and 4,713 ± 387 for 2007, with a total 2007 population estimate of 534,029. The 25-year population trend showed no significant change (85 Camp, R. J., T. K. Pratt, A. P. Marshall, F. Amidon, and L. L. Williams (2009). Recent status and trends of the land bird avifauna on Saipan, Mariana Islands, with emphasis on the endangered Nightingale Reed-warbler Acrocephalus luscinia. Bird Conservation International 19(4):323–337. Close ). The most recent estimate for Saipan is 4,079 ± 313 for 2018 (86 Bak, T, S. Mullin, E. Koehler, B.A. Eichelberger and R.J. Camp. 2024. Forest bird population status on Saipan, a small oceanic island. Global Ecology and Conservation. 56:e03273. Close ). Using similar updated methodology on Tinian, estimates were 3,508 ± 344 for 1982, 2,997 ± 305 for 1996, and 3,275 ± 338 for 2008, with a total 2008 population of 225,360, and with the 26-year population trend showing no significant change (74 Camp, R. J., F. A. Amidon, A. P. Marshall, and T. K. Pratt (2012). Bird populations on the Island of Tinian: persistence despite wholesale loss of native forests. Pacific Science 66:283–298. Close ). On Aguiguan, updated methodology yielded density estimates of 1,798 ± 280 in 1982, 3,787 ± 712 in 1995, 2,427 ± 306 in 2000, 1,933 ± 200 in 2002, and 7,882 ± 1,233 in 2008, with a 2008 total population estimated at 50,205. The difference in estimates among years was significant (73 Amidon, F., R. J. Camp, A. P. Marshall, T. K. Pratt, L. L. Williams, P. Radley, and J. B. Cruz (2014). Terrestrial bird population trends on Aguiguan (Goat Island), Mariana Islands. Bird Conservation International 24(4):505–517. Close ). However, the wide differences in estimates among years suggests that surveys conducted by multiple and differing observers with varying experience and perceptual abilities may have obscured any actual temporal change in population size. Additional analysis of 1991−2010 standard U.S. Fish and Wildlife Service roadside surveys demonstrated that the Bridled White-eye was the most abundant species and that no significant long-term trends occurred in counts (77 Ha, J., J. B. Cruz, S. Kremer, V. A. Camacho, and P. Radley (2018). Trends in avian roadside surveys over a 20-year period on Saipan, Commonwealth of the Northern Mariana Islands. Pacific Science 72:81–93. Close ).

In a 1990−1993 seasonal variable circular plot study of bird populations in native limestone forest on Saipan, densities were 6,027 ± 1,529 birds/km² for the wet season and 6,303 ± 1,172 birds/km² for the dry season. In contrast, there were 3,189 ± 707 birds/km² for disturbed habitat in the dry season. On Aguiguan in the dry season of 1992, density was estimated at 5,806 birds/km² for limestone forest. All these studies were performed by a single observer (48 Craig, R. J. (2021). The structure and dynamics of endangered forest bird communities of the Mariana Islands. Pacific Science 75:543−559. Close ). A difference among Saipan habitats in population densities also was noted by Camp et al. (85 Camp, R. J., T. K. Pratt, A. P. Marshall, F. Amidon, and L. L. Williams (2009). Recent status and trends of the land bird avifauna on Saipan, Mariana Islands, with emphasis on the endangered Nightingale Reed-warbler Acrocephalus luscinia. Bird Conservation International 19(4):323–337. Close ). In addition to these surveys, in 1991−1992, standard U.S. Fish and Wildlife Service roadside surveys were conducted quarterly on Saipan. January counts averaged lowest, probably because higher winds at this season reduced the detectability of this canopy species. Like most passerines, variation in counts was relatively low (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ).

From February 1992 to June 1993 on Saipan, a mark-resighting study of the Bridled White-eye in mixed native and non-native forest used combinations of color bands that permitted identification of individuals in the field. The frequency of banded birds in the population was assessed by determining the proportions of banded vs. unbanded birds at 50 m intervals to 300 m from the banding site. Based on the locations of resightings, banded birds declined in frequency of occurrence, p, from the banding site in an empirically fitted quadratic relationship: p = 1.47x ² − 1.21x + 53.82, where x had values from one for the basal zone (0−50 m from the banding site) to six for the outermost zone (251−300 m). Based on this relationship, a Jolly-Seber estimate of population density was 7,770 birds/km². Population densities of this species as determined through this study, and variable circular plot surveys are among the highest ever recorded for birds (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 ).

Based on 2008−2018 capture-recapture data from six mist net sites on Saipan, population-growth rate estimates suggested a population decline in the Bridled White-eye, with survival the largest contributor to annual-growth rate. Recruitment was particularly important in driving population growth in years of population increase. Spaced at approximately annual intervals, the mean population growth rate was 0.93, the adult apparent survival probability was 0.71, the recruitment rate estimate was 0.23, the proportionate contribution of survival to population growth was 0.76, and the proportionate contribution of recruitment to population growth was 0.24 (87 Saracco, J. F., L. Helton, J. Liske-Clark, and P. R. Radley. (2021). Recent dynamics and trends of landbird populations on Saipan, Northern Mariana Island. Pacific Science 74:319−329. Close ). Variable circular-plot survey and U.S. Fish and Wildlife Service roadside survey data do not support the occurrence of a population decline, however (85 Camp, R. J., T. K. Pratt, A. P. Marshall, F. Amidon, and L. L. Williams (2009). Recent status and trends of the land bird avifauna on Saipan, Mariana Islands, with emphasis on the endangered Nightingale Reed-warbler Acrocephalus luscinia. Bird Conservation International 19(4):323–337. Close , 74 Camp, R. J., F. A. Amidon, A. P. Marshall, and T. K. Pratt (2012). Bird populations on the Island of Tinian: persistence despite wholesale loss of native forests. Pacific Science 66:283–298. Close , 73 Amidon, F., R. J. Camp, A. P. Marshall, T. K. Pratt, L. L. Williams, P. Radley, and J. B. Cruz (2014). Terrestrial bird population trends on Aguiguan (Goat Island), Mariana Islands. Bird Conservation International 24(4):505–517. Close , 77 Ha, J., J. B. Cruz, S. Kremer, V. A. Camacho, and P. Radley (2018). Trends in avian roadside surveys over a 20-year period on Saipan, Commonwealth of the Northern Mariana Islands. Pacific Science 72:81–93. Close ).

Cover conversion from forest to anthropogenic-dominated habitats on Saipan has been implicated in producing declines in some of Saipan’s bird species, although the Bridled White-eye population showed no evidence of decline. Moreover, the invasive non-native vine Coccinia grandis, introduced to Saipan ~1998, has altered bird habitat by smothering woody and other vegetation. Fruit-eating birds may have benefited from the fruit-producing vine’s spread (85 Camp, R. J., T. K. Pratt, A. P. Marshall, F. Amidon, and L. L. Williams (2009). Recent status and trends of the land bird avifauna on Saipan, Mariana Islands, with emphasis on the endangered Nightingale Reed-warbler Acrocephalus luscinia. Bird Conservation International 19(4):323–337. Close ), and fruit comprises a portion of the diet of the Bridled White-eye (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ). Causes for the decline of several bird species on Tinian are thought to be predation and site-specific habitat loss/degradation, such as that due to the expansion of Tinian airport (74 Camp, R. J., F. A. Amidon, A. P. Marshall, and T. K. Pratt (2012). Bird populations on the Island of Tinian: persistence despite wholesale loss of native forests. Pacific Science 66:283–298. Close ), although evidence for this is conjectural. Again, however, there is no evidence for a decline of the Bridled White-eye on Tinian. The greater 2008 population on Aguiguan compared with previous years has been attributed to expansion of secondary forest and particularly Lantana camara thickets into abandoned agricultural land (73 Amidon, F., R. J. Camp, A. P. Marshall, T. K. Pratt, L. L. Williams, P. Radley, and J. B. Cruz (2014). Terrestrial bird population trends on Aguiguan (Goat Island), Mariana Islands. Bird Conservation International 24(4):505–517. Close ). Based on mist-netting studies and foraging observations, Lantana is actively fed upon by the Bridled White-eye (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 ).