Conservation and Management

The Bridled White-eye subspecies conspicillatus is extinct, with the last individuals observed in 1984 (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 ). The subspecies saypani is listed as Near Threatened on the IUCN Red List (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, it is not federally listed as endangered or threatened. Evidence from capture-recapture studies indicate a Saipan population decline (88 Saracco, J. F., P. Radley, P. Pyle, E. Rowan, R. Taylor, and L. Helton (2016). Linking vital rates of landbirds on a tropical island to rainfall and vegetation greenness. PlosONE 11:e0148570. Close , 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 ). However quantitative population surveys from Saipan, Tinian, and Aguiguan show no decline or a possible increase. Still, the species’ range is small and threatened by the possibility of the introduction of the predatory brown tree snake to islands in its range. This could result in rapid elimination of populations, as it did for land birds on nearby Guam (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 ). Based on the most recent estimates from transect counts, the current total Saipan, Tinian, and Aguiguan population is 809,594 (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 ).

Habitat Loss and Degradation

Habitat loss has been cited as a cause of concern for the Saipan and Tinian populations of Bridled White-eye. Since the 1980s, considerable uninhabited land on Saipan and Tinian has been developed for residential, commercial, and tourist-related purposes (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 , 48 Craig, R. J. (2021). The structure and dynamics of endangered forest bird communities of the Mariana Islands. Pacific Science 75:543−559. Close ). Moreover, on Tinian, airport-related development has occurred, and much of the island is leased by the U.S. military for training purposes (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 ). In contrast, on uninhabited Aguiguan, abandoned agricultural land has reverted to thickets and secondary forest (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 ), which likely has benefitted the population there. Despite the potential for forest cover reduction to influence populations, the species’ ecological versatility (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 , 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 ) suggests that it is likely to persist in the face of such change, albeit at reduced densities in alien habitats (48 Craig, R. J. (2021). The structure and dynamics of endangered forest bird communities of the Mariana Islands. Pacific Science 75:543−559. Close ). Changing climatic conditions related to fossil fuel consumption also might affect populations, as wet and dry seasons are predicted to be wetter and warmer in the western tropical Pacific (40 BirdLife International 2024. Species factsheet: Saipan White-eye Zosterops saypani. https://datazone.birdlife.org/species/factsheet/saipan-white-eye-zosterops-saypani/text. Close ). Survival of the Bridled White-eye was positively related to remotely sensed dry season greenness and negatively related to wet-season greenness. Survival also showed evidence of a positive response to overall greenness, thereby highlighting the potentially important role of rainfall regimes in affecting population dynamics of species on oceanic tropical islands. Greater rainfall is associated with increased greenness at all but the highest rainfall levels (88 Saracco, J. F., P. Radley, P. Pyle, E. Rowan, R. Taylor, and L. Helton (2016). Linking vital rates of landbirds on a tropical island to rainfall and vegetation greenness. PlosONE 11:e0148570. Close ).

Effects of Invasive Species

Perhaps the most ominous threat to Bridled White-eye survival is the brown tree snake (89 Rodda, G. H., and J. A. Savidge (2007). Biology and impacts of Pacific Island invasive species. 2. Boiga irregularis, the brown tree snake (Reptilia: Colubridae). Pacific Science 61:307–324. Close ). Once the snake became established on the southernmost Mariana Island of Guam in the 1940s, most endemic land birds declined to extinction within ~40 years (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 ). Accidental introduction via cargo ships and planes has been the primary dispersal mechanism from Guam. All goods received in the Northern Mariana Islands are shipped through Guam, with most arriving on Saipan. Saipan was feared to have an incipient population of brown tree snake, and there have been over 70 reports of brown tree snakes on Saipan, including sightings away from port areas (89 Rodda, G. H., and J. A. Savidge (2007). Biology and impacts of Pacific Island invasive species. 2. Boiga irregularis, the brown tree snake (Reptilia: Colubridae). Pacific Science 61:307–324. Close , 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 ). However, based on available evidence and modeling of surveillance efforts, there is presently a low probability that Saipan has an incipient population (90 Yackel-Adams, A. A., P. D. Barnhart, G. H. Rodda, E. T. Hileman, M. G. Nafus and R. N. Reed. (2021). Can we prove that an undetected species is absent? Evaluating whether brown treesnakes are established on the island of Saipan using surveillance and expert opinion. Management of Biological Invasions 12:901–926. Close ). In contrast to Saipan, the risk of snake introduction to uninhabited Aguiguan, Sarigan, or Guguan is low.

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 Bridled White-eye’s diet (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ). In contrast, the demise of Erythrina variegata due to invasion of the Marianas by an alien gall wasp (Quadrastichus erythrinae, 91 Rubinoff, D., B. S. Holland, A. Shibata, R. H. Messing and M. G. Wright. (2010). Rapid Invasion despite lack of genetic variation in the Erythrina Gall Wasp (Quadrastichus erythrinae Kim). Pacific Science 64:23–31. Close ) may negatively impact populations. This large native tree with large flowers and the eighth most important tree in native forests in the 1990s (51 Craig, R.J. (1992). Ecological characteristics of a native limestone forest on Saipan, Mariana Islands. Micronesica 25:85-97. Close ) drops its leaves and flowers heavily during the dry season at a time when other tree species flower less. The Bridled White-eye feeds at these flowers during this time (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ).

Conservation Areas

Protected areas have been established by the Commonwealth of the Northern Mariana Islands on Saipan (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 ), and Aguiguan is set aside as a reserve (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 ). Additionally, Guguan, Asuncion, Maug, and Uracas have been designated as conservation areas to be used only for the protection of natural resources. Furthermore, Asuncion, Maug, and Uracas are federally protected within a 153,235 km² Marine National Monument (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 ).

Conservation Measures and Habitat Management

Because of the limited range of the Bridled White-eye and its potential for extirpation from this range by introduction of the brown tree snake, a captive breeding and translocation program has been developed. The captive program began in 2006 when 38 birds were captured on Saipan and placed at two zoos. In 2010, 30 additional birds were captured on Tinian and placed at three additional zoos. The captive population presently numbers 30, and these are held at four zoos. Managers plan for a target captive population of 200. Although the species has been maintained in captivity easily, it has proven difficult to breed, with only two young hatched at one zoo and a clutch of fertile but unhatched eggs produced at another. To improve breeding success, the Toledo Zoo has initiated research to determine the cause of low reproductive success and to establish guidelines for breeding the species in captivity (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 ).

In May 2008, 50 Bridled White-eyes were translocated from Saipan to Sarigan. In 2009, 50 more were translocated from Tinian. In 2010 and 2012, surveys on Sarigan demonstrated that the population was growing (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 ). The 2010 surveys yielded a density estimate of 1.3 birds/ha (95% CI = 0.4‒2.8) and a total population of 77‒495 (mean = 234.2) individuals. Surveys in 2012 produced a density estimate of 16.8 birds/ha (95% CI 10.6–24.8) with a total population of 1,897–4,302 (mean = 3004.5) (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 ). In June 2016, surveys on Sarigan yielded a population estimate of 8,239. In 2015 and 2016, translocations of 96 birds also were made to Guguan (40 BirdLife International 2024. Species factsheet: Saipan White-eye Zosterops saypani. https://datazone.birdlife.org/species/factsheet/saipan-white-eye-zosterops-saypani/text. Close ). Translocations are also planned for Pagan (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 ). However, it is conceivable that a large population of a socially dominant species could interfere with the successful establishment of a translocated subordinate species. For this reason, the socially subordinate Bridled White-eye should be translocated to islands before socially dominant 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 ).

To prevent establishment of the brown tree snake on Saipan, cargo arriving from sea and at airports is checked for snakes. Moreover, traps have been installed to catch any snakes that are missed and barriers have been constructed at docks to allow escaping snakes to be contained. Sniffer dogs have been trained to detect snakes at Saipan airport. Port officers have been trained in prevention of snake establishment, and educational programs have been developed to increase awareness among the population of the importance of reporting sightings (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 ).