Breeding

On Guam, nests were first recorded in February−March (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close ). In May−July, one nestling and three nests with eggs also were reported (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ) and in October, 1945, an adult feeding a fledgling was observed (7 Stophlet, J. J. (1946). Birds of Guam. Auk 63(4):534–540. Close ). Although 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 that limited evidence of breeding was found in late May−July, three males taken in June and July had enlarged testes. Similarly, field notes recorded a nest in June (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). Hence, the subspecies conspicillatus appeared to breed year-round (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ), although nesting may have been concentrated in winter−spring (i.e., the dry season) (15 Baker, R. H. (1948). Report on collections of birds made by United States Naval Medical Research Unit No. 2 in the Pacific War area. Smithsonian Miscellaneous Collections 107(15):1–74. Close ).

The first recorded nests and young on Saipan were in May−July (65 Oustalet, M. E. (1895). Les mammifères et les oiseaux des Iles Mariannes. Nouvelles Archives du Muséum d’Histoire naturelle (3)7:141–228. Close ). Of 18 birds collected on Tinian in September, 1931, half had enlarged gonads (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 ). Yamashina (66 Yamashina, Y. (1932). On a collection of bird’s eggs from Micronesia. Tori 7:393–413, pl. 7. (In Japanese and English.) Close ) recorded three nests on Tinian in January, 1932. On Saipan, breeding was recorded in January (carrying nesting material), February (nestlings, carrying nesting material), August (eggs, carrying nesting material), and October (carrying food). Moreover, food begging by juveniles was observed year-round (47 Craig, R. J. (1996). Seasonal population surveys and natural history of a Micronesian bird community. Wilson Bulletin 108(2):246–267. Close ). Hence, breeding appeared to occur year-round (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 , 67 Pyle, P., J.F. Saracco, P. Radley and D.R. Kaschube. 2012. The tropical monitoring avian productivity and survivorship (TMAPS) program on Saipan, Commonwealth of the Northern Mariana Islands: 2011-2012 Report. The Institute for Bird Populations, Point Reyes Station, CA. Close ). However, on Saipan evidence of birds being in breeding condition (presence of cloacal protuberance, brood patch, eggs) occurred on 15 of 60 (25%) wet season and 27 of 54 (50%) dry season captures, indicating that, as on Guam, a peak of breeding occurred during the dry season of December−early June (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close ). Similarly, in 2004, a model based on the densities of located nests predicted that nest densities peaked in late February−early March. However, 2003 data did not verify this trend. In 2004, estimated nest densities generally declined in the native and non-native forest from February−March to April−May survey periods. This decline was more pronounced in nonnative forest, where no nests of any species were found in April−May (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 ).

Site Characteristics

On Guam, nests were placed 3−4 ft from the ground in shrubs and trees (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close ). Nests also were usually found to be built in the Ingadulus (a common name not known to relate to an extant taxon but possibly Pithecellobium dulce, based on other nests found in this species) tree (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ) hidden among leaves. The nest was usually placed far out where several branches come together some distance from the ground. One nest was 2.4 m up in a Leucaena leucocephala (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). On Tinian, a nest was built on a fork of a tree, commonly called Yamaichibi (also a common name not known to relate to an extant taxon), 2 m from the ground. A second nest was hung like a cradle on a fork of a horizontal branch of Pithecellobium dulce 4 m from the ground. A third nest was on a horizontal branch of Pithecellobium dulce 4 m from the ground (66 Yamashina, Y. (1932). On a collection of bird’s eggs from Micronesia. Tori 7:393–413, pl. 7. (In Japanese and English.) Close ). Three active Saipan nests were at 2.3–6.0 m above ground in Casuarina equisetifolia and Phragmites karka (69 Amidon, F. A., C. A. Haas, and J. M Morton (2004). Breeding biology of the endangered Rota Bridled White-eye. Wilson Bulletin 116(4):342–346. Close ). Another nest with two eggs was 2 m up in a Guamia [= Meiogyne] mariannae (R.J. Craig, personal observation).

Sachtleben (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 ) also documented the characteristics of 115 Bridled White-eye nests on Saipan, reporting nest height = 2.3, range = 0.7–5.2 m, nest tree height = 4.3 m, range = 1.2–10.8 m, distance of nests from the boles of nest trees = 42 cm, range = 0–263 cm, number of branches used for nest support = 3, range = 1–7 and diameter of these branches = 2 mm, range = 1–6 mm (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 ).

Construction Process

Nest construction begins by both the male and female making loops out of long fibrous materials such as coconut fibers and weaving them into a base. Once the base is made, the male and female alternate sitting in the nest and weaving small cotton-like material through the gaps (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 ).

Structure and Composition

On Guam, a nest was described as being a fairly deep cup, placed in the fork of a branch, woven together with fine grasses and roots, ornamented with cobwebs, wool, and cottonwool on the exterior (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close ). Another was constructed of fine fibers and grass, the outside being covered with green moss, which rendered it almost indistinguishable from below (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ). Another nest was composed of fine fibers and rootlets woven into a hanging basket (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ).

Three nests have been described from Tinian, which were also cup-shaped, but were so roughly built that their interior could be seen through their side wall. The chief construction materials were fine roots and fine fibers mixed with a small quantity of cotton wool and feathers. The interior was lined with only a small quantity of fibers (66 Yamashina, Y. (1932). On a collection of bird’s eggs from Micronesia. Tori 7:393–413, pl. 7. (In Japanese and English.) Close ). In captivity, subspecies saypani builds an open cup nest.

Dimensions

A Guam nest varied in width from 8 to 5 cm (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close ). Another’s size internally was 4.83 x 4.19 cm with a depth of 2.54 cm, whereas externally it was 7.49 x 5.56 cm with a depth of 4.45 cm (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ). A later nest was externally 4−5 cm in diameter by 7−8 cm deep (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). Three Tinian nests had an outer diameter of 5.5–6.5 cm, height of 4.0–5.5 cm, inner diameter of 4.0–4.5 cm, and internal depth of 2.5–3.0 cm (66 Yamashina, Y. (1932). On a collection of bird’s eggs from Micronesia. Tori 7:393–413, pl. 7. (In Japanese and English.) Close ).

Size

Eggs measured 16.9 mm ± 0.9 mm x 12.9 mm ± 0.5 (n = 5) (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close ), and ~16 x.12.7 mm (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ). On Tinian, eggs measured mean = 15.2 x 11.4 mm (n = 3) (66 Yamashina, Y. (1932). On a collection of bird’s eggs from Micronesia. Tori 7:393–413, pl. 7. (In Japanese and English.) Close ).

Color and Surface Texture

On Guam, the eggs were first described as pale blue, like all Zosterops eggs (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close ). Seale (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ); however, described the eggs as being white with a slight tint of blue and shaped like the eggs of a robin. Another nest contained two light blue-green eggs (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). On Tinian, the color of the eggshell was uniformly pale blue (66 Yamashina, Y. (1932). On a collection of bird’s eggs from Micronesia. Tori 7:393–413, pl. 7. (In Japanese and English.) Close ).

Clutch Size

All described clutches on Guam consisted of two and three eggs (42 Hartert, E. (1898). On the birds of the Marianne Islands. Novitates Zoologicae 5(1):51–69. Close ). On Tinian, three nests contained one, two, and three eggs, respectively (66 Yamashina, Y. (1932). On a collection of bird’s eggs from Micronesia. Tori 7:393–413, pl. 7. (In Japanese and English.) Close ).

Egg Laying

Egg laying ranges from 2–3 days (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 ).

Incubation Period

On Saipan, incubation was documented to take from 9−12 days (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 ). No information was said to be available on Guam incubation (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ).

Information needed.

Growth and Development

Chick growth as reported by the MAC Working Group (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 ) is as follows:

:

Day 0. Chicks are ~1.5 cm (1–2 cm), naked, with light-medium pink skin and two tufts of downy feathers on their heads (appearance-wise, between horns and eyebrows).

Day 1. ~2 cm long and naked with medium-dark pink skin; otherwise, little change from day 0.

Day 3. ~2.5 cm (2–3 cm) long, medium-dark pink skin, wing pins 2–5 mm, head and back pins visible under skin but not erupted or barely so; tufts on head either remaining or no longer present.

Day 4. ~3.5 cm long, medium-light pink skin, back pins 1–2 mm, wing pins ≥3 mm.

Day 6. ~3.5 cm (3–4 cm) long, wing pins 6–7 mm, greenish feathers possibly erupted from wing pins and 1–2 mm, back pins 2–4 mm, greenish feathers possibly erupted from back pins and 1–2 mm, head pins 3−4 mm, white belly feathers in two lines, exposed skin light or medium pink, eyes still closed or cracking open.

Day 8. ~4.5 cm long, fully feathered, olive gray green, eyes open.

Day 9. ~4–4.5 cm long, mostly feathered, olive gray green, eyes open, wing feathers dark gray.

Day 10. ~5–5.5 cm long, fully feathered, wings dark gray, back gray green.

Day 12. ~5–5.5 cm long, greenish and fully feathered, belly appearing downy, often perching on rim of the nest. Chicks will force-fledge at this age and fly well.

On Guam, a pair was observed feeding a young bird out of the nest in a thicket in October, 1945 (7 Stophlet, J. J. (1946). Birds of Guam. Auk 63(4):534–540. Close ). On Saipan, food begging (wing fluttering, crouching, and gaping) by juveniles with no evidence of natal feathers was observed frequently. Moreover, pairs of birds were observed together (foraging, gathering nesting material, responding in pairs to playback of calls), as were family groups of three (apparently a male, female, and juvenile as identified by food-begging) (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 captivity, the male and female of subspecies saypani alternate in incubation. Typically, one bird at a time sits in the nest to keep the 1–3 eggs warm for development. A mate begins by approaching the attending parent sitting on the egg. The attending mate leaves once its partner is within several inches of the nest. The newly incubating bird adjusts the egg with its beak, fluffs its breast, belly, and flank feathers and settles its brood patch over the egg. Both sexes also participate in brooding, with attending parents taking turns keeping the chick warm. The brooding parent does not leave until the other comes within close proximity of the nest. The newly brooding bird adjusts the chick with its bill, puffs its breast, belly, and flank feathers, and settles its brood patch over the chick. The brooding bird adjusts by shifting side to side several times. Birds stay alert with heads upright and eyes open. Both sexes participate in nest maintenance. Using their beaks, the attending parents grab fecal pellets out of the nest and move them elsewhere, typically during feeding times. Both sexes participate in feeding nestlings. Attending parents prepare live waxworms and maggots, bring them whole, and place them in a chick’s mouth. Parents also deliver smaller items to the nestling, such as fruit flies, bean beetles, and fruit pulp (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 ).

No information available.

No information available.

On Saipan, fledging occured 11−14 days post-hatching (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 ). No information was said to be available on Guam nestling or fledgling periods (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ).

In captivity, chicks leave the nest 11−14 days post-hatching and are usually incapable of flight for several days, making them especially vulnerable to exposure and predation. Young birds generally stay high in dense foliage to develop their flying skills and independence. Parent birds actively feed young for about two weeks post-fledging. During this time, fledglings begin to flutter among higher perches to develop their flight skills (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 , 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 ). The duration of post-fledging parental care is unknown (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 ).