Plumages, Molts, and Structure

The Bridled White-eye has 9 full-length primaries (numbered distally, from innermost p1 to outermost p9), 9 secondaries (numbered proximally from outermost s1 to innermost s9 and including 3 tertials, s7–s9 in passerines), and 12 rectrices (numbered distally, from innermost r1 to outermost r6 on each side of the tail). Geographic variation in appearance is slight (see Sytematics: Geographic Variation); the following covers both subspecies and is based on plumage descriptions in Marshall (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ), 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 ), and Pratt et al. (3 Pratt, H. D., P. L. Bruner, and D. G. Berret (1987). A Field Guide to the Birds of Hawaii and the Tropical Pacific. Princeton University Press, Princeton, NJ, USA. Close ), along with examination of Macaulay Library images; see Pyle et al. (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close ), Radley et al. (5 Radley, P., A. L. Crary, J. Bradley, C. Carter, and P. Pyle (2011). Molt patterns, biometrics, and age and gender classification of landbirds on Saipan, Northern Mariana Islands. Wilson Journal of Ornithology 123(3):588–594. Close ), and Craig (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close ) for information on ageing and sexing this species. See Molts for molt and plumage terminology. Appearance of sexes is similar in all plumages; definitive-like appearance is assumed at the Formative Plumage in most individuals and definitive appearance is assumed at the Second Basic Plumage in some individuals. Seasonal variation in plumages (e.g., fresh vs. worn) based on timing of molts, which is unknown but may occur year-round based on year-round breeding (see Breeding: Phenology) and/or extended periods of molt observed (see Molts).

Natal Down

Occurs in the nest. Natal down is undescribed in Bridled White-eye; chicks appear to hatch naked (cf. ).

Juvenile (First Basic) Plumage

Juveniles are grayer above and paler below than in later plumages (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close ) and can show dusky streaking to the crown and underparts (see image below). On Guam, a juvenile was observed to be lighter green above and lighter yellow below compared to its parents (7 Stophlet, J. J. (1946). Birds of Guam. Auk 63(4):534–540. Close ). The lores also appear paler, not as dusky as those in later plumages (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 ). Juvenile body feathers are weaker and more filamentous (barb density sparser) than in later plumages, especially the undertail coverts, and juvenile primaries and rectrices are thinner and more tapered or pointed at the tips than basic feathers. Iris color of juveniles may also be duller (see Bare Parts).

Formative Plumage

This plumage appears to be distinguishable in only a proportion of birds when the Preformative Molt is incomplete (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close , 5 Radley, P., A. L. Crary, J. Bradley, C. Carter, and P. Pyle (2011). Molt patterns, biometrics, and age and gender classification of landbirds on Saipan, Northern Mariana Islands. Wilson Journal of Ornithology 123(3):588–594. Close ). An unknown proportion of birds can retain contrastingly worn and narrow juvenile outer primaries, primary coverts, secondaries among s4–s6, and/or outer rectrices following this molt and can be aged as first-year birds. Formative Plumage following a complete Preformative Molt is indistinguishable from Definitive Basic Plumage.

Definitive Basic Plumage

Definitive Basic Plumage overall can vary from grayish green to yellowish green even within subspecies; factors for this variation are not known. The crown, upperparts, upperwing coverts, and sides of head vary from mixed green and dusky to uniformly green or greenish yellow; a paler cream to pale yellowish band crosses the forehead above the bill, and the nape is slightly paler greenish, greenish yellow, or gray. A full and broad, prominent white eye ring is a conspicuous plumage feature. The lores are dusky, this color usually extending across the eye ring and sometimes under the eye. Remaining sides of the head are washed green to greenish yellow. Rectrices and remiges are dusky with olive to yellow-olive edging. Underparts are variable, primarily buff to whitish or cream, usually with a dull to bright yellowish wash to the throat (can be absent or very faint), ventral feathering, and undertail coverts, occasionally tinged yellowish elsewhere. Definitive Basic Plumage is characterized by having all upperwing coverts and remiges uniform in wear and quality, without retained feathers; basic outer primaries and rectrices broad, more truncate (less pointed), and relatively fresh compared with retained juvenile feathers. No differences between sexes have been confirmed; females on Guam have been described as being lighter on the underparts (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 ) but more recent examination of sexed Saipan individuals revealed no consistent differences in plumage (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close ).

Molt and plumage terminology follows Humphrey and Parkes (8 Humphrey, P. S., and K. C. Parkes (1959). An approach to the study of molts and plumages. Auk 76(1):1–31. Close ) as modified by Howell et al. (9 Howell, S. N. G., C. Corben, P. Pyle, and D. I. Rogers (2003). The first basic problem: A review of molt and plumage homologies. Condor 105:635–653. Close ). Under this nomenclature, terminology is based on evolution of molts along ancestral lineages of birds from ecdysis (molts) of reptiles (cf. 10 Pyle, P., S. N. G. Howell, D. I. Rogers, and C. Corben (2024). Moult terminology: Envisioning an evolutionary approach. Journal of Avian Biology 2024:e03169. Close ), rather than on molts relative to breeding season, location, or time of the year, the latter generally referred to as “life-cycle” molt terminology (11 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ; see also 12 Pyle, P. (2022). Defining moults in migratory birds: A sequence-based approach. Journal of Avian Biology 2022:e02958. Close ). Bridled White-eye exhibits a Complex Basic Strategy (cf. 9 Howell, S. N. G., C. Corben, P. Pyle, and D. I. Rogers (2003). The first basic problem: A review of molt and plumage homologies. Condor 105:635–653. Close , 13 Howell, S. N. G. (2010). Peterson Reference Guide to Molt in North American Birds. Houghton Mifflin Harcourt, Boston, MA, USA. Close ), including complete prebasic molts and a partial-to complete preformative molt in the first cycle, but no prealternate molts (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close , 5 Radley, P., A. L. Crary, J. Bradley, C. Carter, and P. Pyle (2011). Molt patterns, biometrics, and age and gender classification of landbirds on Saipan, Northern Mariana Islands. Wilson Journal of Ornithology 123(3):588–594. Close ). Timing of molts may occur primarily in January–June (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close ) but also may occur year-round following year-round breeding (see Breeding: Phenology); see below for more details.

Prejuvenile (First Prebasic) Molt.

Occurs in the nest. Sequence of juvenile feather development not studied in Bridled White-eye.

Preformative Molt

Typically occurs within 3 mo of fledging. The Preformative Molt appears to be partial to complete, with all or some secondaries, rectrices, and primaries retained in some but not all birds. Often 1–3 secondaries among s4–s6 (the last scondaries replaced in sequence) could be the only juvenile feathers retained following this molt (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close , 5 Radley, P., A. L. Crary, J. Bradley, C. Carter, and P. Pyle (2011). Molt patterns, biometrics, and age and gender classification of landbirds on Saipan, Northern Mariana Islands. Wilson Journal of Ornithology 123(3):588–594. Close ; see image under Formative Plumage). The proportion of individuals that undergo a complete Preformative Molt is unknown.

Second and Definitive Prebasic Molts

Prebasic molts are complete and typically may occur 1–3 mo following breeding, although this may be complicated in Bridled White-eye by potential bimodal breeding seasons. In some cases molt may be suspended for breeding, as has been observed in other Pacific island species (14 Pyle, P., K. Tranquillo, K. Kayano, and N. Arcilla (2016). Molt patterns, age criteria, and molt-breeding dynamics in American Samoan landbirds. Wilson Journal of Ornithology 128(1):56–69. Close ). On Saipan, examination of 53 specimens found that 15 collected in active molt all occurred January–April i.e., the dry season, and the remainder were not in active molt, but banding data indicated molt also occurring in April–July (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close ). Of 114 Saipan birds banded in 1988−1993, there were 29 instances of individuals displaying molt of flight and contour feathers. Most instances were from September−October, the end of the wet season, which lasts from late June−November, although five were from late June. There were five instances of primarily flight feather molt during the dry season, although two of these were in early June— the end of the dry season (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close ). Similarly, four specimens collected during the wet season in October were all in molt (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 ). These observations may suggest a tendency to segregate the energetically expensive activities of molt to the wet season and breeding to the dry season but that molt timing can also occur year-round.

During complete molts, primaries (and corresponding primary coverts) are often replaced distally (p1 to p10), secondaries are replaced proximally and distally from the central or innermost tertial (s8 or s9)proximally from s1, and rectrices are generally replaced distally (r1 to r6) on each side of the tail, though variation in sequence of rectrix molt may occur. However, odd sequences of retained remiges have been observed in Bridled White-eye, where old and new primaries in various positions were observed that were not always symmetrical (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close ); further study on molt sequences needed in this sand other white-eye species. Odd sequences may occur following suspended molts, as noted above.

Bill and Gape

Guam adults had an upper mandible that was olive and the base of the lower mandible was yellow (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). Juveniles have been described as having a light yellowish-brown upper mandible (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 ), although photographic evidence from Saipan shows the bill of a fledged juvenile to be similar to that of the adult (R.J. Craig, personal observation). However, a juvenile Guam bird was also observed to have a bright orange-red bill (7 Stophlet, J. J. (1946). Birds of Guam. Auk 63(4):534–540. Close ), which could be an anomaly. Saipan and Tinian adults have an upper mandible that is shiny black but a lower mandible that is lighter gray (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close , R.J. Craig, personal observation). In hatchlings, the bill and swollen gape are bright yellow (cf. ) and a yellow gape may continue to be visible in recently fledged juveniles. In adults, the gape and perhaps inside of the mouth may be gray (cf. ).

Iris

The iris of Guam birds was described as light brown (16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ) or gray (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close ). That of Saipan birds can be a rich reddish-brown (R.J. Craig, personal observation; see image below). The irides of juvenile passerines are often duller and grayer brown than in adults and this may be the case with Bridled White-eye as well.

Tarsi and Toes

The legs and feet of Guam birds are variously described as dark gray, sometimes tinged olive green (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 ) or dark olive in both adults and juveniles (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close , 16 Jenkins, J. M. (1983). The native forest birds of Guam. Ornithological Monographs 31:1–61. Close ). On Saipan and Tinian, adults and juveniles have bluish-gray feet (2 Marshall, J. T. (1949). The endemic avifauna of Saipan, Tinian, Guam and Palau. Condor 51(5):200–221. Close , R.J. Craig, personal observation).

Linear Measurements

The earliest reported measurements, although the details of measurement procedure are lacking, are for Zosterops conspicillatus conspicillatus:

• Females (n = 4): total length = 109.3 ± 3.4 mm, wing length = 68.1 ± 26.6 mm, tail length = 38.6 ± 0.7 mm, tarsus length = 20.3 ± 0.3 mm, culmen = 11.6 ± 0.5 mm, mid-toe = 15.3 ± 0.5 mm (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ).
• Male (n = 1): total length = 108.0 mm, wing length = 58.4 mm, tail length = 38.1 mm, tarsus length = 24.9 mm, culmen = 11.9 mm, mid-toe = 14.7 mm (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ).
• Juvenile (n = 1): total length = 76.2 mm, wing length = 38.1 mm, tail length = 14.2 mm, tarsus length = 20.3 mm, culmen = 8.9 mm, mid-toe = 15.7 mm (17 Seale, A. (1901). Report of a mission to Guam. Occasional Papers of Bernice P. Bishop Museum 1:17–128. Close ).
• Unsexed (n = 43): flattened wing length = 56 mm, range = 52−59 mm; full culmen = 13.5 mm, range = 13−14.5 mm; tarsus length = 19 mm, range = 18−20 mm; tail length = 41 mm, range = 37−43 mm (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 ).

Zosterops conspicillatus saypani, again with measurement details lacking (reported from [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 , 18 Mees, G. F. (1969). A systematic review of the Indo-Australian Zosteropidae (Part III). Zoologische Verhandelingen 102:1–390. Close ]):

• Unsexed (n = 29): flattened wing length = 52 mm, range = 50−55 mm; tail length = 38, range = 35−40 mm; tarsus length = 18 mm, range = 17−19 mm; culmen = 12.5 mm, range = 12−13.5 mm (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 ).
• Tinian (n = 23): flattened wing length = 51 mm, range = 50−53 mm; tail length = 38 mm, range = 35−41 mm; tarsus length = 18 mm, range = 17−18 mm; culmen = 12.0 mm, range = 12.0−13.0 mm (18 Mees, G. F. (1969). A systematic review of the Indo-Australian Zosteropidae (Part III). Zoologische Verhandelingen 102:1–390. Close ).
• Saipan (n = 6): flattened wing length = 54 mm, range = 52−55 mm; tail length = 37 mm, range = 35−39 mm; tarsus length = 18 mm, range =17−19 mm; culmen = 13.0 mm, range = 13.0−15.0 mm (18 Mees, G. F. (1969). A systematic review of the Indo-Australian Zosteropidae (Part III). Zoologische Verhandelingen 102:1–390. Close ). Birds from Saipan averaged slightly larger than birds from Tinian.

Captures on Saipan in 1988−1993 with full measurement details provided (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close ) showed that:

• Females (n = 12): wing chord = 49.4 ± 1.2 mm, tail length = 40.7 ± 2.6 mm, tarsus length = 17.5 ± 0.9 mm, bill length from proximal nares = 6.9 ± 0.3 mm, bill depth at proximal nares = 2.6 ± 0.2 mm (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close , R.J. Craig, unpublished data).
• Males (n = 30): wing chord = 50.2 ± 1.3 mm, tail length = 40.2 ± 2.3 mm, tarsus length = 18.2 ± 1.2 mm, bill length from proximal nares = 7.0 ± 0.4 mm, bill depth at proximal nares = 2.6 ± 0.1 mm.

Captures on Saipan in 2008−2009 using standard U.S. Fish and Wildlife Service procedures showed that (reported from [ (5 Radley, P., A. L. Crary, J. Bradley, C. Carter, and P. Pyle (2011). Molt patterns, biometrics, and age and gender classification of landbirds on Saipan, Northern Mariana Islands. Wilson Journal of Ornithology 123(3):588–594. Close ]):

• Females (n = 70): wing chord = 51.5 ± 1.4 mm.
• Males (n = 64): wing chord = 52.3 ± 1.6 mm.

Male Saipan captures in 2008, also using standard U.S. Fish and Wildlife Service procedures had (reported from [ (4 Pyle, P., P. Radley, J. Bradley, and C. Carter (2008). Manual for aging and sexing birds on Saipan, with notes on breeding seasonality. The Institute for Bird Populations, Point Reyes Station, CA, USA. Close ]):

• Females (n = 29): wing chord = 49−54 mm.
• Males (n = 17): wing chord = 51−54 mm.

Discriminant function analysis demonstrated that means for Saipan Bridled White-eye sexes differed significantly, albeit weakly (n = 114, Wilk’s λ = 0.64, P < 0.01; canonical correlation = 0.60), with 90.0% of selected but only 58.3% of unselected grouped cases correctly classified. Based on tests of equality of group means, only wing chord contributed substantially to the function. From the 95% confidence interval, birds with wing length >50.1 mm were males, and birds with wing length ≤ 50.1 mm were females (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close ).

Captures in May 1992 of largely unsexed individuals on Aguiguan, using the same procedures as 6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close , had:

• Males (n = 3): sexed by cloacal protuberance; wing chord = 51.1 ± 0.3 mm, tail length = 43.1 ± 1.8 mm, tarsus length = 18.4 ± 0.7 mm, bill length from proximal nares = 7.7 ± 0.4 mm, bill depth at proximal nares = 2.9 ± 0.1 mm (19 Craig, R. J., A. Ellis, and R. Kaipat (1993). Genetic isolation and the comparative morphology of small passerines on Aguiguan and Saipan. Proceedings, Marianas Research Symposium 1:22−25. Close ).
• Unsexed (n = 15): wing chord = 50.3 ± 1.0 mm, tail length = 43.0 ± 1.6 mm, tarsus length = 17.5 ± 0.7 mm, bill length from proximal nares = 7.6 ± 0.4 mm, bill depth at proximal nares = 2.8 ± 0.2 mm (19 Craig, R. J., A. Ellis, and R. Kaipat (1993). Genetic isolation and the comparative morphology of small passerines on Aguiguan and Saipan. Proceedings, Marianas Research Symposium 1:22−25. Close ).

No birds displayed brood patches. Discriminant function analysis demonstrated a significant difference between Saipan and Aguiguan birds. Univariate f tests demonstrated that the Bridled White-eye had significantly larger wing, bill, and tarsus measurements on Aguiguan compared with Saipan (19 Craig, R. J., A. Ellis, and R. Kaipat (1993). Genetic isolation and the comparative morphology of small passerines on Aguiguan and Saipan. Proceedings, Marianas Research Symposium 1:22−25. Close ).

Mass

Birds from Guam had a female mass x = 9.3 g, range = 8–10 g (n = 3) (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 ) and a male mass x = 10.5 g, range = 9.5–14 g (n = 11).

Saipan captures from 2008−2009 had a female mass of 8.0 ± 0.7 g (n = 59) (5 Radley, P., A. L. Crary, J. Bradley, C. Carter, and P. Pyle (2011). Molt patterns, biometrics, and age and gender classification of landbirds on Saipan, Northern Mariana Islands. Wilson Journal of Ornithology 123(3):588–594. Close ) and a male mass of 7.4 ± 0.5 g (n = 48).

Saipan captures from 1988−1993 had a female mass of 8.3 ± 0.9 g (n = 11) and a male mass of 8.0 ± 0.4 g (n = 30).

May 1992 Aguiguan captures had a mass of 7.4 ± 0.5 g (n = 14), which was significantly lower than that of birds from Saipan. Three of these were sexed as males and had a mass of 7.8 ± 0.4 g (6 Craig, R. J. (2021). External morphology of Mariana Island passerines. Micronesica 2021-04:1−9. Close , 19 Craig, R. J., A. Ellis, and R. Kaipat (1993). Genetic isolation and the comparative morphology of small passerines on Aguiguan and Saipan. Proceedings, Marianas Research Symposium 1:22−25. Close , R.J. Craig, unpublished data).