Arctic Warbler Phylloscopus borealis 極北柳鶯
Category I. Common passage migrant, more numerous in autumn, very rare in winter; widespread in wooded areas.
IDENTIFICATION
May 2019, Roman Lo.
11.5-13 cm. The most numerous Phylloscopus warbler in May, September and early October. Relatively large and sturdy, noticeably so at the base of bill, it has a long primary projection. Long narrow supercilium that usually does not reach the bill, broad dark eye stripe includes all lores, and diffuse streaking on the whitish chest. Pale lower mandible with obvious dark subterminal mark. Short, diffuse wing bar on outer greater coverts; may have second wing bar on median coverts. Legs brownish. Call distinctive (see below). Moves slowly compared to most other Phylloscopus warblers in HK.
VOCALISATIONS
Distinctive call a short, buzzing ‘dzeep’, occasionally uttered as a pair ‘dzi-dzip’.
The song is a somewhat harsh slightly modulated trilling phrase.
DISTRIBUTION & HABITAT PREFERENCE
Arctic Warblers occur in all types of shrub and wooded habitat where there is sufficient cover for migrants.
OCCURRENCE
A common migrant in spring and autumn, though passage is more intense and prolonged in autumn, but very rare in early winter. Extreme spring dates are 25 March 2007 and, in four years, 27 May. The earliest record in autumn is of one on 20 August 2015, while the latest occurred on 8 December 2016.
Arctic Warbler is rare for the first three weeks of April, and main passage occurs from the final week of April to the third week of May, peaking in the first week of May (Figure 1). The highest spring count is 43 on Chi Ma Wan on 5 May 2013.
In autumn numbers are relatively low until the first week of September but increase rapidly thereafter to peak in the third week of the month. The main passage period continues until the first week of October, and from the second week of the month Arctic Warbler is uncommon. After the middle of November, it is rare.
The highest counts in autumn are 60 at Tai Po Kau on 18 September 1988, and 52 between Mai Po and Lok Ma Chau on 24 September 1995. Since 1999 the highest count is 40 at Mai Po on 20 September 2003, with the highest count since 2010 being only 17 between Tung Chung and Sha Lo Wan on 19 September 2016. Given the increase in observer activity this may suggest a decline in the number of birds passing through.
There are very few adequately substantiated midwinter records. The first was at the former Victoria Barracks on HK Island (the site of HK Park) during 24-25 December 1987. In addition, in winter 2016/2017 singles were present at Ho Man Tin from 3 January to 23 March, Kai Tak on 9 January and Stanley on 25 January.
Noted by Vaughan and Jones (1913) and Dove and Goodhart (1955) as a passage migrant occurring in very large numbers in autumn in some years. Likewise, Walker (1958) noted ‘immense numbers’ during autumn 1956 on dates between 7 September and 20 October; in contrast, spring passage the following year involved only ‘a few birds’ between 21 April and 15 May.
BEHAVIOUR, FORAGING & DIET
A slow forager in the canopy searching for insects and caterpillars. Has been noted dead in the web of Golden Orb Spider Nephila pilipes.
RANGE & SYSTEMATICS
Monotypic. Very extensive breeding range from northern Scandinavia east across Siberia to western Alaska and south to north central Mongolia, northeast China and south Ussuriland; winters in southeast Asia (Lowther and Sharbaugh 2020). The pattern of records in HK suggests only very small numbers winter in south China, with most likely to be west of here.
CONSERVATION STATUS
IUCN: Least Concern. Population trend increasing.
Figure 1.
Dove, R. S. and H. J. Goodhart (1955). Field observations from the Colony of Hong Kong. Ibis 97: 311-340.
Lowther, P. E. and S. Sharbaugh (2020). Arctic Warbler (Phylloscopus borealis), version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.arcwar1.01
Vaughan, R. E. and K. H. Jones (1913). The birds of Hong Kong, Macao and the West River or Si Kiang in South-East China, with special reference to their nidification and seasonal movements. Ibis 1913: 17-76, 163-201, 351-384.
Walker, F. J. (1958). Field observations on birds in the Colony of Hong Kong. Hong Kong Bird Watching Society, Hong Kong (duplicated).
