Willow Warbler Phylloscopus trochilus


Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
In Europe, the breeding population is estimated to number 62,200,000-97,100,000 pairs, which equates to 124,000,000-194,000,000 mature individuals (BirdLife International 2015). Europe forms c.30% of the global range, so a very preliminary estimate of the global population size is 413,000,000-647,000,000 mature individuals, although further validation of this estimate is needed. The population in Russia has been estimated at < c.100,000 breeding pairs and < c.1,000 individuals on migration (Brazil 2009).

Trend justification
In Europe, trends between 1980 and 2013 show that populations have undergone a moderate decline (EBCC 2015).


During the breeding season this species inhabits deciduous and mixed forests in which birch (Betula) largely predominates, but also birches and willow (Salix) thickets and scrub on arctic tundra. In central areas it is found in temperate heathland, forest clearings, damp areas with alders (Alnus) and willows as well as secondary growth, scrub and shrubby areas away from trees. It generally uses a wide selection of locations, including large overgrown gardens, orchards, hedges, railway embankments, and rough pastures with tussock grassland. It breeds from April to July and lays four to eight eggs. The nest is a ball of dry grasses, leaves, plant fibres, moss, strips of bark, animal hair and feathers and is placed on the ground, usually well concealed in vegetation. It feeds mainly on insects and their eggs and larvae and some plant material. The species is migratory and winters in Africa, south of the Sahara (Clement 2006).


The species is affected by drought conditions in its wintering quarters and habitat alterations due to human population expansion (Thingstad et al. 2014). In the southern U.K., habitat loss linked to modern forest management techniques that alter the woodland vegetation structure is likely to have caused declines (Stostad and Menéndez 2014). In addition, declining oak tree health may have impacted the species as oak is typically a major source of invertebrates (Peach et al. 1995).

Conservation actions

Conservation Actions Underway
CMS Appendix II. Bern Convention Appendix II. There are currently no known conservation measures for this species within Europe.

Conservation Actions Proposed

This species would benefit from the preservation and restoration of its woodland habitat, including the maintenance of traditional management techniques.


Text account compilers
Ashpole, J, Butchart, S., Ekstrom, J.

Recommended citation
BirdLife International (2019) Species factsheet: Phylloscopus trochilus. Downloaded from on 18/04/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from on 18/04/2019.