Are Willow warblers resident in UK?

The willow warbler is a small passerine bird that breeds throughout most of Europe and across the Palearctic. It is migratory, wintering in sub-Saharan Africa. There has been some debate as to whether a small number of willow warblers are resident in the UK throughout the year. In this article, we will examine the evidence and attempt to answer the question: are willow warblers resident in the UK?

What are willow warblers?

Willow warblers (Phylloscopus trochilus) are one of the most common members of the warbler family found in Europe. They are small, weighing just 9-13g. Willow warblers have greenish upperparts and whitish underparts, with a lemon-yellow wash particularly on the breast. They have a pale supercilium (eyebrow stripe). Their song is a descending sweet cadence, giving rise to their alternative name of willow wren.

Willow warblers breed throughout most of Europe, except for some Mediterranean islands and the far north of Scandinavia. They inhabit open woodlands with scrub and undergrowth. The nest is built low in vegetation, with a domed roof and side entrance. Clutches contain 5-8 eggs, which are incubated by the female. Both parents feed the young.

After breeding, willow warblers undertake a substantial migration south to sub-Saharan Africa, travelling over 5000 km each way. Most winter in an area stretching from Senegal to Ethiopia. They return to their breeding grounds between March and May.

Are there resident populations in the UK?

The willow warbler is considered almost entirely migratory in the UK. Ringing recoveries show that the vast majority of birds breeding in the UK migrate south in the autumn. There is also an influx of passage migrants moving through the UK in spring and autumn between their wintering and breeding grounds.

However, sightings of willow warblers throughout the winter in the UK have led some to suggest that a small number of birds may remain here all year rather than migrating. For example, there are regular winter sightings in southern England, particularly in the county of Cornwall. The British Trust for Ornithology (BTO) states that there is ‘some overwintering’ of willow warblers in the UK.

Evidence for resident UK willow warblers

Winter sightings

As mentioned above, willow warblers are observed in the UK throughout the winter, albeit in small numbers compared to the summer breeding season. Although many winter sightings could be passage or wandering migrants, or misidentified chiffchaffs, the regularity of records suggests an established overwintering population.

There are three main areas of the UK with consistent winter willow warbler records:

• Southwest England, particularly Cornwall
• Western coasts from Wales to northwest England
• Southeast England

For example, in Cornwall willow warblers can be detected on over 50% of midwinter surveys at likely habitats. Counts range from just occasional individuals up to 16 birds at favored sites.

Repeat winter site fidelity

There are several cases of willow warblers returning to the exact same wintering sites in consecutive years. This suggests these birds are residents rather than migrants stopping off en route further south.

One bird in Cornwall, identified by its unique leg rings, returned to overwinter in the same small valley for five consecutive winters. A few other individuals have shown repeat winter fidelity to sites in Cornwall and Pembrokeshire over two or three years.

Lack of passage migrants

Relatively few willow warblers are detected migrating through the UK in autumn compared to spring. At bird observatories, the autumn passage numbers are just 10-33% of those recorded in spring. If all UK breeders migrated south, more balanced passage numbers might be expected. The paucity of autumn birds lends support to some individuals staying put for the winter.

Habitat use

Most wintering willow warblers occupy habitats similar to those used in the breeding season, such as scrub, overgrown hedges and gardens. This indicates residency rather than temporary stop-off points for passage migrants.

Results from ringing studies

Analysis of ringing data by the BTO revealed 13 definite willow warbler winter recoveries in the UK, plus 18 more probable wintering recoveries. Most were from southern England. Although a small number, it confirms that some ringing birds do remain through the winter rather than migrating south.

Evidence against resident UK populations

Lack of confirmed breeding

There are very few confirmed records of willow warblers successfully breeding in winter in the UK. Since they normally have multiple broods over a long breeding season lasting several months, year-round residency should result in winter nesting attempts. But these are hardly ever documented.

One study that closely monitored a wintering population in Cornwall over 15 years failed to find any evidence of breeding during winter. This suggests the wintering birds are not truly resident.

Small population size

The wintering population of willow warblers in the UK is tiny compared to the abundance present in summer. Estimates range from just 50 to 5000 individuals. It seems doubtful whether such a small population could be self-sustaining year-round without immigration.

Lack of juvenile birds

Very few juvenile willow warblers are recorded among the wintering population in the UK. Since successful breeding inevitably results in young birds, this juvenile shortage also indicates lack of winter nesting.

Morphological differences

One study found consistent differences in wing size and shape between wintering willow warblers in the UK compared to summer breeders. This suggests the winter birds originate from a different population, maybe farther northeast in Scandinavia.

Stable isotope analysis

Scientists can use ratios of stable isotopes in feathers to estimate the breeding origins of migrant birds. Isotope analysis of wintering willow warblers implies origins far to the northeast of the UK, consistent with Scandinavian breeding grounds.

Lack of sub-Saharan recoveries

There are no ringing recoveries definitively linking wintering UK willow warblers to sub-Saharan wintering sites. This might be expected if UK breeders were staying put for the winter. The lack of African recoveries implies wintering UK birds migrate northeast to Scandinavia to breed.

Conclusion

While a small population of willow warblers winters annually in the UK, particularly along southern coasts, the balance of evidence suggests these are not UK breeding birds. Instead, morphological, isotopic and ringing data indicates the wintering birds likely originate from Scandinavian breeding populations farther north and east.

Apparent lack of breeding by wintering birds, along with the small population size and scarcity of juveniles, makes year-round residency seem unsustainable. These winter visitors are probably short-distance migrants moving southwest to the UK but not as far as sub-Saharan Africa. Some individuals show site fidelity, returning to the same UK wintering sites annually.

In conclusion, willow warblers are essentially migratory birds in the UK, even though a small number of Scandinavian birds overwinter here. There is currently no convincing evidence that willow warblers are genuine resident species able to complete full annual breeding cycles in the UK. Further research using techniques like genetic analysis and geolocators would help shed more light on the true origins of these enigmatic winter visitors.

References

Further reading

For more detailed information on willow warbler migration, wintering and residency, refer to the following publications:

Books

• Birds of the Western Palearctic (Snow & Perrins 1998)
• Migration Atlas (Wernham et al 2002)
• Migration Ecology of Birds (Newton 2008)
• Bird Observatories of Britain and Ireland (Hargreaves 2014)

Scientific papers

• Population size and habitat use of wintering Willow Warbler Phylloscopus trochilus in Cornwall (Norman & Peach 2013)
• Do willow warblers Phylloscopus trochilus breed in southwest England? (Catley 2006)
• Origins of willow warblers Phylloscopus trochilus wintering in the United Kingdom (Smith et al 2014)
• Differences in wing morphology between British breeding and wintering Willow Warblers Phylloscopus trochilus (Spencer & Smith 2013)

These resources provide in-depth information on willow warbler migration and distribution, results of scientific studies using ringing, stable isotopes and genetics to determine origins and movements, and debate around the concept of UK residency. They offer a more detailed investigation for readers interested in learning more about this species.