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Common Grasshopper Warblers and Eurasian Tree Sparrows Declined Sharply at a Portuguese Ringing Station

Common grasshopper warbler singing from vegetation
Conservation · Population & status

Twenty-seven years of standardized autumn ringing at Santo André in southwestern Portugal showed steep declines in several passerines. All five long-distance migrants declined, and common grasshopper warbler (Locustella naevia) was among three species that now barely occur at the site. Among resident birds, Eurasian tree sparrow (Passer montanus) showed the steepest decline and was not detected after 2022.

The researchers analysed capture data from 1997 to 2024 for 16 passerine species. Annual abundance indices were corrected for differences in ringing effort, allowing long-term changes at the station to be compared. The results describe capture-based abundance at Santo André during autumn migration and do not by themselves measure population size across the wider region.

All five long-distance migrants declined

Common reed warbler (Acrocephalus scirpaceus), sedge warbler (A. schoenobaenus), Savi’s warbler (Locustella luscinioides), common grasshopper warbler and willow warbler (Phylloscopus trochilus) all declined over the study period. Savi’s warbler, common grasshopper warbler and sedge warbler showed the most severe losses and now barely occur at Santo André.

The three short-distance migrants showed a different pattern. Bluethroat (Luscinia svecica) declined, while common chiffchaff (Phylloscopus collybita) and Eurasian blackcap (Sylvia atricapilla) increased.

Long-term abundance trends of five long-distance migratory passerines at Santo André, Portugal
Abundance indices for the five long-distance migratory passerines monitored at Santo André from 1997 to 2024. All five declined. Figure 2 from Almeida et al. (2026), Conservation, CC BY 4.0.

Tree sparrows were not detected after 2022

Among the eight resident species, Eurasian tree sparrow declined most steeply. Captures peaked in 2004, remained comparatively high until 2012 and then fell towards zero. The authors interpret its non-detection after 2022 as possible local extinction or a marked contraction in use of the site, not evidence that the species has disappeared from the wider region.

Sardinian warbler (Curruca melanocephala) also declined. European robin (Erithacus rubecula) and Eurasian blackbird (Turdus merula) increased, while Cetti’s warbler (Cettia cetti), European greenfinch (Chloris chloris), Eurasian blue tit (Cyanistes caeruleus) and great tit (Parus major) showed no statistically supported long-term trend.

Long-term abundance trends of resident passerines at Santo André, Portugal
Observed abundance trends for resident passerines at Santo André from 1997 to 2024, including the steep decline of Eurasian tree sparrows. Figure 4 from Almeida et al. (2026), Conservation, CC BY 4.0.

The authors link many declines to dependence on insects

The authors argue that the concentration of declines among insect-dependent birds is consistent with wider trophic disruption associated with insect losses. They point to habitat loss, pesticide use, pollution and climate change as major drivers reported for insect declines elsewhere, but insect abundance was not measured at Santo André.

The study’s own diet classifications make the pattern more complex than a simple insectivore-versus-non-insectivore split. Cetti’s warbler was classified as insectivorous but showed no statistically supported decline, while the declining Eurasian tree sparrow and Sardinian warbler were classified as omnivorous.

Local habitat quality, weather, competitors and predators can also affect capture rates. The authors report, however, that the habitats used by the ringing station remained broadly similar over the monitoring period and argue that minor local changes are unlikely to explain the strongest long-term patterns.

Sources and image credit

  1. Almeida, A. P., Araújo, M., Encarnação, V., & Ramos, J. A. (2026). Steep population declines in insectivorous passerines, irrespective of their migratory strategies. Conservation, 6(1), 19. https://doi.org/10.3390/conservation6010019Primary peer-reviewed open-access article published 5 February 2026 under CC BY 4.0. Standardized autumn ringing data from 1997–2024 covered 16 passerine species at Santo André National Ringing Station, Portugal. All five long-distance migrants declined significantly; grasshopper warbler was among the species now barely occurring locally, and tree sparrow showed the steepest resident decline with no detections after 2022.

Image: Keith Gallie / Wikimedia Commons — common grasshopper warbler (Locustella naevia) at Woolston Eyes, Cheshire, United Kingdom, photographed 11 May 2017. Representative species image, not a bird from the Portuguese ringing study. · CC BY 2.0 — https://creativecommons.org/licenses/by/2.0/ · Source

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Population & status

SPECIES IN THIS STORY

Species in this story

Common Reed Warbler Acrocephalus scirpaceus Explore species European Greenfinch Chloris chloris Explore species Eurasian Blue Tit Cyanistes caeruleus Explore species European Robin Erithacus rubecula Explore species Common Grasshopper Warbler Locustella naevia Explore species Bluethroat Luscinia svecica Explore species Great Tit Parus major Explore species Eurasian Tree Sparrow Passer montanus Explore species Common Chiffchaff Phylloscopus collybita Explore species Willow Warbler Phylloscopus trochilus Explore species Eurasian Blackcap Sylvia atricapilla Explore species Eurasian Blackbird Turdus merula Explore species

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