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Blackcaps Advanced the Start of Spring Passage by Nine Days at Poland’s Baltic Coast Without Widening the Sex Gap

Male Eurasian blackcap perched on a branch
Research · Ecology

A 44-year ringing series from Poland’s Baltic coast shows that male and female Eurasian blackcaps (Sylvia atricapilla) have advanced the beginning of their spring passage at almost the same pace. The preprint, posted on 5 August 2026, analysed standardised spring ringing data from Bukowo-Kopań between 1982 and 2025. Four years with fewer than five females were excluded, so the phenology analyses used 38 spring seasons and 7,257 birds: 4,345 males and 2,912 females.

Both sexes advanced in parallel

Males passed through earlier than females at the start of the season, a pattern known as protandry. The gap averaged about 2.2 days, but it did not increase over the study period because the first 10% of migrants of both sexes advanced by about nine days. The median passage date moved forward by roughly seven to nine days, while the end of passage showed no long-term trend.

Different climate patterns tracked different phases

The climate indices used in the study describe recurring large-scale atmospheric or ocean–atmosphere patterns that influence weather across broad regions. A positive winter phase of the North Atlantic Oscillation and a positive spring Scandinavian Pattern were associated with an earlier start of passage. Median passage was earlier with positive North Atlantic Oscillation phases in both winter and spring and with a positive spring Scandinavian Pattern. A positive winter Indian Ocean Dipole was associated with a later end of passage. These relationships did not differ between males and females. The selected climate models accounted for about 48% of year-to-year variation in the start of passage and 39% in the median, but only 7% in the end of passage, so climate explained much less of the variation in when migration finished.

The authors interpret the stable early-season sex difference as evidence that both sexes have shifted spring timing in parallel. They connect the persistence of this early protandry to the mating opportunity hypothesis, in which selection can favour males that arrive early enough to secure high-quality territories. They also suggest that the different climate signals affecting the beginning, middle and end of passage may reflect successive groups of birds arriving through different routes and from different parts of the wintering range.

Sources and image credit

  1. Pinszke, A., & Remisiewicz, M. (2026). The Influence of Winter and Spring Large-Scale Climate Patterns on the Timing of Male and Female Blackcap Spring Passage at the Baltic Coast [Preprint]. Preprints.org. https://doi.org/10.20944/preprints202608.0278.v1Primary public version posted 5 August 2026. The study analyses standardised spring ringing data from Bukowo-Kopań in northern Poland over 1982–2025 and tests long-term phenological trends and relationships with large-scale climate indices.

Image: Luiz Lapa / Wikimedia Commons — male Eurasian blackcap in Serra da Arrábida, Portugal, 23 February 2022; representative image, not a study bird · CC BY 2.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

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