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Inland Mute Swans Had Higher Winter Body Condition Despite Harsher Winters and Supplemental Feeding

Pair of mute swans on the Vistula River in Tyniec, Kraków, Poland
Research: Physiology, Ecology

Mute swans (Cygnus olor) wintering in inland Poland had higher body condition than birds at milder coastal sites, even though winters were harsher inland and swans at all four study regions were regularly fed by people. Researchers compared 853 swans captured between mid-November and mid-March in the Gulf of Gdańsk, Świnoujście, central Poland and Upper Silesia. Sampling years differed among regions and together spanned 1994–2015, so the comparison was not a single simultaneous coast-versus-inland winter.

Body condition was estimated relative to structural size

The researchers used a scaled mass index, which relates body mass to structural size and serves as a proxy for energetic reserves. It is not a direct measurement of fat or muscle. This correction matters because a heavier swan is not necessarily in better condition if it is also structurally larger. Total head length was used as the size measure because it is taken between fixed anatomical points and changes little after juvenile development.

Inland birds and adults had higher condition

Across age classes, swans at the two inland sites had higher scaled mass indices than birds from the two coastal regions. This was notable because the inland winters had lower temperatures and more days below freezing. Adults also had higher condition than juveniles in every region, with immature birds generally intermediate where the data allowed comparison. Sex differences were small and not consistent across the study population.

Condition declined through winter despite supplemental food

Body condition declined from mid-November towards mid-March. Among adults, the decline was steeper inland than on the coast, while juveniles showed more similar seasonal declines across the two region groups. The pattern persisted even though people regularly supplied food, mainly bread and grain, at every study site.

The authors interpret the higher inland condition as consistent with swans maintaining larger energy reserves where winter conditions are harsher, while the seasonal decline may partly reflect endogenous regulation of body mass rather than food availability alone. Supplemental feeding therefore did not erase the patterns associated with region, season and age. The quantity of food supplied was not measured, however, so the study cannot show that provisioning fully met the birds’ energetic needs. Competition may also contribute to the poorer condition of juveniles, which are subordinate to adults in winter flocks.

Sources and image credit

  1. Meissner, W., Betleja, J., Kiljan, G., & Włodarczyk, R. (2026). Environmental versus endogenous drivers of body condition in the Mute Swan (Cygnus olor): a comparison of coastal and inland wintering populations. Journal of Ornithology.Primary open-access research article published and released as the version of record on 18 September 2026. Full text checked. The study analysed 853 Mute Swans captured in four Polish wintering regions. Inland birds had higher scaled mass index than coastal birds despite harsher winters and regular supplemental feeding at all sites; adults were consistently in better condition than juveniles, and body condition declined through winter.

Image: Pair of mute swans (Cygnus olor) on the Vistula River in Tyniec, Kraków, Poland, photographed 28 February 2023. Photo: Jakub Hałun. Source: Wikimedia Commons. CC BY-SA 4.0. Representative winter image from Poland; not identified study birds. · CC BY-SA 4.0 · Source

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