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Male Mute Swans Were Larger Than Females in Every Linear Measurement

Mute swan swimming on blue water
Research · Anatomy & morphology

Measurements of wintering mute swans (Cygnus olor) from four regions of Poland found that males were larger than females across every linear body measurement examined. The study, published online on 9 January 2026, also found differences related to age and geographic region.

The researchers measured 938 ringed swans from four Polish wintering regions, comparing total head length, forearm length, wing and primary-feather length, and the combined length of the tarsus and toe. Sexual size differences were strongest in tarsus-plus-toe length.

Leg growth differed between males and females

Tarsus-plus-toe length increased with age only in males. The authors suggest that this may reflect differences in body mass and parental roles. Male mute swans are heavier and take a prominent role in territorial defence, and larger feet may help with rapid take-off, movement on water and the characteristic foot-clapping display used during territorial encounters.

The fourth primary feather increased in length with age in both sexes, while forearm length increased with age only in females. Total head length did not change with age in either sex, which the authors link to the importance of the head and bill reaching functional size early for feeding.

Some measurements also varied geographically

The swans came from four wintering regions in Poland. Birds from the two northern coastal areas, the Gulf of Gdańsk and Świnoujście, had greater total head lengths than birds from Central Poland and Upper Silesia across age and sex classes. Other measurements showed less consistent geographic variation.

The study also tested how repeatable the different measurements were. Total head length, forearm length and tarsus-plus-toe length were the most repeatable, while wing length and fourth-primary length varied more between repeated measurements. The authors recommend the three more repeatable measurements for future morphological monitoring of waterbirds.

Body measurements are widely used to study sex, age, condition and geographic variation in birds. The results show that mute swan morphology varies not only between males and females but also with age and location. The authors therefore emphasise that population-specific variation should be considered when body measurements are used to classify sex or compare populations.

Sources and image credit

  1. Size variation in the Mute Swan Cygnus olorOpen-access peer-reviewed primary study of sex-, age- and geographic variation in linear body measurements of wintering mute swans from four regions of Poland; published online 9 January 2026; CC BY 4.0.
  2. Size variation in the Mute SwanSpecialist-group summary highlighting the 2026 study.

Image: Bengt Nyman / Wikimedia Commons — mute swan photographed in Sweden, 1 May 2017; representative species image, not a bird from the Polish measurement study. · CC BY 2.0 · Source

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