New Study Reveals Surprising Flexibility in Black Swan Partnerships

By Vincent Knowles & Raoul Mulder, The University of Melbourne

Published in Emu – Austral Ornithology, 2025

Swans are renowned for forming long-lasting partnerships, characterised by continuous close association. However, new research from the University of Melbourne challenges this assumption, revealing unexpected flexibility in how Black Swan pairs associate within and between seasons.

Using cutting-edge GPS biologging technology, researchers tracked both members of 10 Black Swan pairs at Albert Park Lake, Melbourne, over an entire year. Each bird wore a lightweight, solar-powered collar that recorded its position multiple times per day, generating nearly 6,000 proximity estimates per pair. This dataset allowed the team to examine in great detail how closely paired individuals stayed together, used space, and moved across the landscape.

A pair of Black Swans (Cygnus atratus) wearing biologger collars. Photo by Raoul Mulder.

The study revealed two strikingly different partnership types. Some pairs, dubbed “continuously coupled”, remained within 200 metres of each other year-round. Others, labelled “seasonally uncoupled”, separated for weeks or even months at a time, with partners sometimes ranging up to 37 km apart before eventually reuniting. Such variation has not been documented before in swans or geese, and raises new questions about how monogamous bonds are maintained.

Illustration of movements of one ‘seasonally uncoupled’ pair. The male (V02) left the study site shortly after the breeding season and remained 10-35km away for three months before reuniting with his mate (T02). .

Importantly, closeness between partners was not a reliable predictor of reproductive success. While continuously coupled pairs were slightly more likely to attempt breeding, seasonally uncoupled pairs also nested successfully. Likewise, the length of time a pair had been together did not consistently predict whether they would breed or remain together. This suggests that factors beyond simple proximity (such as individual experience, age, or resource availability) play key roles in breeding outcomes.

The biologging data also highlighted sex differences in space use. Females consistently maintained larger home ranges than males, though both sexes expanded their ranges around the breeding season. Despite their close associations, male and female home ranges overlapped by only about half on average, suggesting some degree of spatial independence even within partnerships.

The findings challenge the traditional view of Black Swan partnerships as uniformly close-knit. Instead, pair cohesion appears far more flexible, with some partners separating seasonally without undermining the long-term bond. This flexibility may represent an adaptive response to changing ecological conditions in urban wetlands, or individual differences in strategy.

While the study focused on a single urban population, it demonstrates the potential of modern tracking technology to shed new light on the dynamics of monogamous partnerships in swans. Further work across different habitats will be needed to see whether this surprising variation is widespread.

Citation:

Knowles, V. G. G., & Mulder, R. A. (2025). Year-long location tracking reveals temporal and spatial variation in cohesion, space use and movements of paired Black Swans. Emu – Austral Ornithology, 125(2), 160–172. https://doi.org/10.1080/01584197.2025.2508371