Ensuring the continued availability of the crops eaten by swans is critical to the long-term persistence of their populations. The amount and quality of the crops available to the swans can be highly variable in both space and time, however, due to factors such as farming practices, the availability of subsidies, market forces, and weather conditions. The conservation and management of farmland-dependent herbivores such as swans would therefore benefit from predictions about how species will respond to changes in their environment, including the loss of key crops or increased competition from other swans.

In a recently-published study (Wood et al. 2021), we investigated how potential future changes in food availability and competition would affect the three species of swan that feed on agricultural land in eastern England during winter. In our study area, the food resources available to swans include sugar beet Beta vulgaris and potatoes Solanum tuberosum, along with some maize Zea mays, which are harvested during autumn and early winter, after which the unharvested remains are eaten by swans. Many of these fields are subsequently re-sown with wheat Triticum aestivum or oilseed rape Brassica napus, which are also eaten by swans.

To predict the sensitivity of swans to future changes in food availability and competition, we developed an individual-based model (IBM). IBMs are simulation models used to make predictions about the behaviour, movement and state of individual animals within a population, based on our understanding of how individuals make decisions about where and when to forage. Once we had built our IBM, we tested its predictions against observed field data from our study system to ensure that our model accurately reflected the real world.

We used our IBM to predict how potential future changes in food availability and competition would affect four properties of our study system: (i) the proportion of the current swan population that could be supported, (ii) the proportion of swans that successfully departed on migration at the end of winter, (iii) swan daily foraging effort, and (iv) late winter crop biomasses.

Our IBM predicted that swans could compensate for the effects of greater competition and reduced food availability by increasing the proportion of time spent foraging. Even in scenarios of very high competition (i.e. up to 12 times the current numbers of swans) and reduced food availability (i.e. total loss of post-harvest remains of potatoes, sugar beet, and maize), individual swans could compensate by increasing the proportion of the day that they spent foraging. Current daily foraging effort is relatively low when compared with other sites that these species feed in. However, our IBM also predicted that a consequence of increased foraging effort by the swans would be additional grazing damage to agricultural crops, which could exacerbate conflicts with farmers.

Our model predictions of swans’ low sensitivity to increased competition and reduced food availability suggest that the recent c.40% decline in Bewick’s Swan numbers was unlikely to be linked to changes in arable food resources or competition with other swans on the winter grounds. Our findings do not, however, preclude the possibility of impacts of competition and food shortages in other parts of the species’ range, such as stopover sites and breeding areas, which should be studied in future research.