Autumn-sown crops, an overwinter food resource, facilitate recent mute swan population increase

The British mute swan population has increased dramatically since the 1980s. Using variation in population change across regions in England and Wales, we asked what factors could have led to this increase. Recent autumn-sown cropland expansion can explain part of the story.

In a changing world where many species are in dwindling numbers, many conservation interventions target specific species in hopes to facilitate their population recovery. But often identifying the factor that pushed the species into its decline is very hard. One way of approaching this task is to look at population increases, to understand what factors are closely associated with the increase and examine if this knowledge can be transferred to help other species in decline.

Setting the scene…

British mute swan population have increased dramatically since the 1980s (Breeding Bird Survey). The low numbers prior to this are attributed to the high prevalence of lead fishing weights in the waterways deposited from coarse fishing, which were ingested by the mute swans when foraging. In the 1980s, there was considerable research linking the high mortality observed in mute swans to the use of lead angling weights, which resulted in the national ban (e.g. Birkhead & Perrins, 1985, Biological Conservation). The increase since the 1980s has been largely attributed to the success of this ban.

The increase in mute swan population number, the decrease in incidence of lead poisoning found in mute swans and the decrease in proportion of deaths attributable to lead poisoning over time does support this explanation (e.g. Sears & Hunt, 1991, Wildfowl). And there is clear evidence that the ingestion of lead from angler’s weights and ammunition kills birds and has a detrimental impact on local mute swan populations (e.g. Newth et al., 2013, Eur J Wildl Res).

However, there are signs that this might not be the whole story, as although mute swan blood lead levels dropped in the years following the ban, the decline of lead levels did not subsequently continue, despite the expected decrease in ingestion of legacy lead angling weights with time (Perrins et al., 2003, Avian Pathology). During this period, there were also other changes that may have contributed to the observed population change. Winters have become warmer, which may improve overwinter survival; autumn-sown crop area has increased, which may have improved overwinter survival and breeding success; and waterways have improved in water quality, which may benefit mute swans by allowing their natural food resources, submerged aquatic macrophytes vegetation, to grow better (e.g. Rowell & Spray, 2004, WWT/JNCC).   

With my supervisors, Jenny (UEA), Rhys and Debbie (Cambridge), we looked at the relationship between mute swan abundance change and the factors described above at a regional level, to ask which factors were likely to explain the observed change. 

How do the regional changes in these factors relate to the changes we see in the mute swan population? Is there a factor’s map that looks rather similar to the swan change map?

Results

Analysing the spatial variation shown in the data above, we found that increased autumn-sown crop area was strongly linked to mute swan abundance increase. This makes sense because autumn-sown crops are used as an overwinter food resource for the mute swan, and more food improves survival and allows females to reach peak breeding conditions earlier on in the breeding season. This allows earlier laying of eggs, larger clutch sizes and greater fledgling success, improving breeding success. 

What does this mean for conservation?

Removal of poisons from the environment is clearly important for the conservation of species as it allows increased survival, however, our findings suggest that other factors (i.e. autumn-sown cropland expansion) may be important to facilitating population recovery. Conservation interventions, to facilitate population recovery, therefore need to both remove poisons from the environment as well as focus on other factors, such as improving productivity.

Summary

Autumn-sown cropland expansion has helped the mute swan population recovery. However, this is only part of the story – there are some regions that have increased in mute swan abundance without the corresponding increase in autumn-sown cropland. Other factors are also likely to have contributed to the population recovery of the mute swan, but were not detected in our data. Future studies done at even smaller spatial scale, e.g. waterways, will likely reveal more factors.