Analyses of two mute swan populations and the effects of clutch reduction: implications for population management

Published source details Watola G.V., Stone D.A., Smith G.C., Forrester G.J., Coleman A.E., Coleman J.T., Goulding M.J., Robinson K.A. & Milsom T.P. (2003) Analyses of two mute swan populations and the effects of clutch reduction: implications for population management. Journal of Applied Ecology, 40, 565-579.
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Summary

Populations of mute swans Cygnus olor in some areas of Western Europe and heavily graze aquatic vegetation, compete with other waterbirds and damaged crops. As a method of controlling numbers, oiling a proportion of eggs in each clutch to prevent hatching was trialed in Wiltshire and the West Midlands, England.

A licence was issued by the Ministry of Agriculture, Fisheries and Food to oil a number of mute swan eggs on the Rivers Wylye and Avon in Wiltshire, southwest England, in 1997. In each treated clutch, all but two eggs were oiled using paraffin. Two sections of river were chosen as controls (clutches untreated). Between May 1997 and September 2000 effects of the egg-oiling was monitored. This proved inconclusive due to various confounding factors. An alternative approach was therefore adopted.

A computer population model was developed to examine the effectiveness of clutch reduction on swan populations. Model parameters were derived mainly from a long-term study of individually ringed birds. Between 1974 and 2000, 117 juveniles, 254 first years, 219 non-breeding adults and 51 breeding adults were ringed in the Wylye, and 121 swans that had been ringed outside the Wylye were also recorded in the study area. In July 2000, 87% (n = 48) of breeding swans and 100% of non-breeders (n = 89) were colour-ringed. More than 500 swans had also been ringed and monitored on the nearby River Nadder and River Avon, and Christchurch Harbour since 1974 which provided further data to enhance the computer model.

The model focused on non-breeding mute swans, considered to have a negative impact on local fisheries due to overgrazing of aquatic macrophytes. The model was also parameterized for another swan population in the West Midlands characterized by rapid growth over a much larger area, compared with the smaller, comparatively stable population in the Wylye Valley.

Mute swan survival rates: Annual mute swan survival was high in both populations (Wylye Valley and West Midlands), between 68% and 73% for juveniles, first-years and non-breeding adults, and 72% to 90% for breeding adults. Immigration was an important factor in the dynamics of both populations.

Wylye Valley: The effects of different levels of clutch reduction on the Wylye mute swan population were simulated. Reducing clutches to 2 eggs/clutch lowered non-breeding numbers by 30% over a 10-year period. Total destruction of eggs in a clutch stabilized the non-breeding sub-population. The effects of clutch reduction were offset by high survival rates and immigration of swans.

West Midlands: The simulated restriction of clutches to two eggs stabilized the non-breeding subpopulation but did not affect breeding numbers. Total destruction of all eggs in each clutch markedly reduced the non-breeding sub-population. However, immigration was under-estimated in this model.

Conclusions: Clutch reduction is labour intensive and its effects may vary between swan populations depending on immigration rates. This study demonstrates that reduction of breeding output did not alleviate localized crop or habitat damage.

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