The Comparative Population Ecology of Mallard and Tufted Duck.
Nest predation, duckling mortality and overwinter loss were shown to be density dependent in a mallard population in Kent and were therefore capable of some compensation. Predator control was shown to increase nest success. Duckling mortality was identified as the key factor influencing total annual fluctuations in mortality. In chapter 3 first egg date was shown to be earlier and season length longer in years with warm springs in mallard. In tufted duck first egg date and date of the last clutch were dependent on mean April temperature. In chapter 4 I showed that nest success was higher in tufted duck than in mallard because they nest later in the season, taking advantage of tall vegetation. Island nests were more successful than mainland nests. In chapter 5 I discussed factors influencing the survival of ducklings; early hatched ducklings of both species survived better than those hatching later because of food supplies. Mortality was higher under poor weather conditions and feeding activity of mallard ducklings was suppressed in poor weather. In chapter 6 brood range size was shown to be larger than that of incubating females during feeding bouts and brood mortality increased as their range size increased. Post-breeding female ranges are also discussed. Chapter 7 investigates density dependent overwinter loss, which in mallard, occurred in 80 per cent, and in tufted duck 66 per cent, of 35 sites analysed. Competition for nest sites appears to exist in mallard such that nests are spaced in relation to the available cover to reduce the affects of density-dependent nest predation. In both species overwinter loss has a stabilising influence on the size of the breeding population - tufted duck populations have been increasing in southern England because of the weaker levels of overwinter density dependence than exist in mallard populations. In chapter 8 a mallard population model is constructed which shows that, because of overwinter compensation, changes in nest success or duckling survival do not influence greatly the size of the breeding population but yields can be increased. Density dependent overwinter loss can compensate for shooting mortality. The concept of maximum sustainable yield is discussed.