Tufted Duck Aythya fuligula [Linnaeus, 1758]
Dam, Blagdon Lake © Nigel Milbourne, 2012
Tufted Ducks prefer lowland eutrophic waters of 3-5 metres in depth, so Blagdon provides an obvious attraction to them. However, an island or abundant marginal and emergent vegetation is required for breeding, so unless the water level stays at top level breeding is sporadic at the lake. The increase in the Northwest European population coincides with freshwater eutrophication and the spread of the introduced Zebra Mussel Dreissena polymorpha.
Tufted Ducks have been counted at the lake since 1938, which gives us an incredible data series to look at and analyse. There have been five counts in excess of 1000 individuals, four of which were in August, and all within the last 8 years. The most recent large count, in November 2013, was probably swollen by birds temporarily displaced from Chew Valley Lake by angling & / or sailing pressure.
I have plotted the last five years monthly counts to demonstrate that there is a significant increase in Tufted Duck numbers in early autumn, usually peaking in August. This is due to the moult migration of males which commences in June and peaks at the end of July, resulting in a large, mainly single-sex, gathering that comes to shed wing feathers in a safe environment, with plenty of available food. There is a smaller peak, about two months later, which is the result of females moult migration. This peak is not as pronounced as the earlier one because there are generally more males in the population as a whole, and female migration may differ to males in direction and length. The peak may also be obscured in some years by the movement of local birds seeking good winter feeding conditions.
Here is a chart of the regular winter WeBS counts made since 1960 with each winter presented as an average of the counts made from September to March inclusive, with the first winter period plotted against the x-axis viz. winter 1963-64 is shown against 1963 on the x-axis.
I have superimposed what I regard as moult counts, given by the average of counts made from August to October inclusive for the last 20 years, to show that these appear to be undergoing a significant increase in the last 10 years, or so. Tufted Ducks are omnivorous and dive to the bottom for stationary or slow-moving food items, so it has been suggested that the rise in numbers probably relates to an increase in weed growth at the lake, with a concomitant increase of invertebrate biomass e.g. freshwater snails Gastropoda. It is also likely that numbers are affected by a combination of two other factors, namely water depth and angling disturbance. In years when the water level drops sufficiently for the Top End to be unfishable by boat (east of a line drawn between Rainbow and Rugmoor Points) due to the combination of depth and weed, duck numbers appear to be higher during the moult period, as you can see by the peak in 2013 when the level dropped to approx 60%. If the water level continues to fall over subsequent weeks, the birds move out of Top End towards deeper water and utilise new areas such as Holt Bay, and Rugmoor to Ash Tree where there are more large weed beds. If the level carries on dropping, they will move on towards the dam, feeding off Green Lawn and Butcombe Bank. However, I will add the caveat that feeding areas are exploited subject to boat angling activity during the moult period. If boat anglers are fishing in the preferred feeding areas, the majority of ducks seek refuge at Top End, moving out to feed over the rest of the lake when the anglers return to the Lodge.
Bibliography (sources of information):
- British Trust for Ornithology (BTO) website
- Cramp S. & Simmons K.E.L. (eds.) 1978. Handbook of the Birds of Europe, the Middle East and North Africa. The Birds of the Western Palearctic. Volume I. Oxford.
- Kear, J. (ed). 2005. Bird Families of the World. Ducks, Geese and Swans. Vol. 2. Oxford University Press.