31. Small Bird Population Changes
HQP Crick

An index of population size for Wrens on farmland and woodland in the UK shown in relation to mean December-February temperatures in Central England

Specific Indicator
Since 1962, the British Trust for Ornithology has run a Common Birds Census of about 60 species, counted systematically during the breeding season in 200-300 plots around the UK. Small birds, like the wren, need to feed continuously during the winter and their numbers are know to be adversely affect by severe winter weather. Thus, the indicator chosen is the population change in numbers of Wrens (Troglodytes troglodytes) assessed on both farmland and woodland measured over the UK.
[Source: Census Unit, British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU.]

Relevance
Bird populations are an important part of the UK=s biodiversity which the UK Government seeks to maintain or enhance as part of the Biodiversity Convention and European Wild Birds Directive. Birds are often sensitive bio-indicators of many types of environmental change and pollution. They tend to occur near or at the top of food-chains and by monitoring populations of birds it is possible to monitor the functioning of the ecosystems upon which those birds depend.
Sensitivity to climatic and other factors

Resident populations of birds may suffer high mortality in severe winters to the extent that breeding populations are reduced. Small-bodied birds are particularly prone to prolonged spells of cold, wet, snowy or frosty weather because of their inability to obtain enough food during short winter daylight to maintain their body temperatures.

The Wren is one of the UK=s smallest birds (weighing c. 8 g) and occurs throughout the UK in a wide variety of habitats. It is known to be affected strongly by snow cover and cold winter temperatures with breeding populations severely reduced after particularly cold winters. Numbers can recover quickly, however, because under most conditions Wrens can produce two broods per year of around 5-8 young, each of which survive well if winters are mild. Changes in population size between years are strongly related to mean temperatures in December, January and February.

Change over time
Wren populations fluctuate widely between years, mainly due to severe falls after cold winters, followed by relatively rapid population recovery. Populations tend to fall when the average temperature from December to February falls below the 3EC mark. There was a particularly big fall after the severe1962/63 winter. Other big population declines have occurred after the winters of 1978/79, 1981/82, 1985/86, 1990/91 and 1995/96.

The population failed to recover in 1997 after the drop that followed the 1995/96 winter. This was unlikely to be due to the winter weather in 1996/97, which was mild enough to have allowed population recovery, but was probably due to the unusually cold wet spring of 1996 which caused poor nesting success and poor survival of young after leaving the nest. The poor nesting season of 1996 impacted severely on a large number of species of breeding birds, but such events are relatively rare and do not interfere with the generally over-riding effect of winter weather on Wren populations.

Populations can increase, but they are eventually limited by food and other resources. Thus, Wren populations will not increase indefinitely in series of mild winters: they will tend to stabilise at a relatively high level. Indeed, there is some evidence for this now, because relatively high populations have been maintained during the 1990s.