The reeds jostle in the breeze, their dry stalks pale against the darker alders that fringe these lagoons. Stripped of the vibrant greens of summer the reedbeds appear lifeless. This, however, is a deception and instead they shelter many thousands of overwintering insects and a few still active flies, beetles and spiders. A scolding wren, one of several dozen heard here this morning, underlines the invertebrate life that remains available to insectivorous birds at this otherwise difficult time of the year.
While some of these wrens are local birds that have bred on the site, others have arrived from elsewhere. Being small, wrens lose body heat quickly and are unable to build up stores of fat to see them through the challenging winter months. This prompts them to move to those sites and habitats, such a reedbeds, where invertebrate food remains available throughout the winter.
So important is this access to food that the wrens establish winter territories, smaller than those used in the breeding season but still defended against other individuals. Many wrens will remain on these territories through into March when, with the approach of the breeding season, they will return to their former haunts. Holding a small territory at this time of the year secures access to both feeding and roosting opportunities, the wren becoming increasingly familiar with the resources available within its small patch of East Anglian reedbed. Competition for territories can be fierce and research has demonstrated that birds which fail to secure a winter territory tend to suffer increased levels of overwinter mortality.
Despite maintaining a winter territory, individual wrens may come together in late afternoon to share a roosting site. Communal roosting enables these small birds to share their warmth and reduce the amount of body heat lost during the long winter nights. Several dozen wrens may cram themselves into a nest box or other cavity, and I wonder to what extent the wrens wintering around these pools make use of the nest boxes we have erected here over the years. The mild start to the winter will certainly have benefitted the wrens but if it turns significantly colder then even here they are likely to struggle.