At least one small group of Crossbills has been active around Croxton Heath over recent days, their chirpy calls expressed like a group of overexcited children on their first independent outing. I seem to catch up with them most mornings and they provide a new backdrop to my early morning walks. Their loud, metallic-sounding calls are a welcome addition to a soundscape that is much diminished with the approach of winter (the wistful Robin and ever present Wren my usual companions).
The presence of these robust little birds owes everything to the dark plantations of pine and, in particular, spruce, upon whose seeds the birds depend. The changing fortunes of the Crossbill within Norfolk owe much to the establishment and subsequent maturing of these conifers, an increase in feeding opportunities and nesting habitat. The primary source of these birds, however, is the great tract of boreal forest that stretches across Scandinavia and east towards the Pacific. This great forest supports the Crossbill populations from which our far smaller population is derived and, in many years, supplemented.
Crossbill populations are so dependent upon the conifer seeds that they will quickly vacate large areas within which seed supplies have been exhausted. This results in large-scale irruptive movements, the birds tending to move in early summer once they have finished breeding. Breeding itself normally takes place incredibly early in the year, during late winter to be precise, and there are even old Norfolk records of incubating females seen with fresh snowfall around them. Such early breeding means that some pairs will be egg-laying in the latter half of December. The irruptive nature of Scandinavia populations brings new arrivals into Norfolk in numbers that can vary dramatically from one year to the next. These birds can turn up in unusual situations – we once caught one in a reedbed – but ultimately they supplement our key breeding populations in Thetford Forest, in the woods around Sandringham and along the Holt to Cromer ridge. Breeding was first documented within Breckland in 1815, when a nest was taken at Livermere, and is certainly considered an annual occurrence now. As well as winter breeding records, there are occasional records of birds breeding in late summer, some of which may include youngsters breeding in the same year in which they were born.
The future of our breeding Crossbills is dependent upon a number of different factors, including the form and age of our conifer plantations. Many of those within Breckland are currently at an ideal age but these will soon be felled and this will diminish the resources available to these delightful little birds. If we want to keep them then we need a good mix of trees of differing age.