Two-barred Crossbills

The final weeks of July saw an influx of crossbills arrive in the county, with small parties of birds much in evidence here in the Brecks and also noted elsewhere across Norfolk and Suffolk. That these were newly arrived birds rather than local breeders was apparent from the upturn in the number of individuals seen and heard. It had been a quiet year for the species locally and crossbills had been only an occasional sighting on my local forest patch during the first half of the year.

Arrivals of this kind tend to occur in late summer and reflect a good breeding season to the north (into Norway, Sweden, Finland and Russia) followed by a poor cone crop, which forces the birds to move further afield in search of food. The recent arrivals may well prove to be the early part of a larger influx over the coming weeks, and it is likely that some of these birds will remain to breed next year, topping up the population breeding within Thetford and other blocks of conifer forest.

Perhaps the most exciting aspect of the arrival has been the presence of another species of crossbill within these small parties of birds. The two-barred crossbill is a superficially similar bird but has two broad white wing bars, white edges to the tertial feathers and a somewhat different call. Even so, it requires patience to work through a party of feeding crossbills with a telescope and to pick out this the rarer of the two species. There is the added complication that some common crossbills show similar barring on their wings, although this is less extensive and seen in only one in a thousand individuals.

Fortunately, I managed to catch up with one of the juvenile two-barred crossbills that have been present within the common crossbill flocks feeding in the larches at Lynford Arboretum. A late afternoon stakeout delivered the goods, so to speak, and I was treated to a wonderful 20 minutes of the bird feeding alongside some common crossbills, even managing to secure a ‘record’ shot with a camera pushed up against the scope. It was amazing to think that this bird had been born earlier this year somewhere in the boreal forests of northern Scandinavia.

A special treat

The other weekend I was treated to a very special sight. I’d been lucky enough to go out into the forest with a couple of nest recorders, specially trained and licensed individuals who monitor bird nests for the British Trust for Ornithology (BTO). Nest recording is an extremely important component of the BTO’s work, providing information on those changes in breeding success that might result in population decline or flag up the worrying effects of human activities. It was nest recording that highlighted the impact of organochlorine pesticides on the breeding success of birds of prey like Sparrowhawk and Peregrine.

As well as helping to locate nesting birds, I was taken to see two rather special nests, both of species whose nests I had never seen before. The two species were Siskin and Crossbill, both of which are very early nesters and their breeding attempts were now well advanced. There seem to be very good numbers of both species in the forest this year. Crossbill numbers in particular tend to rise and fall, with increased numbers breeding here in years when we have had a good influx of birds from Scandinavia during the previous winter. We were certainly treated to excellent views of both species as they fed, perched and called in the firs.

Although Crossbills are fairly ‘tame’ when nest building and appear to disregard the observer, they are rather furtive when they have eggs or young in the nest. Visits to the nest to feed are rather infrequent and the birds are very careful in their approach. The nest itself is also rather hard to spot, placed on a branch high in a conifer. We had to strain our necks to spot where the nest was located, the slight bulge of material silhouetted above the branch on which it was placed. The same was true for the Siskin nest which, though placed lower down in this instance, was rather more compact. We managed to get a good view of the Siskin nest from slightly below the horizontal, standing well back but using our telescope to good effect.

Conditions must be difficult for these birds, nesting so early in the season, and even though the day of our visit was a warm and sunny one, there were still the remnants of the gusty winds that had been present over previous days. One of the Crossbill nests that we went to check on had been blown down by the wind, a small amount of debris left in the tree, with nest and eggshell remains scattered across the forest floor. There was a good chance that the birds from this failed attempt would have another go so all was not lost for them.

Some chirpy birds

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.