Recently I was in a local woodland when high-pitched bird calls nearby caught my attention, they were the trilling contact-calls of Long-tailed Tits and I hastened over to where the calls were coming from as I knew what they signified. As I approached I raised my gaze into the canopy of birch, oak and pine and looked for movement; I first saw one Long-tailed Tit, then two, then four, then six, then ten and then too many to count. It was a large feeding flock; the branches and twigs above my head were alive with tiny spoon-shaped bodies as the tits passed by in their constant search for food.
As the wave of Long-tails passed by I noticed the small, familiar forms of cerulean Blue Tits hunting for prey alongside them, they only called occasionally, spending most of their time performing acrobatics from the end of thin twigs. Then through the branches my eyes caught the movement of a very small but short-tailed bird quite high up, silently pecking in bark crevices and fluttering among the pine needles. Through my binoculars I could see its black and white humbug-patterned head – making it a Coal Tit; as the flock moved through the trees around me I spotted a couple more individuals of this pretty little species busily probing the dead leaves still hanging from the oaks.
The Long-tailed Tits had mostly passed beyond me by now and only a few Blue and Coal tits remained in the branches – yet it was now that I noticed yet another species bringing up the rear of the flock. Far smaller than even the Coal Tits; with a very short tail and tiny buzzing wings these were Goldcrests, Britain’s smallest bird, it would have been very difficult to spot these attractive little fairies if they hadn’t been uttering very high-pitched twitterings as they foraged in the wake of their larger fellows. Within a few minutes the whole mixed flock had passed out of sight and hearing; disappearing far into the wood in their continuous search for food, I was left standing in an empty wood, which now felt very quiet and desolate after the noise, movement and excitement which had come as the birds moved overhead.
But why do these small birds mix together in spread-out flocks at this time of year? Why are they not separated into their respective species? In fact the reasons for these ever-moving flocks during the winter are many, the most obvious being the whole reason they are in flocks in the first place – safety in numbers. A single little passerine moving through a woodland canopy or hedgerow, almost all of its attention on searching for food, is an easy and vulnerable target for any predator. Birds form flocks throughout the year but in winter when the trees are bare (reducing hiding places) and the cold makes predators hungry it is even more important to crowd together to reduce the risk of getting eaten and increase the chances of a predator being spotted.
But why are different species flocking together? It would seem that some species are far more observant, or more cautious, than others – Long-tailed Tits for example are always looking about them and readily utter alarm calls at any hint of a predator. Other species take advantage of this by travelling with them, enabling them to focus more on eating than on looking around for raptors, more eyes also means an ambush is far less likely. Long-tailed tits also call constantly to one another so that the rest of their group knows where they are without having to look, other species travelling with them use these calls to stay with the main flock without having to call themselves – which might risk them being spotted by a sparrowhawk.
These traits of Long-tailed tits makes them ‘nucleus’ species, they will attract other small birds to their family flocks that want a safe group to feed with; some of these birds only travel with them as long as they are in their territory, then leave if they move away – others that don’t keep winter territories may stay with them throughout the day. In very cold weather or larger woodlands these mixed flocks may become quite large with a lot of attendant species – Treecreepers, Nuthatches, Goldcrests, Coal tits, Great tits, Blue tits, Lesser-spotted woodpeckers, Marsh tits etc.
What connects these species is that they are all insectivores; consuming spiders, earwigs, woodlice, flies, mites, moths and any other mini-beast hiding in the trees – which brings me to another reason they flock together in winter. The theory is that as this large group of birds moves through the canopy poking their bills everywhere, they are likely to disturb any invertebrates hiding in the bark or moss, if one bird misses one then another is likely to spot it – therefore group feeding could increase a birds chances of finding prey.
This mixed-species small bird feeding flock behaviour is not confined solely to British woods and hedges, it is a phenomenon observed in almost any forest habitat that has small insectivorous birds living in it. In tropical rain-forests it is obviously not even a seasonal occurrence, flocks of exotic forest warblers are a regular and year-round spectacle that liven up the Amazon as much as tit-flocks liven up an English copse.