It’s that time of year when there’s less incentive to be enjoying the great outdoors. Temperatures are dropping, days are drawing in and many of our summer visitors have headed for warmer and drier climes. However, there’s one natural winter spectacle that never fails to draw an audience – starling murmurations. Thousands, or hundreds of thousands, of birds flying in one synchronous, swirling cloud, creating fluid shapes against the darkening late afternoon light before pouring out of the sky into their chosen roost.

The ‘how’ and ‘why’ of starling murmurations are age-old questions – how do so many birds manage to coordinate their movement without crashing into one another and why do they do it? The answer to the first question appears to be down to quick reactions, high levels of self-awareness and careful attention to what six or seven of their immediate neighbours are doing. Of course, I’ve just grossly paraphrased a whole industry of scientific research that not only draws on behavioural ecology but also involves physics and maths, neither of which were my strongest subjects at school. As such, I’m happy to take the final conclusions of ‘how’ at face value, and will leave it to you to take your own foray into the scientific literature if you want to become more learned on the subject!

The ‘why’ of starling murmurations (and indeed of flocking, herding or shoaling behaviour of many different species) has various explanations and one common to all of these examples of aggregative behaviour is that of predator avoidance. There’s safety in numbers - not only are the odds of being scoffed much reduced if you’re just one tasty morsel amongst thousands of other tasty morsels, but it’s a lot harder for a predator to select which tasty morsel to grab when faced with a moving mass of them. It’s also easier to spot a potential predator if there are many pairs of eyes looking out for them. However, a fuller explanation is more nuanced and benefits from a delve in to W.D. Hamilton’s Selfish Herd Theory, first published in 1971. Hamilton proposed that it’s not just a case of being one of many, but also of using the rest of the crowd to distance yourself from the predator and ensure that there are others between you and them. In many cases, being in the middle of the aggregation is the best place to be – being on the edge puts you at greater risk of being predated; in migrating herds, your best bet is to be at the front, not trailing along at the rear. In the case of starlings, researchers have shown that at roost sites with a low perceived predation risk, flocks tended to be smaller or even had singleton birds whereas higher predation risk was associated with bigger and more compact flocks. The benefits of being in the middle of a flock rather than on the edge are self-evident when there’s a hungry sparrowhawk scouting the skies, but even being on the edge is still better than being out on your own.

That provides an explanation for some of the ‘why’ but surely, if it’s all about avoiding being eaten, you’d be better off heading straight into the roost and getting a prime spot in the middle rather than spending time showing off your aerial acrobatics to an audience that is intent on grabbing you and eating you? In fact, studies suggest the opposite, with birds being likely to spend longer in their aerial display when predators are about, and then all dropping into their chosen roost location in a single synchronised movement. It seems to be all about sticking together, attracting as many birds as possible to join in, and delaying the descent into roost for as long as possible, all reducing the chances of any one individual being picked out.

The other theory that has been proposed for the advantages of communal roosting is that the roost offers warmth - the more bodies there are in the roost the warmer it is, hence time spent in the pre-roost display serves as a means of advertising the site to other potential roost mates as a great place to spend a cold night. However, little evidence has been found to support that theory for starling roosts, with much stronger evidence to support the reduced risk of predation being the main driver for the behaviour. There are species that do clearly benefit from huddling together and a quick browse of the literature threw up immediate examples of sociable weavers, acorn woodpeckers and, here in the UK, long-tailed tits, all of which have documented studies demonstrating higher ambient temperature within communal roost sites than outside. Of course, there is the old question of cause and effect (or chicken and egg, if we want to maintain the bird theme). All roosting individuals will seek out places that offer some degree of shelter from the elements and so these roosts will inherently offer more clement conditions than locations deemed unsuitable for roosting. For starlings, they’re quite adaptable and will happily roost in a forest plantation, reedbed, or in the understructure of Aberystwyth pier!

However, being one of the gang is not all to do with sharing a sheltered spot or avoiding becoming someone’s dinner; it can also help out with your own foraging efforts. The Information Centre Hypothesis was developed in 1973 by Ward & Zahavi, who theorised that individuals within a roost may learn from their roost mates where the most productive foraging areas are. Such a strategy has been demonstrated in certain species, particularly those that rely on ephemeral food sources such as carcasses, with ravens being a good example. For a full insight into the complexities of their foraging behaviour and how it is influenced by where and with whom they are foraging, Bernd Heinrich’s ‘Ravens in Winter’ provides an enthralling read that combines mystery, engaging science writing and an insight into just what levels of personal discomfort an ecologist with an enquiring mind is prepared to go!

The role that information sharing may play in the communal roosting behaviour of some other species is less compelling. However, variables such as time of coming to roost (those that have fed well during the day may be able to return to roost sooner than those who are struggling to achieve their daily energy needs); body condition (individuals that are well-padded are obviously feasting well); or direction and speed of flight from the roost in a morning (why loiter when you know where there’s lots of grub to be had?) may all act as cues as to which birds to follow if you want to find rich pickings.

In trying to understand the mechanisms of and reasons for starling murmurations, it’s clear that there’s a lot more to it than meets the eye, with the behaviour driven by a number of factors not all of which may yet be fully understood. Nevertheless, I’m content to just enjoy them for what they are – fantastic, awe-inspiring natural spectacles that provide a reason to venture out on a cold winter’s afternoon. We’re lucky to have the opportunity on our doorstep, with wonderful footage often captured of thousands of birds displaying at our Low Barns Reserve. Hopefully, they’ll be back again this year so there’ll be no excuse not to get out and experience the spectacle for yourself!