Basin Notes - February 2008

Flocks of Birds

The Basin has been home to a few unusual species of bird recently. These vagrants are Bewick's swans, lesser yellowlegs and avocet. They are wandering about in the company of similar species of birds that are on the Basin in larger numbers (mute and whooper swans, redshank and curlew respectively). It always pays to examine large flocks of any bird as they may contain a similar looking bird that really belongs elsewhere.

The most visible flocks of birds on the Basin are the pink-footed geese when they are here in the largest numbers in October. Other winter migrants to the Basin are less noticeable but very characteristic - they are called knot. This little grey wader flies in a "smokey" cloud that swirls back and forth changing shape so rapidly and so smoothly that the watcher can only believe that it has been professionally choreographed.

The amazing fact is that, of course, it is not! There is no leader and no single bird gives directions. The fact that this flying ballet is natural and different for each species can be seen when the flock of knot has some dunlin mixed in with it. These, even smaller, grey waders find it hard to follow the gyrations of the main flock and slowly fall behind and can be seen as a less agile clump of birds below and behind the knot. They can still follow the general direction of the knot but are unable to match them in grace and manoeuvrability.

Anyone seeing such aerial artistry always asks "How do they do it without bumping into each other?". It seems impossible to be able to fly so fast and still avoid birds flying inches away from you and changing direction every few seconds. (Go to to see some video of this amazing feat.)

There have been a number of attempts to produce computer models of flocking behaviour in birds and fish to try to understand how they achieve such a feat. The most famous of these models is probably "Boids" by Craig Reynolds, which produces very convincing animations of flocking and has been used in films like Batman Returns and The Lion King. The model uses some simple rules to mimic flocks of birds and its main rule is that each bird follows the birds it can see a small distance from it.

Some recent research studying flocks of starlings (project "Starflag") has come up with the different theory that the birds monitor just six or seven birds around them and follow their lead in the jinking dance in the sky. This model differs from Boids because it ignores the distance to the nearest birds and concentrates on just the seven nearest birds. The researchers think this simple difference gives a much better animation of flocking behaviour. Whether this is what birds actually do is still open to debate.

What is more certain are the reasons for flying in closely packed flocks - survival. If a flock of knot is chased by a peregrine falcon then the flock provides a confusing mass of targets which reduces the chances of the predator isolating an individual bird and catching it. It also improves the odds of survival for each of the prey as they try to keep in the middle of the flock.

Another benefit accrues when roosting in a flock, as the birds in the centre of the roost are warmer than those on the outer edges. There is also a theory that when a roost breaks up in the morning to head out for the feeding grounds, the dominant and healthiest birds head for the best feeding areas so other birds follow them and benefit from a better feeding opportunity.

What many Montrosians may not realise is that from the Visitor Centre the local starling flock can be seen flying around the rooftops of the town at dusk ready to descend onto their night-time roost. While not numbered in the thousands seen at some very large roosts across the country, it is still a natural spectacle to be marvelled at.

And next time you see a flock of knot darting around low over the water, look carefully and you might see a peregrine chasing along behind them.