Many birds tend to flock in winter, either in single- or multi species groups, because they are making use of a food source that is unpredictable and concentrated at scattered localities and therefore undefendable as individuals. In summer, when food is plentiful everywhere, birds split up and invariably become territorial. Finding food is also possibly another major reason for moving in flocks. In this way, the collective instincts and senses of the group can be used, with inexperienced birds then able to derive benefit and advantage from those that have more experience in finding food.

  Information on food concentrations can be obtained at communal roosts. This is one of the ways that seabirds find food in the open ocean, resulting in flocks descending in one place. Gannets, gulls and cormorants can be seen following shoals in this way close to the shore.

  Winged termite (alates) irruptions during the spring and summer also result in temporary flocks of many species (including eagles), concentrating so that they take advantage of this rich but temporary food source. Most birds will probably flock for the principal reason of safety – having strength in numbers, more eyes and ears for danger and an intimidating effect on enemies. This enables them to effectively mob enemies and to create the dilution effect (the more individuals, the lower the individual risk) and the confusion effect (too many for predators to choose from allows them all to get away). Migrating birds, especially, tend to fly in flocks and this probably has the advantage of offering guidance to inexperienced migrants in the flock as well as giving reassurance to all present when crossing over or stopping in unfamiliar territory.

  Many water birds, including ducks, geese, and grebes, will undergo a seasonal simultaneous moult of their flight feathers, confining them to the water for a few weeks where they concentrate in numbers for safety. Large birds can often be seen flying in flocks that are turning in circles. This is a behaviour called soaring and is utilised by a number of species for energy-efficient flight. They are using rising currents of warm air called thermals to get to a height in the quickest time so that they can start searching for scattered food supplies. Some species will form small flocks because they are what are referred to as co-operative breeders. These small flocks – usually not numbering more than about 10 individuals – are family units with a single breeding pair (the parents) and previous offspring. By consolidating their efforts at territorial defence, they are able to secure enough resources to ensure the survival of the entire group.

Why don’t they fly into each other in flocks?

  Flocks of larger species usually only number a few individuals flying at a leisurely pace on a direct and fairly constant flight path, and they are consequently not in serious danger of flying into one another. Smaller bird species tend to flock in much bigger numbers with erratic flight paths, and the attacks of predators, such as falcons, make collision likely. In autumn to early winter, when grasses are receding, red billed queleas take this to extremes with dense flocks numbering millions, resembling swarms of locusts.

  Birds have excellent vision and can detect even the most subtle changes in space and movement. Each bird, whether at rest or in flight, has an exact personal space or ‘zone’ around its body in all directions – called the ‘individual distance’. They don’t like the space being encroached upon. Smaller birds are at higher risk to predation than larger birds and their eyes are placed on the side of the head so they can see all around themselves for danger. When an individual within the flock moves towards and encroaches on the personal space of the bird next to it, the neighbour immediately (within milliseconds) detects the closure of space and moves in the opposite direction to open the gap again. In so doing, it then encroaches on another bird which reacts in the same manner, and this goes through the entire flock in a chain reaction that from the outside makes it look like the flock is moving in waves. Because individual birds towards the edge of the flock are more at risk to the influence of predators and wind, they are the ones that are prone to erratic movements and ultimately, therefore, determine the direction of movement of the entire flock.

Excerpts from ‘Beat about the Bush – Mammals and Birds’ by Trevor Carnaby, Published by Jacana Media, Second Edition reprint 2018.