Bird Behaviour: Territory
If you are watching almost any aspect of a bird’s breeding biology and behaviour and, indeed, sometimes its winter behaviour, sooner or later, you need to understand the part that territory plays in its life. Although Aristotle made some reference to the territory of eagles and a number of other writers have since commented on the fact that many breeding males cannot tolerate the presence of another male close to them, it was a businessman and amateur ornithologist, Eliot Howard, who first developed the idea of territory in his book, Territory in Bird Life. Since then territory has attracted the attention of birdwatchers from amateurs to highly erudite scientists and yet, there are still points of interpretation on which there is no agreement. Indeed, there remain species, even common ones, whose territories have never been studied in detail. So there is room for birdwatchers, not only to learn for themselves about territory and all its ramifications, but perhaps to add something new to science.
No single definition of territory covers all its different manifestations. Probably the simplest and most generally accepted definition at present is, ‘any defended area’, but even that has some drawbacks as a definition, because a pair may occupy a territory and ‘enjoy’ its benefits but may never have to defend it, as they may not have any others of the same species close enough to defend it from.
Nowadays, there are generally considered to be six or seven types of territory: the first is a mating, nesting and feeding territory, which is the type of territory occupied by Robins, Wrens, Blackbirds and Wheatears. The second type is a mating and nesting territory, here, a typical bird is the Goldfinch which leaves its territory in order to find food. The third type of territory is the mating territory, and the leks of Ruffs and Blackcocks are good examples of this. The fourth type is a simple nesting territory and here we think of the seabird in their dense colonies. The fifth type is a feeding territory but there really is not much evidence about this; whether it is really a territory, in the strict sense, or a form of ‘individual distance’ is open to debate. ‘Individual distance’ is an area about one body length around a bird and which moves with the bird; although it has no visible boundaries it is defended. It would be interesting to know how closely related this type of territory is to the sixth type, the so-called winter territory, which several species hold during the winter months. In Britain a notable example of a species which holds this type of territory is the Robin. However, a number of migratory species, for example, Wheatears and thousands of Starlings roost in the centres of cities in Britain where the night temperature is often higher than in woods. Notice how regularly they space themselves; this is their ‘individual distance’.
Shrikes hold territories in their winter quarters, and also on migration. Some of the leaf warblers seem to have at least temporarily restricted territories in their winter quarters and, perhaps, whenever they rest on migration.
This type of territory could also be linked with the seventh type – the roosting territory. For instance, Starlings are known to return to the same perch to roost at night and to defend the spot. Treecreepers return to the same hollow in the soft bark of Redwood trees and attack other Tree-creepers which come close to it. While the first four types of territory seem to be fairly straightforward, I have some unease about the ordering and nomenclature of the last three groups. The Reverend E. A. Armstrong differentiated between territory used for reproductive purposes and those used for self-preservation. This seems to be a logical split between the first four types and the last three.
The function of territory varies enormously between species and in addition to the important function of restricting disturbance to a breeding pair, territory also spaces out birds and allows individuals to become attached for a while to a particular site.
The size of the territory varies, not only between species but also within species. The size of the breeding territory of Razorbills, or other seabirds may be as little as 0.3 square metres around the nest or egg; where the territory is a nesting and mating area it may be slightly larger. The Goldfinches, had territories which averaged 250 square metres when they were building the nest and courting vigorously. However, once they were engaged in feeding the young the size of the territory dwindled to between 9 and 12 square metres although as the nest was up a tree it would perhaps be more accurate to say about 20 cubic metres around the nest. The territories of Pheasants also decrease in size as the breeding season progresses. There may also be reasons for size differences of individual territories depending on availability of nest-sites and so on.
If a pair of birds has to find food, mate and nest in the same territory, as well as bringing up their young, the size is even greater. The territory of the Wheatear on Skokholm varied between 0.49 hectares and 3.28 hectares, averaging 1.54 hectares. Professor Hinde listed the average territory size for four species of tits. The 14 territories of Great Tits that he measured averaged 0.8 hectares; the three territories of Blue Tits covered 0.2, 0.4 and 0.8 hectares; the three territories of Marsh Tits covered 1.8, 1.5 and 2.2 hectares; and the territories of two pairs of Willow Tits both covered about 8 hectares. In Cambridge, the territory of male Wrens who were unrestrained by rivals in neighbouring territories patrolled over 2.4-2.8 hectares, whereas elsewhere Wren territory size could be as low as 0.4 hectares.
Not all birds of prey have territories and, for most species, ‘home range’ is a better term. Buzzards have territories which they defend but Kestrels do not. Golden Eagles may be dispersed 5 or 6 kilometres apart and the area which may be available for each pair to range over may be between 39 and 60 square kilometres.
Normally the males defend their territories – they sing and proclaim their presence. Generally they are also more brightly coloured than the females and these colours are often used in displays. The males attack both male and female intruders, particularly when the male is unmated. Once, however, a pair occupy a territory, a female will approach another intruding female and see her off. As the female does not usually have the bright colours of the male she does not normally use the same posturing, so their routine is a dominance display followed by a chase; if the intruding female does not retreat, then a fight ensues.
The type of habitat, population density and the individual aggressiveness of the owner (these last two factors may be linked), leads to variations in territory size. Indeed, in a way, individual aggressiveness is probably the important factor in the variation of territory size. It would seem that while the shortage of food within a habitat may occasionally be responsible for a low population, the chief way in which a habitat affects population is its ability to screen the view of one aggressive male from another. The territories of the Skokholm Wheatears were much smaller — about o-8 hectares in the rocky south-east part of the island near Spy Rock- than on the more open northern plains where they could cover as much as 3 hectares. The reason for this was, I believe, that the broken ground prevented neighbouring males from seeing each other. Foliage density is also important, because it reduces the chances of one owner seeing another. As I have mentioned in the section on **habitat, the distance the bird can see can affect its behaviour in a number of ways that would not appear to have been studied yet, both in relation to habitat selection and size of territory.
Variation of population density in waterbirds such as the Great Crested Grebe is a well known factor affecting territory size. The less successful pairs of some species in terrestrial, rather than aquatic habitats, will move to less suitable ground when the population density is high and there will be a little reduction in territory size. The territories of Robins and many other species are also compressible in response to population pressure and there can also be seasonal changes in area, too. There does not seem to be any direct evidence that territory limits the total breeding population in all habitats. However, where there is clearly a dense population of birds already, the chances that a latecomer will be able to carve a territory out for himself are fairly small.
Whilst keeping an area free of intruders is one important characteristic of territory, another is site attachment. A male and female will for part of the year become attached to, and isolated in, one particular part of a habitat which they will often defend. This is thought to have many advantages: it has been suggested that for some species territory is important in the formation and maintenance of the bond between the pair. Philip Brown and Gwen Davies, who studied the Reed Warbler, emphasize that the female wandered about rather widely when she first arrived and that, therefore, the male’s territory must be large enough to contain her. Ruffs and Blackcocks only use part of their territory or ‘lek’ for the display in which they indulge, and once the female has been fertilized and has laid her eggs it is abandoned. In a few species, territory may be important in ensuring a food supply for the young. Indeed, the territories of some hummingbirds are centred on a food supply, but it is clear that generally, this is of little importance.
An attachment to one site is also important in another sense: the owners gain familiarity – particularly if they return year after year – with places where food is available or where shelter can be found. This, for example, increases their ‘confidence’, to use a very anthropomorphic term; an older Wheatear which had bred successfully in one year was able, when it returned rather late to Skokholm, to dislodge a younger Wheatear, which had already established itself in the territory occupied by the older bird in the previous year.
Some Wheatears live until they are five years old and perhaps longer. In spring they usually return to the same territory and pair with the same mate if it has also returned. I came to the conclusion that familiarity with a previous year’s mate was also useful. However, although Wheatears tended to be faithful to a mate, the evidence really showed that they were more faithful to the territory. At the end of each breeding season they would part, and disperse independently from the nest area.
Dr David Lack, in his book Swifts in a Tower, tells how Swifts return to a previous nest site and if the previous year’s mate returns then the bond is reformed. Several species as different as Bearded Tits and ducks form pairs as many as six months before they actually nest. In spite of this betrothal period they split and go their own ways at the end of the nesting period. Jackdaws may be one of the few species where the evidence suggests that they mate for life.
Some of the more obvious manifestations of territory are song and the various display patterns which birds use to defend their territory. These manifestations enable us to determine where the boundaries of the territories are, how large they are and what uses pairs make of them. Most male birds advertise their presence by a song which identifies their species, their sex and their sexuality. Also, there is a tendency to attack individuals which intrude in the territory and, if they leave their own territory, a tendency to flee from other birds that attack them.
The song is usually loud and often delivered from a conspicuous perch or during a display flight, such as the ‘butterfly flight’ of the Greenfinch. Other examples are the song flight of the Skylark, circling over the fields and the tumbling flight of the Whitethroat.
The tendency to attack individuals which intrude is usually manifested by a variety of threat displays; actual fighting only occurs when the threats have failed. The most obvious examples are found in our own gardens: each territory owner seems to try to assert his own dominance over the area first by song and then by display. When an intruder comes into a territory the bird, such as a Robin or Blackbird, hops towards it in a somewhat upright position and at first, patrols alongside it. Often this approach is sufficient to make the intruder flee.
If, however, this display of dominance does not have the desired effect, the owner will start to adopt various extravagant postures: the Robin, for example, lifts its head and neck and displays its red breast, keeping it towards the intruder, whether the latter is perched above or below. The Blackbird adopts various aggressive postures in an upright position with body and neck feathers fluffed out so that the whole body appears larger, also, the tail is usually depressed and slightly fanned. The Wheatear has a somewhat similar upright position and, like the Robin, shows off its pale sandy buff throat and breast, as well as the black and white markings on the head, wings and tail.
Other birds in similar situations also make use of exaggerated postures and emphasize prominent patches and colour. Great Tits, for instance, have what is called the ‘head up posture’, which reveals more clearly the black throat and belly markings, the sudden revelation of which is often enough to make an attacker hold off in mid-air.
If these displays fail the owner may eventually fly at the intruder and a chase may ensue. Sometimes the intruder leaves the territory but then tries to come back by another route, in this case the chase is prolonged. Finally, if the owner is still determined to hold on to the territory and the intruder remains reluctant to leave a real fight can ensue, with the birds attempting to peck and fly at each other. Very occasionally, these battles result in the death of one or other of the combatants.
During late winter and early spring, many resident birds begin to show signs of establishing their territories and, for the curious ornithologist, there is a large field for observation and interpretation. Much has been written on the subject of territory and it has occupied the minds of amateur and professional ornithologists alike. But there is still much to be described and interpreted, even for common species.