Bird Pair Formation and Courtship Displays
Song tells an unmated female, newly arrived from her winter quarters, of a territory-holding male which is ready for a mate. But when the newly-arrived female comes into a territory she is often attacked as if she is an intruding male, and if she is not ready to mate she may well move on. If, on the other hand, the female holds her ground and does not leave the territory or, as in some species, lowers her head, exposing the nape of her neck in a submissive posture, the male will eventually stop attacking her and accept her as a mate. The females of some species, for instance the Chaffinch and the American Song Sparrow, respond with a special call, when they are prepared to accept the attacking male as a mate. The pair then settle down and initially seek out the boundaries of their territory and discover what it contains in the way of food and shelter. Indeed, one gets the impression from watching newly paired Wheatears that they are learning not only about their territory but also about each other.
Once the birds are paired they will usually remain so until the end of the breeding season and then return to their wintering areas independently. Amongst some species which raise two broods, the female will occasionally take a new mate for the second, if the first male has started his moult and has deserted her. In the Wheatear this led to polygamy: the female which had been deserted formed a pair with a neighbouring male which had not deserted his territory or mate. Not all birds pair for the season; the male and female Ruff and Blackcock meet at the lek, copulate there and part, and the male takes no further interest in the female, eggs or young, so this must be one of the shortest pairings.
Birds which mate in the flock in winter follow a rather different pattern from those where migrant males set up their territory. Pairing up in a flock which may be moving from place to place is a bit more difficult to study and there are few details about this type of pair formation. Ian Newton, whose book entitled Finches which is already a classic study, says that the act of forming pairs is a less definite affair than in territorial species because the relationship between the two birds develops over several days or weeks and is not marked by any particular event. Cock Greenfinches become more aggressive towards other cocks in the flock and begin to relinquish their dominance over females. They court them by crouching in a special posture and reaching out and attempting to touch or nibble at their bills. Gradually, as time passes, the pairs are drawn closer together and the hen assumes dominance from the male, finally leading the cock in search of nest sites.
Whilst song is one way of manifesting sexual feelings, courtship displays are another. In some species they may reinforce the effect of song but in others they are the main method by which the male communicates to the female his need to mate with her.
During these courtship displays the males demonstrate patches of bright colours to their mates by twisting themselves into odd and sometimes fantastic positions. Some of this colour may often remain concealed until the bird takes up this posture. Other patches of colour are only revealed when the tips of the dull autumn feathers, which helped to camouflage the birds during the winter, wear off. Some plumages used in displays are grown every spring between January and April, like the plumes of the Great White and Little Egret, as well as the variously coloured necklet feathers of the Ruff. Not only feathers but other parts of the body are used in displays; certain areas of skin, such as the brilliant red-skinned throat pouches of the frigate birds of the tropical seas, are inflated during their display. Puffins grow their large and multi-coloured bills just before they return to their nesting slopes, and some of the herons, and in particular the Little Bittern, can change its bill colour quite quickly when the male encounters the female at the nest.
The courtship displays have a number of important functions. They stimulate sexual readiness, sometimes not only in the bird being courted but also in the bird doing the courting. Often seabirds nesting on cliff-face colonies will not have such a wide range of sexual displays as, for instance, smaller song birds and a number of social displays appear to compensate for this. These large social displays, such as the head-flicking or hiccuping of the Puffins, may also stimulate sexual readiness, not only in their own mates but in other members of the colony. At a seabird colony in the evening you may often see three-quarters of the Puffin colony on a clifftop head-flicking as though they all have hiccups.
Amongst the many displays are those in which the male brings material to the nest, usually with excitement and sometimes with some ceremony. Gulls, terns, gannets, herons and indeed many passerines all bring material and may display it with some excitement to the female. I have even seen cock Goldfinches and Wheatears, which do not normally build, occasionally bring material to their nests, but they did not always persevere with attempts to weave in the material; often these small offerings were dropped or carried away again.
Most of the true courtship displays, therefore, bring birds towards the fertilization of the female. Day length and temperature have already affected their sex organs and the pair have sought out their special area for breeding which will bring the maximum quantity of food for the young birds when they hatch. They have met and overcome their hostility to the close proximity of each other by the first of the sexual displays – the sexual chase – and their ensuing behaviour is gradually working towards the production of eggs and young. After the establishment of the pair they search for nest sites and then build the nest. At intervals they break off to relax, preen and court each other. This may include courtship feeding in which the male brings food and gives it to the female, which, in addition to stimulating and strengthening the pair bond, is thought by some authorities to be helpful to the female at a time when the eggs are forming within her body.
The male also becomes accustomed during this period to bringing food to the female so that when the eggs are laid he can feed her on the nest without hesitation. The male normally approaches the female with food in a rather upright posture, showing excitement by flicking his wings. The female in return crouches and stretches out her wings and quivers them. She is now rather below the male, and holds her beak skywards to receive the food. This is an attitude of solicitation which is common, with variations, to many bird species. The male then feeds her and she relaxes.
This is the high-intensity display but courtship feeding does not begin like this. In its low-intensity form, in Goldfinches, for instance, I watched a pair sitting side by side on a twig and at intervals in nest-building, they turned their heads and began ‘kissing’ each other, touching the tips of their bills. They did this for a minute or so at a time. No excitement was evinced and no food was passed. Later the pair separated and eventually went to their nest-building. A day or two later I found the female crouching slightly by the side of the male and once more they touched beaks as though feeding. There was only a little more excitement – still no food was passed. Courtship feeding began properly when food was passed. My point is that many of the displays that you may see in gardens and elsewhere develop from very small, less obvious beginnings.
The next stage in this sequence of events is copulation. In the case of the female Goldfinch she solicits with her legs bent, wings out of the coverts and quivering, head up, beak open, calling a high-pitched ‘tee-tee-tee-tee’. The male approaches her excitedly as usual, but instead of feeding her he mounts her and they copulate. This is a rather general picture of the way that courtship displays lead up to copulation and there are a number of variations between species. Some experts think that courtship feeding, acts as a releaser for coition, and it occurs just prior in a number of species.
The act of copulation may be very beautiful, although perhaps for most passerines after the short preliminary display it is rather abrupt and may only take a few seconds. On the other hand, the Avocet goes through quite a long routine. A pair may be feeding vigorously in a shallow lagoon, sweeping their bills through the mud below the water. After a time they will stop feeding and, while standing in shallow water, begin preening, at first fairly normally, then exaggeratedly. Suddenly, the female will lower her head and neck until her chin is resting on the water and remain in this position, occasionally jerking her head a fraction from side to side, to solicit the male. He will continue preening even more vigorously and move towards her, finally, standing quite close. After a moment or two he will move behind and always touching her, will then move to the other side while still preening and splashing her with water. He will continue moving from one side to the other for three or four minutes, and then without any warning, will suddenly jump sideways onto the female’s back and, with wings raised almost vertically, sink down on his tarsi and tuck his cloaca under her body, until they touch. Coition lasts only four or five seconds and after it is over the male slithers forward and sideways off the female and, almost always, the pair momentarily cross bills as they run apart for a few metres before they begin preening again in a more normal manner.
When the young start begging for food their soliciting postures are virtually the same as that used by the female. The Reverend E.A. Armstrong recorded that courtship feeding in adults is a recrudescence of the begging behaviour of the young.
Several species of birds have displays, in which the bills are touched, which lead to courtship feeding and copulation. The billing of the Puffin where a pair or up to six or so individuals may knock their bills together or where Gannets spar with their bills are examples of this.
Related to the solicitation posture are certain types of flight. Many land and water birds, use a ‘moth’ flight in which the bird flies comparatively short distances with wings quivering rapidly, but not very deeply. This flight is often used, if not always, in circumstances which have some sexual connotation, for instance often just before, or just after, copulation. There are variations too on this flight: for instance, the Wheatear flies fast and erratically over his territory in what I call the ‘zig-zag’ flight.
The ‘butterfly’ flight or ‘bat’ flight, in which the wings beat deeply and more slowly, is more often used in territorial circumstances. The Greenfinch uses it regularly in spring. Besides finches it is used by the auks as they fly onto their cliffs. Hen Harriers and many other species have similar display flights.