(182) NEST-CONSTRUCTION BY BIRDS BY LT.-COLONEL B. H. RYVES. IN the present paper, which is prompted by a realization that published matter on the subject is extraordinarily meagre, I shall endeavour to outline a general picture of the one phase of the breeding cycle of birds—nest-construction—which for absorbing interest cannot, I believe, be surpassed by any other. To describe the many and varied types of nests built by the varied species, whose requirements differ to a marked degree, is outside the scope of this paper. Indeed, it would almost be necessary to write a book and devote a chapter to each species or at least to each group of species. This will be readily understood when one considers that, exclusive of geese, ducks and game-birds, over 160 species have been recorded to breed in Great Britain. Before discussing such points as the use some birds make of certain materials and their well-defined purposes, and the essential features of a nest, one or other or all of which must apply to the nest of every species, I propose at the outset (after a few introductory remarks) to approach the subject of the r61es of the sexes in con­ structional operations and then to follow with an account of the actual procedure of building (with as precise particulars as it is possible for me to give) under a few species, each providing a typical illustration of the peculiar part taken by each sex. At the close of each illustration will be found some general notes attempting to amplify it. It must be obvious that the close study of a bird at work is beset with many difficulties. Although many birds lose a good deal of their innate shyness when absorbed in their task and permit closer human approach than at ordinary times, it is extremely hard to follow their rapid and intricate actions. In any case one cannot see exactly what is being done within the nest when only a beak or tail is visible. However, by connecting observed movements with changes at the nest after a spell of work, some of the movements can sometimes be interpreted with a measure of accuracy. At the same time a sense of bewilderment pervades one's faculties, en­ hanced, no doubt, by the knowledge that no human being is capable of reproducing the amazing structure even with tools that might specially be fashioned for the purpose. One finds oneself constantly wishing that the bird would repeat its operations again and again in the manner of a slow-motion film. ROLES OF THE SEXES. I have little hesitation in saying that there is no phase in nest- construction that we less understand or more misunderstand than the parts played by the sexes—none certainly that calls more urgently for clarification. VOL. XXXVII.] NEST-CONSTRUCTION BY BIRDS. 183 To start with, I submit that it is only a specially inherited craft that enables a bird to build its specialized nest. All birds, regardless of sex, do not inherit it and it cannot be acquired except, perhaps, in the course of evolution. If this view be accepted, it is reasonable to argue that a bird that actually builds and one that only provides materials are performing entirely distinct functions which should not be confused. If I may be allowed the metaphor, the former is a craftsman and the latter a navvy. It thus logically follows that the fact of seeing a bird with materials in its beak does not justify the assumption that it is a craftsman until proved to be so by its subsequent actions at the nest. From recorded observations in the field we are, however, justified in pronouncing that nearly every bird falls into one or other of four clearly defined categories though, with our present state of know­ ledge, we cannot place every species in its appropriate place. The consummation of so desirable an end will not be reached without an immense amount of careful research. Category i.—Species in which both sexes build. (a) Species in which both sexes take a full and equal share (namely both inherit the craft) ; (b) species in which the male builds so-called " cocks' nests." Category 2.—Species in which the female alone builds and the male alone provides the materials. Category 3.—Species in which the female alone builds, the male taking no consistent part. Category 4.—Species in which the female alone builds and both consistently provide materials. [There are a few species, comparatively negligible in number, in which the r61es of the sexes in Category 3 are reversed. I have not placed these in a separate category in order to avoid multiplication and confusion. I omit altogether those species which build no nest of any kind, not even a scrape.] ILLUSTRATIONS. (1) LONG-TAILED TIT (Mgithalos caudatus rosaceus) (Category 1 (a)). The following case is, I think, fairly representative of the species. But, before giving details, I will describe in a nutshell the chief features of the birds* methods. Both birds appear to be equal partners throughout operations and no " master craftsman " is discernible between them. They work in unison as a single bird. Building normally begins when the sun is well up and continues, with intervals up to an hour for feeding, until the late afternoon. A whole day may be lost in really inclement weather, but a day seldom passes without an hour or two of activity. Each bird generally waits for the other to finish its job, the waiting one often becoming very impatient, but it is not uncommon to see them in the nest 184 BRITISH BIRDS. [VOL. XXXVII together. All through operations they keep up a ceaseless chatter. Shifts at the nest are remarkably short in comparison with the lengthy stays of some other species. This, I suppose, is due to the extraordinary rapidity of their movements. Work is carried on from within the nest. An occasional feather is brought before the dome has been formed, but the bulk of feathering is done when the outer nest appears to be finished. Both birds begin roosting in the nest as soon as it has been domed. In the present case the pair were occupied for 27 days in completing their nest, but from other records secured, the period has been about 23 days. I may be allowed to record here that 73 days had elapsed from the commencement of building to the " fledging " of the young. On March 10th, 1938, I watched a pair site-hunting and on the nth they began building in my privet hedge, 4J feet above the ground. After three days constant work, fine lichens and cobwebs tightly woven round two twigs forming a V, and a small platform in the apex of the V, became clearly visible from a distance of six feet or so. These formed the framework and foundation of the final nest. As work progressed the weaving round the forks and the platform rose higher and thickened. It seemed to me that the weaving was accomplished by dexterous manipulation with the beak (from a later illustration it will be noted that the beak is but little used except for bringing materials). By the 19th the nest had assumed the shape of a shallow, open cup. From this point onwards the beak appeared to be less in use and body work came into play. A bird would subside on to the nest and proceed with a series of shufflings, twistings, turnings and gyrations. Sometimes the head would be lowered and the tail elevated. Suddenly a wing would shoot out here or there at an angle that gave the bird the appearance of being injured. On the 23rd I watched one bird tugging at a cocoon in the crevice of a tree and pulling out long strands of silk. Very fine lichens were now being collected from the trunk of an old willow and green moss was often in evidence in their beaks. On the 25th a dome was beginning to rise and the wall was distinctly higher on the side opposite to where the entrance hole eventually was built. The birds shaped the dome by pressing the felting in­ wards with their chins. When the dome became too high for them to continue this procedure they butted it upwards and inwards with the crowns of their heads. On the 29th the hole was discernible, the wall on its side haying been built up to the level of the rest of the dome and, on the 31st, it seemed to have taken full shape. I could no longer see what was going on inside, but materials (chiefly moss) were still taken in, presumably to thicken and reduce the size of the cup. On April 3rd feathering was in full process and after the 6th building ceased and the nest appeared at last to be completed. [It would appear to me correct to include in this category Swallow and House-Martin and, as expert diggers, Sand-Martin, Kingfisher, the woodpeckers and shearwaters.] VOL. XXXVII] NEST-CONSTRUCTION BY BIRDS. 185 (2) WHITETHROAT (Sylvia c. communis) (Category 1(6)). (a) On June 6th, 1931, I watched a solitary male commence building a nest which he completed on the 8th. The nest appeared to be a perfect one and I awaited the sight of an egg in it, but none was ever laid. Indeed, although the bird remained in the territory, constantly singing, for several days longer, no female ever materialized. (b) On May 7th, 1935, after courtship and site-selection had been observed, I noted the commencement of building.