The general coloration of waxwings is cryptic, that is to say, concealing or blending. The lighter color of the venter, especially of the belly, contrasts with the duller, darker vinaceous color of the dorsum. Several ruptive marks tend to obliterate the outline of the body. The crest of the head, when elevated, tends to elongate the body, making the outline less like that of a normal bird. The facial mask effectively breaks up the outline of the head, and conceals the bright eye, which would otherwise be strikingly distinct. The white spots on the distal ends of the secondaries of B. garrula and the yellow color on the distal ends of the rectrices (red in B. japonica) are also ruptive. These ruptive marks on an otherwise blending type of plumage might be important to waxwings, and probably are more effective when the birds remain motionless in either a well-lighted area or in one that is partly in shadow, rather than in one that is wholly in shadow.

The red wax tips on the secondaries of the flight feathers, and sometimes found on the ends of the rectrices in Bombycilla, are puzzling and no wholly convincing reason has been suggested for their occurrence. Two instances are known of yellow instead of red-colored wax tips in B. cedrorum (Farley, 1924). It is well known that many individuals, especially of B. cedrorum, do not possess these tips; they are absent in a smaller proportion of individuals of B. garrula. Of the 53 skins of B. cedrorum available in the University of Kansas Museum of Natural History, which might be taken as a sampling at random of the general population of this species, only 17 possess wax tips. A few specimens are unilateral, and the tips are of varying sizes in different individuals. Of these 17 birds, 6 are female and 7 male, the others being unsexed at the time of skinning. This proportion is, roughly, half and half. Of the seven skins of B. garrula pallidiceps in the same Museum, five possess the tips, and two that are females have no trace of the red tips at all. Of the five which do have the tips, two are males, two are females, and one is unsexed. In a series of 13 specimens of the three subspecies of B. garrula, loaned by the United States National Museum, all but two individuals possess the tips on the secondaries, and, in addition, four specimens, equally divided between the two sexes, have color on the rachis of some rectrices, and small appendages of pigment extend beyond the feathers. Stevenson (1882) found that among 144 specimens of B. garrula garrula killed by storms in England in the winter of 1866-67, 69 individuals had wax tips. Of these, 41 were males and 27 were females; the remaining one was of uncertain sex. Among 38 definitely sexed B. garrula pallidiceps in the California Museum of Vertebrate Zoölogy, Swarth (1922:276) lists tips in 22 males and 16 females. These data indicate that the proportion of birds with the wax tips is higher in B. garrula than in B. cedrorum. The potentiality for wax tips is possibly inherited according to Mendelian ratio.

Bombycilla japonica is of interest in that the adults, at least, seldom have the waxy appendages. Nevertheless, in the specimens observed, the entire distal ends of the feathers normally possessing the tips in other species are suffused with red color. This may be the original condition of all waxwings, or perhaps, instead, this species is in a transitional stage in the development of the tips. Swarth (1922:277) says concerning the probable derivation of the wax tips in B. garrula (and in B. cedrorum): "the ornamentation, in fact, may well have begun with the coloring of the shaft, spreading later over adjoining feather barbs. The last stage would have been the coalescing of the barbs, forming the waxlike scale as is now seen. Various steps of this hypothetical development are supplied in the wing and tail feathers of different birds of this series." Bombycilla japonica thus may be close to the ancestral condition in the waxwing stock in the development of the waxy appendage.

The rectrices of all three species of waxwings seldom possess the wax tips, unless the secondaries have the maximum number of tips. In these individuals, the pigment seems to "spill over" onto the tail feathers. Eight is the maximum number of tips found on the secondaries. Rectrices with wax tips are more frequently found in B. garrula, and only occasionally in B. cedrorum. The pigment in the tip of the tail of B. japonica is red rather than yellow as it is in the other two species, and some individuals of the Japanese Waxwing show a slight amount of coalescence of wax in the tail feathers as well as in the secondaries.

If the tips were present in all members of the two species, it could be postulated, in line with recent investigational work by Tinbergen (1947), that the tips are in the nature of species "releasers," facilitating species recognition. Such recognition is now regarded as of prime importance in the formation of species. It is improbable that sex recognition may be aided, as there is no evidence to indicate that the tips are found predominantly in either sex.

The wax tips are not limited to the adult birds in the species B. garrula. Swarth (op. cit.) mentions the capture of several young Bohemian Waxwings, and describes them as "possessing all the distinctive markings of the most highly developed adult." This includes wax appendages, and several citations are given (Wolley 1857, Gould 1862) to indicate that this is the rule rather than the exception, not only for the American subspecies pallidiceps, but at least for the European subspecies garrula as well. On the other hand, the young of B. cedrorum lack the wax tips, at least as far as available data show.

Some characteristics of living animals are of the "relict" type; that is to say, they were developed in ancient times when some unknown ecological factor was operative which is no longer demonstrable, and the characteristic is now neutral or at least not detrimental, although of no positive value to the organism. Possibly the wax tips of waxwings are thus to be explained. I am more inclined to the opinion that the wax tips are adaptations to present-day ecological conditions for the birds.

The wax tips are ruptive in effect, since the birds, especially in winter, are habitués of bushes and trees that have berries, and the tips, on the otherwise dull body, suggest berries. The red tips tend further to disrupt the body outline at the midline, or slightly posterior to this. Perhaps the wax tips on the rectrices emphasize the end of the tail, the region of the body that is the least vital and that may be expendable in times of pursuit by an enemy.

Any characteristic is of survival value to an organism if in any way the characteristic enhances the chances of survival up to the time when the organism can successfully raise even a few young to maturity. If that character, as for example, the red wax tips on the secondaries, helps to maintain the individual until it can raise to independence a greater number than merely a few young, such a character can be said to be of greater survival value. The character may be effective for a brief period of time and may be uncommon; it might be effective for a split second in time, and only at a particular stage in the life history.

The winter period probably is the most hazardous for waxwings, in that they then depend at times upon long flights to find food. The food is vegetable, and thus is comparatively low in food value; the birds must ingest large quantities of berries or dried fruits to maintain themselves. In winter, in northern latitudes at least, predators are more apt to prey upon those species which, like waxwings, do not migrate south. The winter months are those in which waxwings frequent berry bushes, and it may well be that in these months, the wax tips that appear like berries, are especially valuable to the birds, and operate selectively.

It is suggested, therefore, that the wax tips are of positive value to waxwings, rather than being relict characters. Coalescence of pigment has taken place in the formation of the wax tips. B. japonica is closer to the ancestral stock insofar as wax tips are concerned, and generally lacks the tips. B. cedrorum has the tips in approximately half of the adults, and not at all in the young. B. garrula has the tips in almost all the adults, and in a like proportion of the young, and probably has evolved further in the development and retention of the wax tips than has either of the other two species.

The streaked plumage of Dulus is decidedly generalized, and is probably more nearly like the color of the ancestral stock. In this connection it is notable that young Cedar Waxwings are streaked, and young Bohemian Waxwings are streaked to a lesser degree. This streaking is apparently a recapitulation of the feather color of the stock. Perhaps the color of Dulus has not changed, as the streaking would not be a disadvantage to the birds in their environment of light and shadow. In joining together in groups and in the construction of large communal nests, Dulus has evidently gained sufficient protection against predators; other birds solve this problem by modifying their coloration.

Ptilogonys is ruptively colored, but in a different fashion than Bombycilla. The tail markings, the distinct yellow on the under tail coverts, the sharply marked pileum, are all examples of ruptive coloration. The generally lighter venter (especially under tail coverts), the crest that may be elevated, and the generally drab bluish dorsum, are cryptic and serve to hide the animal insofar as is possible considering its habits. The very conspicuous coloration of the male, in contrast to the more drab color of the female, however, would lead one to believe that in Ptilogonys, following the pattern of many passerine birds, the male leads a predator from the nest, leaving the drab female to incubate the eggs, and thus preserve the young.

It is difficult to suggest reasons for the brilliant coloration of the male Phainopepla, unless it is for decoying predators away from the nest. Possibly some birds survive not because of, but in spite of, their coloration, and Phainopepla may be a case of this sort. Anyone who has observed Phainopepla in life will agree, certainly, that the male makes no attempt at concealment, and flaunts his color to all comers.

The coloration of Phainoptila, in contrast to Phainopepla, is much more plain, and is suited to its habits of brush dwelling; in a brush habitat the drab coloration is difficult to detect. The Yellowish Olive under tail-coverts and the Olivaceous dorsum are all evidences of cryptic coloration, and undoubtedly, this bird depends upon hiding for escape from its enemies, since it is a bird of the dense forest cover.

Coloration, which varies relatively rapidly in response to differing ecological conditions, has become more different in the species of Bombycillidae than is true in many other families of passerine birds. The explanation lies in early geographical isolation of the three subfamilies, with consequent radiation in three directions. Waxwings have become adapted by possessing a thick protective layer of feathers and drab coloration broken by ruptive marks. They still retain the streaked plumage, which is probably ancestral, in the juveniles; this is lost at the first molt in the fall. In its evolution, Dulus has developed large feet, heavy decurved beak, and the large communal nest that affords protection from enemies; as a consequence, perhaps Dulus did not need a plumage different from the primitive and streaked one. The survival of Dulus may not have depended on either ruptive marks or on brilliant and outstanding plumage. The large feet and large bill seem to be responses to particular ecological requirements, as will be shown later.

The Ptilogonatinae, with habits paralleling those of the flycatchers, probably are considerably modified from the ancestral stock; the coloration probably is more brilliant and conspicuous. Perhaps this type of coloration and the habit of capturing insects from a perch are correlated. Some amount of territoriality is characteristic of this subfamily and dimorphism in color—the plumage of the male is outstandingly conspicuous—possibly is of selective value to the race. In a tropical forest community, a duller pattern possibly would be more visible and thus would be selectively disadvantageous.

Waxwings are gregarious birds and individuals establish no well-defined territories as do many birds. The nest itself is the only defended territory, and as Crouch (1936) has shown, the Cedar Waxwing will nest in close proximity to others of the same species. Swarth (1932:275) mentions that the Bohemian Waxwing is tolerant of the nests of other pairs near by. The extreme condition is that found in Dulus, in which the territory is not limited even to the nest, but to the individual compartment of the community nest. Phainopepla, a less gregarious bird than Dulus and waxwings, has a much more definite territory, although individuals of Phainopepla are tolerant of others of the same species; no feeding territory is established, and small flocks of birds feed together at any time of the year.

In birds whose territories lack well-defined boundaries, it would be expected that elaborate song would not have evolved, and that most of the recognition of kind and sex would be dependent upon the behavior of the birds. This is the fact; song, as such, is lacking in the three subfamilies Bombycillinae, Ptilogonatinae, and Dulinae. Waxwings utter (1) notes that serve to keep the flock together, (2) calls used by the young in begging for food, and (3) some low notes that Crouch (op. cit.:2) considered as possibly concerned with courtship. Phainopepla has various call notes, and in addition, a succession of notes which are run together. Ptilogonys utters a note which Skutch (MS) characterizes as a loud, not unmusical "tu-whip" that is used as the birds "fly in straggling parties which keep in contact by their constant chatter." Dulus is described by Wetmore and Swales (1931:349) as having only a variety of rather harsh chattering notes in chorus.

The most notable behavior pattern associated with courtship in Waxwings, in the absence of song, is the so-called "mating dance" described by Crouch (1936), and observed by me in Lawrence, Kansas, in the spring of 1948. This consists of one bird of a pair (presumably the male) hopping along a branch toward the other bird (the female), then away again, repeating the procedure for some little time. The female remains motionless until, as the male approaches, mutual fondling of the head and neck feathers takes place, or the birds may peck at each other's bill. A berry may be passed from bill to bill, although generally the berry is not utilized for food, and this can be interpreted as a nervous reaction of the birds. It may be an instance of "false feeding" as is seen in many birds, in which the female begs for food, as a nestling would beg, as a preliminary to the sexual act. I am of the opinion that these reactions are in the nature of behavioristic patterns that bring the birds into the emotional balance for copulation, as copulation follows the "dance." Sometimes, however, copulation is preceded by a "nuptial flight" around the nesting area, at which time the birds utter loud calls. Armstrong (1924:183) is of the same opinion, citing numerous instances in which nuptial flights and elaborate displays have evolved for just this purpose. The birds are then in the proper physiological balance to initiate the complicated sequence of copulation, nesting, incubation, feeding, and brooding of the young.

It would be valuable to know more concerning the life histories of the other birds considered in this paper, since behavior is inherent, and probably can be cited as evidence of close relationship or the opposite. All that I have been able to learn is that Phainopepla has a nuptial flight in which the male chases the female, and that Dulus (Wetmore and Swales, 1931:347) seeks the company of others of its kind at all times, and that two birds, presumably paired, will sidle up to one another when they are perched.

Both species of American waxwings build bulky nests, with the base or platform composed of a large amount of twigs and sticks, from which there often trails a mass of sticks and moss or string. Softer materials such as moss, plant fibers, and string, are placed inside the platform; moss is readily available to, and preferred by, B. garrula according to Swarth (op. cit.:271), and various plant fibers and string are used by B. cedrorum. The inner lining consists of soft plant fibers or down, dry grasses, and feathers. The nest is usually unconcealed in a tree either adjacent to a trunk or on a main side branch, but sometimes in a fork. Nest building by both Cedar and Bohemian waxwings is rapid, taking from three to five days, and is followed immediately by egg laying.

Nesting by waxwings is late in the season; June is the month in which the nest is usually started. This is readily explainable in Bohemian Waxwings, since adverse weather would prohibit earlier nesting in the area in which they spend the summer. Crouch (op. cit.:1) remarks that B. cedrorum possibly evolved in the far north where it was impossible for it to start nesting earlier, and that the habit has been retained. Perhaps, on the other hand, nesting is delayed until the berry crop is ripe, to insure sufficient food for the young.

Desertion of the nest is not uncommon in waxwings, despite the tolerance to other animals that is shown by the birds. A new nest may suddenly be begun before the first one is finished, and all the materials from the first nest may be removed, or the nest may be abandoned before it is completed. The eggs may be left at any time up to hatching, and the young may be deserted, especially in the earlier stages of development.

The very large and bulky communal nest of Dulus is not radically different from the nest of waxwings. In the absence of sufficient nesting sites, a pair of gregarious birds such as Dulus could combine their nest with those of other pairs, retaining for their own territory only the nest cavity, and in this way communal nests might have evolved. The nest of Dulus is communal probably because of the lack of suitable trees for nesting sites, and only incidentally does this type of nest afford better protection from natural marauders. Large numbers of Palm-chats work together in the construction of the nest platform, and both sexes probably take part in the work.

In Phainopepla the nest is built mostly by the male (Merriam, 1896; Myers, 1908), although the female does some of the work, especially in the shaping and lining of the nest. In this genus, the nest is usually a compact structure, but exceptional nests are of considerable bulk. The nest is commonly placed in a fork near the main trunk of a tree, in a conspicuous location, and generally is 10 to 20 feet from the ground. In shape and location, the nest closely corresponds to that of Bombycilla, but the materials used for a base are stems of annual plants, whereas Bombycilla uses more woody twigs. The finer materials used by Phainopepla are more readily obtainable in the ecological association inhabited by Phainopepla than would be heavier twigs such as Bombycilla uses.

Waxwings are typically frugivorous; berries are the staple food. The birds are known to catch insects, especially in the spring and summer, and their insect gathering technique has been likened to that of Tyrannid flycatchers. Nice (1941) experimented with a young captive Cedar Waxwing and found that it had a decided preference for red or blue berries, and that meal worms were utilized as food only when the birds became educated by other captive birds of other species as to the food value of the worms. Post (1916) indicates that the food given to the nestlings of Cedar Waxwings is entirely animal for the first three days, and that a mixed diet of berries and insects is subsequently offered.

In feeding of the young, regurgitation of partly digested food does not take place, according to Wheelock (1905). Rather, the adults "store" food in the form of berries in the expanded esophagus or crop, feeding them whole to the young. Digestion is an unusually rapid process, involving merely minutes for the passage of berries and cherries. This is correlated with a short intestinal tract, which is unusual for a frugivorous bird. Nice's (1940) experiments with Cedar Waxwings revealed that cherries would pass through the digestive tract in 20 minutes, blueberries in 28 minutes, and chokecherries in 40 minutes. Heinroth (1924) states that berries pass through the digestive tract of Bohemian Waxwings in the space of a "few minutes." This rapid digestion is obviously adaptive, since the value of the food is slight and therefore large quantities of it must be ingested; the large seeds would hamper further ingestion until they were eliminated, since they seem not to be regurgitated.

Members of the subfamily Ptilogonatinae are both insectivorous and frugivorous insofar as available data show, although again there is relatively little information available concerning them. Skutch (MS) has found that the Guatemalan Ptilogonys cinereus catches insects by repeated sallies into the air from a perch, after the manner of flycatchers. He notes also that the birds feed on berries of Eurya theoides and Monnina xalapensis. It is well known that Phainopepla catches insects when these are available, and its liking for berries is so apparent that in parts of its range, it is known as the "pepper bird," since it frequents pepper trees (Schinus molle) and feeds on the small red berries. The preserved specimens of Ptilogonys and Phainoptila available for this study contain only berries in the digestive tract. Dulus feeds mostly, if not wholly, on plant food. According to Wetmore and Swales (1931:349), berries, fruits, and parts of flowers are eaten.

A critical analysis of the skeletons provides evidence that aids the student in estimating which differences are merely the result of habits developed in relatively recent geological time as opposed to those which owe their existence to more ancient heritage. Stresses caused by the action of different sets of muscles can apparently stimulate changes in bones to meet new needs, and the evidence from genetics is that such mutations in wild birds are minute and cumulative, rather than of large degree and of sudden appearance. Once adaptive mutations have occurred, if genetic isolation from one source or another accompanies it, a new population different from the parental stock may become established. Study of the skeleton of any species of living bird may indicate those characters identifiable as modifications fitting it to a particular environment. If no distinguishing characters are discovered that may be attributed to environmental factors, such a species can be spoken of as generalized; the inference then is that such a species is not modified for a single, particular ecological niche.

Some parts of the skeleton, obviously, are more adaptable or plastic than others. The beak seems to be the most adaptable part. Probably this results from its frequent use; it is the part of the bird to capture the food. The long bones, meeting the environment as legs which serve as landing mechanisms or as locomotory appendages, and as wings which provide considerable locomotion for most birds, probably come next in order as regards plasticity. In these parts, then, one may look for the most change in birds, which, within relatively recent geologic times, have been modified to fit a particular set of conditions. From the beak and long bones of a species in which habits are unknown, one can infer the habits and habitat from a comparison with the skeletal features of species of known habits.

Skull.—The skulls in all three subfamilies have essentially the same general appearance and structure, the most marked differences being, as would be expected, in the bills and associated bones.

The most specialized bill is to be found in Dulus; its bill is decurved, and the associated bones are correspondingly changed for support of the bill. For example, the palatines and "vomer" are much wider, the palatines are more concave from below and have longer posterior processes than the corresponding bones in Bombycilla. Moreover, the "vomer" in Dulus and in Phainoptila is larger and heavier than in Bombycilla, and the quadrate and pterygoid bones are relatively large for support of the beak. The palatines, however, are weak in Phainoptila. In the Ptilogonatinae, with the exception of Phainoptila, the wings of the palatines flare more than in Bombycilla, but not to the extent that they do in Dulus, nor does the palatine bone present a concave appearance in the Ptilogonatinae. The premaxilla is a relatively weak bone in Bombycilla and Phainopepla, stronger in Ptilogonys, and is notably heavy in Phainoptila and Dulus, and in these latter two genera shows a sharply-ridged tomium. The maxillae connect to somewhat widened nasal and naso-lateral processes in all the genera, and the premaxillae narrow abruptly from this point forward. In the family, Phainopepla and Phainoptila show the least flaring in this region.

This flaring, immediately lateral to the antorbital plate, is common to all Bombycillids and constitutes a major skeletal characteristic useful for recognition of the members of the family, since the swelling is easily discernible both externally and on the cleaned skulls. In Phainopepla there is much variability in this character; some specimens have a narrower antorbital bridge than others. Only one skeleton of Phainopepla n. nitens was available. The flaring in the skull of this specimen is identical with that in Ptilogonys. Among the skulls of P. n. lepida in the University of Kansas Museum of Natural History, is No. 19228, a juvenile, taken 5 miles south of Tucson, Arizona. In this specimen, the flaring in the antorbital region is clearly evident and equal in amount to that in skulls of P. n. nitens, but the bird had not attained full skeletal growth. However, the flaring of the antorbital region appears to be common in the nestlings of many species of passerine birds. Other specimens of the subspecies lepida show a varying amount of flaring, the least (in the series available) being in No. 24754, MNH, in which the proportion of the skull (length divided by width) closely corresponds to that in Phainoptila; the skull of No. 24754 is long and thin, and the base of the bill is only slightly swollen. The skull of Phainopepla nitens lepida is more generalized than that of Phainopepla n. nitens, having a longer and narrower bill like the generalized Phainoptila. In Phainopepla n. nitens and in members of the genus Ptilogonys, more flaring occurs in the antorbital region.

Phainoptila, as noted above, has no great amount of flaring in the antorbital region. When more specimens of Phainoptila are examined, the base of the bill probably will be found to flare more in some individuals than in others; this would be expected if we may judge by the data on Phainopepla. The premaxilla and maxilla of Phainoptila are similar to the same bones in Dulus, and there is a well-marked ridge on the tomium (possibly for cutting flower parts). In Phainoptila, the palatines are narrower than in any other genus of the family and abut the lacrimals. The entire skull appears to be modified along different lines from those of the skull of Dulus; the skull of Phainoptila seems to be modified for a frugivorous rather than an insectivorous diet. The skull of Phainoptila probably is more nearly similar to the ancestral skull than is that of any other living species in the family. The wide gape characteristic of some members of the family is undoubtedly a modification for aiding in the capture of insects, and Phainoptila has progressed less in this direction than have other species in the family.

The mandibles vary somewhat in the shape and proportionate size of the bones. The mandible is proportionately, as well as actually, highest in Dulus. The medial condyle varies to some extent, being slightly flattened mediad in Bombycilla, and less so in the other genera. The mandible of Bombycilla narrows to the symphysis much more gradually than it does in the other genera.

The antorbital plate is large and divides the orbital chamber from the nasal chamber. The small lacrimal bone anterior to the plate articulates with the maxilla and the premaxilla. Shufeldt (1889) states that the free lacrimal ossicle might be of some taxonomic importance in the passerines, since it is found in the generalized Corvids and in nestling Turdids. I find it well developed and identical, with a double articulation and free ends, in all the Bombycillids. There is no significant variability in the family, and this is more evidence of close taxonomic relationship between the members of the family.

The size of the crania is somewhat variable, although the differences seem to be primarily those of proportion. Ptilogonatinae have long crania, whereas the crania of the Bombycillinae and Dulinae are shorter but deeper. I regard the longer cranium as primitive, and it is longest in Phainoptila. In order of decreasing relative length of the cranium, Phainoptila is followed by Ptilogonys caudatus, P. cinereus, and Phainopepla. Bombycilla garrula has the deepest cranium in the family.

The measurements of the lengths and widths of the skulls are given in Table 9. The relative length of the bill and relative width of the skull are given in Table 10. These relative measurements are calculated by using the actual measurements in Table 9 as numerators, the length of the skull from the lacrimal bone to the posteriormost end of the skull being used as the denominator. The data indicate that Phainoptila has a slightly narrower cranium.

Humerus.—Certain families of passerine birds have a noticeable variation in the characteristics of the humerus; the bone varies in length, in diameter, and in the complexity of the processes at either end. In the Bombycillids, however, the amount of variation is relatively small, and the diaphysis of the bone is somewhat twisted, especially so in Dulus. The deltoid tuberosity is variable, being shorter but more elevated in Bombycilla than it is in the Ptilogonatinae and in the Dulinae. The tendon from the pectoralis major muscle, which inserts on this process, probably finds better insertion on a higher process than on a lower but longer one.

Distally, the two major condyles and the intercondylar groove or olecranon fossa that make efficient articulation with the ulnar process, are not variable. The external condyle, however, is significantly variable in the family. This condyle is longest and most pronounced in birds in which the humerus is short in relation to the trunk, as for example in Tachycineta. In the Bombycillidae the condyle is smallest in Phainoptila, where it is a mere suggestion of a process. In the remainder of the Ptilogonatinae, the condyle is larger but rounded, and shows a double process in Ptilogonys caudatus, and a slightly pointed process in P. cinereus. The external condyle in Dulus is not specialized, being low and rounded, but in Bombycilla, it is noticeably elongated, indicating a better attachment distally for the deltoid muscle. (No measurements are tabulated for this condyle, as the percentage of error in measuring this small structure is great.) Table 1 gives lengths of humeri, and Table 2 gives lengths of the humeri expressed as percentages of the length of the trunk, a standard measurement.

The area of insertion of the deltoid muscle is elongated in those birds with shortened humeri; these birds have also greater flight power than do birds with longer humeri and therefore a shorter external condyle.