Phylogeny of the Waxwings and Allied Birds Part 3

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 uns.e.xed 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 uns.e.xed. 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 s.e.xes, 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 s.e.x. Among 38 definitely s.e.xed _B. garrula pallidiceps_ in the California Museum of Vertebrate Zoology, 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 j.a.ponica_ 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 j.a.ponica_ 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. j.a.ponica_ is red rather than yellow as it is in the other two species, and some individuals of the j.a.panese 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 s.e.x recognition may be aided, as there is no evidence to indicate that the tips are found predominantly in either s.e.x.

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 habitues 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 quant.i.ties of berries or dried fruits to maintain themselves. In winter, in northern lat.i.tudes 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. j.a.ponica_ 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 fas.h.i.+on 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 pa.s.serine 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 pa.s.serine 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.

COURTs.h.i.+P

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 s.e.x 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 courts.h.i.+p.

_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 a.s.sociated with courts.h.i.+p 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 pa.s.sed 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 s.e.xual 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 relations.h.i.+p 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.

NEST BUILDING

There are numerous papers concerning the nesting of waxwings. _B.

garrula_, owing to its nesting in the far north, where observers are few, has received less attention than _B. cedrorum_. There is, on the other hand, no literature that deals with the nesting habits of the majority of the Ptilogonatines, with the exception of _Phainopepla_, on which there is considerable literature (Merriam, 1896; Myers, 1907, 1908). No detailed study of the nesting of _Dulus_ has been reported, although Wetmore and Swales (1931) have described carefully the large communal nest of this genus.

In _Bombycilla_, both members of a pair apparently aid in the construction of the nest (Crouch, 1936; Swarth, 1932). Although the s.e.xes are alike in plumage and general appearance, most students of the nesting of waxwings agree that one bird, a.s.sumed to be the female, does most of the arranging of the material, and does the shaping of the nest, whereas both birds carry materials to the nest site. As is characteristic of many pa.s.serine birds, both members of the pair gather materials and fly back to the nest site, where the female takes the more active part in the construction of the nest itself.

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 ma.s.s 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 gra.s.ses, 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 s.e.xes 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 a.s.sociation inhabited by _Phainopepla_ than would be heavier twigs such as _Bombycilla_ uses.

FOOD

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 pa.s.sage 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 pa.s.s through the digestive tract in 20 minutes, blueberries in 28 minutes, and chokecherries in 40 minutes. Heinroth (1924) states that berries pa.s.s through the digestive tract of Bohemian Waxwings in the s.p.a.ce of a "few minutes."

This rapid digestion is obviously adaptive, since the value of the food is slight and therefore large quant.i.ties 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.