Part 7

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.

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 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.

SKELETON

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.

1. Phainoptila m. melanoxantha, sex?, MNH no. 26493, 15 mi. SE Cartago, Costa Rica.

2. Ptilogonys caudatus, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica.

3. Phainopepla nitens, male, MNH no. 24752, Pima Co., Arizona.

4. Ptilogonys cinereus, female, Louisiana State University no. 297, Xilitla Region, San Luis Potosi, Mexico.

5. Dulus dominicus, female, USNM no. 292652, Don Don, Haiti.

6. Bombycilla cedrorum, male, MNH no. 15331, Bexar Co., Texas.

7. Bombycilla garrula, sex?, USNM no. 223895, Bozeman, Montana.]

8. Phainoptila m. melanoxantha, sex?, MNH no. 26492, 15 mi. SE Cartago, Costa Rica.

9. Ptilogonys caudatus, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica.

10. Phainopepla nitens, male, MNH no. 24754, Pima Co., Arizona.

11. Ptilogonys cinereus, female, Louisiana State University no 297, Xilitla Region, San Luis Potosi, Mexico.

12. Dulus dominicus, female, USNM no. 292652, Don Don, Haiti.

13. Bombycilla cedrorum, male, MNH no. 15331, Bexar Co., Texas.

14. Bombycilla garrula, sex?, USNM no. 223895, Bozeman, Montana.]

15. Phainoptila m. melanoxantha, sex?, MNH no. 26493, 15 mi. SE Cartago, Costa Rica.