The digestive tract is relatively uniform in all genera of the family; there are only slight differences between the species. The degree of compactness of the visceral mass varies, Phainoptila and Ptilogonys caudatus having the folds of the digestive tract loosely arranged, whereas Ptilogonys cinereus and Phainopepla have folds which adhere more tightly to the ventriculus and liver. In Dulus and Bombycilla, as compared with the Ptilogonatinae, the visceral mass (primarily liver and ventriculus) is situated more posteriorly in the body cavity, and is more compact, and the intestine is more tightly coiled.
The coiling of the intestine, if its degree of compactness is disregarded, is nearly identical in the birds of the family; there are four major loops between the ventriculus and the anus. The length of this section of the tract is, however, somewhat variable, as can be seen by reference to [Table 13], in which the actual and relative lengths of the intestine are given. It may be seen that in Bombycilla and in Phainopepla, the tracts are much shortened. This is notable, since these are frugivorous birds, and in many frugivorous birds, the tract is lengthened for better extraction of edible portions of the food. Possibly the action of the digestive juices is correspondingly more rapid in Bombycilla and Phainopepla, thereby permitting the necessary nutriment to be extracted by a short digestive tract.
In a migratory bird, or one that depends on flight power to find food and escape capture by predators, as in the case of the waxwings, the compacted and shortened visceral mass would seem to be advantageous, because of the consequent reduction in weight. I consider the longer intestine to be the ancestral condition, and that the intestine has become shorter to meet new environmental conditions.
Table 13. Digestive Tract: Actual Length, and Length Relative to Thoracic Length
| Species | Length in mm. | Relative length (in percent) |
| Ptilogonys caudatus | 134 | 476.9 |
| Ptilogonys cinereus | 111 | 415.6 |
| Phainopepla nitens | 94 | 357.5 |
| Phainoptila melanoxantha | 150 | 457.1 |
| Dulus dominicus | 130 | 451.0 |
| Bombycilla garrula | 102 | 298.2 |
| Bombycilla cedrorum | 95 | 309.5 |
Beddard (1898:30) states that caecae in the tract may be highly variable in a single family of birds. The Bombycillidae is no exception in this regard. At the junction of the cloaca and the large intestine, there are two small caecae, the function of which is unknown to me. The caecae are largest in the Ptilogonatinae, smaller in the Bombycillinae, and smallest in the Dulinae. There may be a correlation between large caecae and more insectivorous diet and small caecae and frugivorous diet; however, the data are not conclusive in this regard.
ORIGIN OF THE SPECIES