A re-examination of the alimentary tract of two species of Asiatic swiftlets (Collocalia) appears to confirm Bernstein's (1859) hypothesis that nest-cement is produced by salivary glands. There is no sign of a crop-gland such as occurs in pigeons and doves.
2
The salivary glands of Collocalia are relatively enormous in comparison with those of the unrelated Swallow Hirundo which has apparently similar aerial-feeding habits yet does not cement its nest. On the other hand, the oesophageal and gastric mucosa in both genera are much the same.
3
A strikingly differential degree of salivary gland development is demonstrated in two species of Collocalia (taken on the same day at the same cavern) which are said to have different breeding dates. No such difference was detected in the oesophageal and gastric epithelium of the same examples. The probability of a hormonal control of the seasonal development of the salivary glands of Collocalia is briefly discussed.