148 F. Finn — Structure and Function of the Tracheal Bulb. [No. 2, 



Oidemia, 0. nigra alone is exceptional in having no difference of this kind 

 "between the sexes. 



Aix galericulata. 



Well-known as is the Mandarin duck, I have been unable to find 

 any reference to a description of the tracheal structure of the male, and 

 I therefore venture to record that there exists in this sex of the species 

 a large, rather flattened, thin- walled bony bulb of rounded outline, some- 

 what resembling that of the Muscovy (Cairina moschata). 



Cairina moschata x Anas boschas. 



It may be of interest to note that the male hybrid between these 

 two domestic ducks, judging from two specimens of each species and of 

 the hybrid examined by me, agrees with the Mallard in the size and 

 form of the tracheal bulb, but has this rather thinner-walled, thus 

 approaching the structure of the very thin-walled bulb found in the 

 Muscovy drake, the very flattened and expanded form of which is 

 characteristic. 



Casarca rutila. 



I have been unable to discover any mention of the form of the male 

 trachea in this species, and so was unable to give any account of it 

 when writing the series above-mentioned. Since then, however, I have 

 obtained a specimen, and find the bulb present and bony-walled, but 

 very small, only about the size of a pea, thus differing very much from 

 the great double inflation found in Tadorna comuta. 



Function op the Tracheal Bulb in Drakes. 



There can be no doubt that the use of this peculiar structure is to 

 modify the voice of the owner, as was pointed out, indeed, by Yarrell 

 (" British Birds," article Mallard) many years ago ; but the fact needs 

 drawing attention to, as it has been doubted, for Coues (fide Dresser, 

 "Birds of Europe," article American Bittern) states that "The curious 

 cartilaginous or osseous bulbs at the lower larynx of most Ducks seem to 

 have no influence on the voice." But nothing could be further from the 

 truth. Yarrell (loc. cit.) approvingly quotes Gilbert White's observation 

 on the sexual difference of voice in Ducks ; and careful study of the 

 living birds will show that in nearly every case where the tracheal 

 enlargement exists in the male, he emits a correspondingly different 

 note from the female — generally one of a weaker character, if not always. 

 The accompanying table will make this point clear. 



