Re: How many cervical vertebrae do ducks, geese and swan have

I’m not sure how taxonomists distinguish ducks from geese and 
swans. Apparently this is a contested issue. Because of the relative 
ease with which these animals can be separated from external 
characters, there is little (none, as far as I found by searching the 
Web) use of internal characteristics for keying them out, either 
naturally or artificially. The classification, which I found on the Web at 
URL:
 http://212.187.155.84/wnv/Lists_specieskingdoms/Living
_Kingdoms.


is something like this:

Class Aves
  Order Anseriformes - ducks, geese, swans
    Infraorder Anserides (excludes screamers and the magpie goose)
      Family Anatidae - ducks, geese, swans (but not including the 
whistling ducks)

There are 4 subfamilies, one of which is the Subfamily Cygninae, 
including all swans, and another of which is the Subfamily Anatinae, 
including all ducks and geese. In the Subfamily Anatinae there are 
two tribes (clusters of genera), the Tribe Anserini, which includes all 
geese and things which I take to be goose-like ducks, and the Tribe 
Anatini, which includes all the ordinary ducks (as well as some you 
have probably never heard of).

The long and short of this is that the situation is far more complicated 
than you have been led to believe. None the less, it can be said, with 
reasonable certainty, that the absolute range of cervical vertebrae in 
birds is from 11 to 25. Generally, Web references I was able to find 
indicate that swans have 24 or 25 neck vertebrae, geese have 18 or 
19, and ducks have 16.

I hope this has helped.