The key on the following pages provides an efficient means of identifying virtually all North American waterfowl that may be examined in the hand. The procedure for using it is comparable to that used for all such dual-choice or "dichotomous" keys. One simply chooses which of the initial descriptive couplets (A or A') best fits the unknown bird. Having chosen one of these, the pair of descriptive couplets (a and a') immediately below the chosen alternative is next considered, without further regard for the rejected one. Subsequent choices, which are sequentially numbered (1 and 1', 2 and 2', etc.) must then be considered until the name of a species has been reached. In no case will more than eleven choices be required to identify any of the 52 waterfowl species or subspecies represented in the key. After having tentatively determined the identity of the unknown bird, one should refer to the appropriate "Identification" sections of the text, to confirm or reject the initial determination. Illustrations in this book or other references should also be consulted, bearing in mind that sexual or seasonal variations in plumage may exist.
Available at: http://works.bepress.com/paul_johnsgard/48/