Beaks consist of two parts, the upper and lower beak. Upper beaks have a long, acute rostrum tip and large rostrum with small wings. On the other hand, lower beaks have a thick, short rostrum tip and small rostrum with large, long wings. Upper and lower beaks are of a similar structure and the same terminology is used for both. For the lower beak the rostrum is anterior, the lateral wall is posterior, and the crest is ventral.

1. The sharp ridge, called the cutting tooth, runs between the rostrum tip to the junction between the wings, making an angle; the junction is called jaw angle.

2. The base of the wing is slightly swollen and is called the wing fold. Inside each wing fold is a small ridge called the wing tooth.

3. The lateral walls merge to form the crest; it looks like an arch in upper beaks and a ridge in lower beaks.

4. On the lateral wall of the lower beaks is a fold or ridge that runs from the crest to the inner corner of the wall, called the lateral wall fold.

When beaks are used for species identification it is better to examine both upper and lower beaks but between the beaks, the lower beak has a much more complicated structure so is better for examining differences between species. In stomach contents, beaks are usually separated so it is difficult to discern which are pairs. Therefore this manual focuses mainly on the identification of lower beaks and only considers upper beaks that have very distinct characters.

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