Two forms of a shorebird called greater and lesser yellowlegs provide a good example of how the relative sizes of body parts can differentiate two species of birds.
At first glance, both forms look very similar (and even share overlapping geographical ranges).
However, close inspection of the relative lengths of the head and bill reveals that the greater yellowlegs' bill is longer than its head whereas the lesser yellowlegs' bill is about the same length as its head:
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