Differentiating the two species of Yellowlegs in the field isn't always an easy proposition. Both Greater and Lesser have a similar plumage, proportions and behaviour.
At Presa Este, I had the two species side by side, and this made things a lot easier.
The picture below highlights the size difference between the two birds (Greater on right)
Lesser, as you can see is a significantly smaller bird. But when you come across an individual bird, the size feature is difficult to ascertain. Other features are sometimes more reliable. The bill is often a good starting point. Lesser Yellowlegs's bill is straight, 'sharp-pointed', and all dark, as seen below.
Lesser Yellowlegs, Presa Este
On the other hand, Greater has a bill which is slightly upturned (almost Greenshank like), blunt tipped, and with a pale grey base during the nonbreeding season.
Greater Yellowlegs, Presa Este
Another useful feature can be the extent of barring on the flanks of the birds. Compare the Lesser Yellowlegs above with this Greater below. Greater often has considerably more barring...
Greater Yellowlegs, Barco
This feature again depends on the time of year during which you see the birds. The Greater legs at Presa Este, for example, does not show this feature, as it hasn't reached breeding plumage yet.
As with many species of birds, call is the most reliable way to distinguish the two species. Greater gives three or four loud shrill notes, whereas Lesser gives a much softer double whistle.
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