Though this one looks very similar to the common sandpiper, the Green Sandpiper is a fairly bulky in comparison. The key differentiation is on the head, we can see a conspicuous white supercilium in front of the eye, and a narrow white eyering. There is a very dark line across the lores, but it does not extend behind the eye.
The straight bill has olive-green base and blackish tip. The eyes are dark brown. Legs and feet are dull greyish-green, but they look dark at a distance.
Male and female are similar, but the female is slightly larger than male.It is typically found nearby inland fresh waters.
The Green Sandpiper’s usual call is iconically high-pitched “twit-wit-wit” and sometimes preceded by a rising “ko-wit”. These calls can be heard even when the birds are flying at great height during the migration. I am given to understand that, during the breeding season, the alarm call is a rapid “tit-ti-tit”.
Unlike most of it’s cousins, this one usually nests in trees, but instead of building a nest, it uses an old one, abandoned by other species. However, it may occasionally nest on the ground among tree roots.
The Green Sandpiper is known to breed in forested habitats across Northern Europe and Asia, and spends the winter from Africa to South East Asia.
During the breeding season, it is interesting to see the way Green Sandpiper male performs a typical display flight, rising first on quickly fluttering or vibrating wings. Then, it flies in circles or semicircular pattern before doing a steep dive on fixed wings. It repeats this display several times, accompanied by song.
I have only watched videos of this behaviour and yet to experience it on the field.
Info source: http://www.oiseaux-birds.com/card-green-sandpiper.html