How do you tell a common redshank from a spotted redshank?

The spotted redshank (Tringa erythropus) is a separate species from the common redshank (Tringa totanus). Breeding spotted redshanks can be almost black, so they are easy to tell apart. Winter spotted redshanks are much paler, though, and are easier to mix up with common redshanks.

The most useful differences:

Wings in flight: the common redshank shows broad white trailing edges on its wings. The spotted redshank lacks these broad white wing bars.
Face: the spotted redshank has a stronger pale stripe above the eye.

Both species have reddish legs, so leg colour alone is a poor way to identify them. Check the wing pattern and the eye stripe instead.

Sources

Redshanks
RSPB: Spotted redshank

How do you tell a common redshank from a spotted redshank?

Sources

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