A migrant bird is said to overwinter if it does not migrate back to the breeding grounds in autumn.
This is a very Southern Hemisphere expression! The annual cycle of a long-distance migrant bird species which crosses the Equator is a confusing array of seasons. In the Northern Hemisphere, summer is in June-July-August; but this is winter in the Southern Hemisphere. To make this clear, we say the seasons are “boreal” in the Northern Hemisphere, and “austral” in the Southern. So the boreal autumn is September-October-November (Northern Hemisphere) and the austral autumn is March-April-May. Long-distance migrants breed in Eurasia in the boreal summer, then migrate south in the boreal autumn, cross the Equator and arrive in their non-breeding areas in the austral spring, and stay for the austral summer. In the austral autumn they migrate north, and arrive in the breeding grounds in the boreal spring.
Those birds which fail to migrate north in the austral autumn are said to overwinter. Here we are talking about winter from the perspective of the Southern Hemisphere; the birds stay in their non-breeding areas for the austral winter, June-July-August.
Once upon a time, overwintering birds were thought to be the “poor quality” birds that were unable to prepare for migration. Mostly, this is not true.
In some species of migrants, and the Ruddy Turnstone is a good example, birds do not breed until they are two years old. So, assuming that Curlew Sandpipers hatch in July, the juveniles are nine months old when when the adults migrate north in April. The stay put, and overwinter. So large flocks of Ruddy Turnstontes in June-July in places like Langebaan Lagoon are an indication that the previous breeding season (a year ago, in the boreal summer) was a good one.
Some of the larger waders, such as Common Whimbrel, don’t breed until they are three or four years old. They then overwinter several times, before their first migration back to the northern hemisphere to breed. Other wader species which overwinter include Curlew Sandpiper, Grey Plover, Sanderling and Wood Sandpiper. In contrast, very few Little Stints and Common Ringed Plovers overwinter; it is likely that these species breeding when they are one year old.