Long flights and rearing families puts a lot of strain on birds' feathers. Each year they have to moult out the old, worn feathers and grow new ones. This poses problems for birds that migrate. Some, such as geese, moult their wing feathers after breeding and grow new ones before the return journey south. Others such as Wood Sandpipers, stop for a partial moult half-way along their migration route.
Because birds are vulnerable when they moult, they need safe places to do it. Some birds, like Shelducks, have traditional moulting places to which they fly every year. This is called a 'moult migration'. Some Shelducks go to Bridgewater Bay in Somerset to moult. Most fly to the North Sea island of Helgoland. After moulting, they return to the coasts where they are normally found.
We are familiar with the idea of birds flying to or from Ireland on migration. Many other birds pass through Ireland on their way to other countries. They are noted wherever birdwatchers gather to record migration. They are called 'passage migrants'.
Birds such as Arctic Terns, which breed both in Ireland and further north, are consequently both summer visitors and passage migrants. Sanderlings, small wading birds, breed in the high arctic regions and many spend the winter off West Africa. They are passage migrants to Ireland and use our estuaries to refuel and rest as they move through (although some stay here for the winter).
For some species, like Swallows, all the birds migrate. For others, like Starlings, some migrate and some do not. In this case, those that migrate are moving away from harsh winter conditions, while those that don't are already living in an area with favourable winter conditions. These birds are 'partial migrants'.
Quite a lot of common birds, such as Robins, Pied Wagtails, Goldfinches, Song Thrushes and Lapwings are partial migrants. The extent of any migration they make will depend on the weather either in Ireland, the British Isles or on the continent. Mild weather will not encourage migration, while bad weather may force birds south and west in 'hard weather movements' which are sometimes spectacular.