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Migration – how it works

Bird migration is an extraordinary phenomenon.  It is incredible to think that millions of birds travel from their wintering grounds to their breeding grounds each year, only to make the return journey a few months later.  How a Willow Warbler is able to return to the same breeding site as the previous year, having spent the winter in sub-Saharan Africa is just amazing!

Migration means the movement of bird populations occurring at predictable times of each year, between breeding and non-breeding areas.  It’s not just birds that migrate though, fish, butterflies and whales amongst others, also migrate.  Birds migrate to improve their chances of survival and also to improve their breeding success; they move to a place where they can rear more young than if they remained resident.  Species that migrate do so because the climate offers a better environment for feeding, and on their breeding grounds better conditions for rearing young.

There are different types of migrant and levels of migration, each suited to the needs of different kinds of birds.  Species such as Swift that return to Britain and Ireland each spring are called summer migrants.  These species stay for our summer and breed here.  In late summer or early autumn they leave to spend the winter elsewhere.

		Some birds choose to spend the winter in Britain and Ireland.  Species such as Redwing and Fieldfare breed in northern Europe and in the autumn they migrate to southern or western Europe where the climate is milder and food more accessible.  These are called winter migrants and will depart for their northern breeding grounds in spring.  In some bird populations, only part of the population migrates.  These are called partial migrants and Goldfinch is a classic example.    Ringing studies have shown that a proportion of the Goldfinch population in Britain and Ireland move south to France and Spain for the winter.
Goldfinch

It is not always necessary for birds to make great journeys.  Some species can find better feeding conditions by just moving downhill.  This kind of migration is called altitudinal migration and the Snow Bunting is a good example.  Snow Buntings breeding in the uplands of Scotland move downhill in winter, and some move as far as the coast.  Their breeding grounds are covered in snow during the winter and food is hard to find.  Lapwings are known to move from their breeding areas at the end of the season to low-lying areas. At times when it is very frosty or there is a lot of snowfall, which makes it difficult for them to feed, they will move out of the area in search of better conditions.  These kinds of journeys are called cold weather movements.  All birds need to replace their feathers, some species do this in stages, replacing 1-2 at a time whilst others lose all their flight feathers at the same time so they can’t fly.  Prior to moulting, birds move to an area that is safe from predators and where there is an abundance of food.  Wildfowl such as Shelduck undertake this moult migration.  In the past, Shelducks have migrated to the Helgoland Bight in north-west Germany.  More recently, Shelducks have formed a new moulting area in Bridgewater Bay in Somerset.

Is it worth it?

There are huge costs incurred by birds during migration.  Not only do birds fly vast distances, there are other problems too.  Poor weather during migration, shortage of food en-route, predators, navigational problems and interference by man all make the journey difficult.

For many species, such as insect-eating warblers like the Willow Warbler, the winters in Britain and Ireland are just too cold to survive.  Some species are hardier and have adapted to the conditions and now Chiffchaffs and Blackcaps regularly winter here.

Birds that migrate to northern countries to breed have longer hours of daylight to feed and rear young, benefit from the seasonal bumper crop of insects and are faced with fewer predators.  Good enough reasons to migrate?