Drift migration is the phenomenon in which migrating

Bird migration

Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

 bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s are blown off course by the winds at the time they are in flight. It is more likely to happen to birds heading south in autumn because the large numbers of inexperienced young birds are less able to compensate than the adults heading north in spring.{http://rspb.royalsocietypublishing.org/content/270/Suppl_1/S8}

Drift is assisted by disorientating conditions such as mist or drizzle, and can result in large numbers of birds arriving together in an area in which they are not normally seen. In the UK this is called a fall, while in the U.S. it's known as a fallout, though in the latter case, it need not involve vagrant birds.

An example would be an east wind in September blowing Scandinavia

Scandinavia

Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...

n migrants such as Bluethroat

Bluethroat

The Bluethroat is a small passerine bird that was formerly classed as a member of the thrush family Turdidae, but is now more generally considered to be an Old World flycatcher, Muscicapidae...

s, Wryneck

Wryneck

The wrynecks are a small but distinctive group of small Old World woodpeckers.Like the true woodpeckers, wrynecks have large heads, long tongues which they use to extract their insect prey and zygodactyl feet, with two toes pointing forward, and two backwards...

s, and the continental race of Robin

European Robin

The European Robin , most commonly known in Anglophone Europe simply as the Robin, is a small insectivorous passerine bird that was formerly classed as a member of the thrush family , but is now considered to be an Old World flycatcher...

 onto the east coast of England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 and Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

, leading to temporary concentrations of these species at headlands like Spurn

Spurn

Spurn Point is a narrow sand spit on the tip of the coast of the East Riding of Yorkshire, England that reaches into the North Sea and forms the north bank of the mouth of the Humber estuary. It is over long, almost half the width of the estuary at that point, and as little as wide in places...

.