How do birds know when and where to migrate?
How birds prepare for, and navigate, their migration journeys
This is really several questions in one. How do birds decide when to migrate? How do they prepare for the journey? How do they know where their destination is? And how do they navigate en route?
How do birds know when to migrate?
Many birds are summer visitors to Britain, joining resident breeders to take advantage of seasonal feeding opportunities – hence returning south before winter arrives.
The seasonality of the northern summer, with its regular cycles in plant growth and invertebrate abundance, has led to largely predictable patterns of bird migration. Spring migration delivers a bird to its breeding grounds, timed so that its chicks hatch when food is most plentiful.
For birds wintering in the temperate zone, changing day-length is a cue to initiate migration. But what about those birds wintering in Africa, where variations in day-length are less marked? How do they know what the conditions will be on their breeding grounds and when to set off (in either direction)?
They don’t – their departure time must be controlled by an innate mechanism. Evidence suggests the existence of an internal clock, which manifests itself in migratory restlessness (or zugunruhe) in captive birds during the period when their wild cousins would be migrating.
How do birds prepare for migrating?
Pre-departure fuelling is one of several adaptations that aid long-distance migrants, and even species that feed on the wing may lay down fat reserves.
The size of internal organs may also be altered – flight muscles and heart increasing in size, but stomach, liver and intestine shrinking. These changes may be reversed when the bird makes a stopover, so it effectively switches between feeding and flying modes.
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For example, an 18g garden warbler can more than double its weight to 37g before departure. During the journey the warbler may lose 3–3.5g for each 1,000km travelled, burning off fat and protein – the latter mainly from the digestive tract but also the liver and leg muscles.
Such changes are extreme in waders that cover very long distances: the bar-tailed godwit, which can migrate up to 11,000km, is up to 55 per cent fat on departure, and absorbs up to a quarter of its kidneys, liver, gizzard and intestine en route.
How do birds know where to migrate?
The decision of whether to migrate and where to winter may vary not only between species but also within one.
Females and young birds often winter farther south than adult males, and an individual might remain in the UK one winter but move south the next (the goldfinch is one such partial migrant). There are also decisions to be made about where to stop to refuel, and about routes.
Navigation presents the biggest conundrums: how do birds assess where they are and how to get to a location they may never have visited before?
Clearly, many birds have innate mechanisms that determine the orientation of their migration – even a cuckoo, raised by foster parents, instinctively knows the right bearing to take.
The innate nature of the migratory journey has also been proved experimentally with young starlings as long ago as the 1950s.
More recent work on a range of species, including white-crowned sparrows in North America, has shown that experienced adults can reorientate their bearings if displaced, rather than following a pre-programmed route, as juveniles do.
Such goal-orientated navigation implies that birds can determine their own location and use navigational tools to reach their intended destination, adjusting their route if they find themselves off course.
These tools include the use of landmarks (seabirds often follow the coast), the Earth’s magnetic field, the position of the sun and, in the case of nocturnal migrants, patterns of stars: an experiment with indigo buntings revealed the use of a ‘star compass’.
Other clues aren’t evident to humans: sound, barometric pressure, polarised light – even smell is thought to be used by seabirds and racing pigeons over short distances.
Main image: Flock of snow geese migrating, Bosque del Apache National Wildlife Refuge, New Mexico, USA © Getty Images
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