Bird migration

Aristotle thought many birds disappeared during cold weather because they were torpid, undetected in unseen environments like tree hollows or burrowing down in mud found at the bottom of ponds, then reemerging months later.

[2][1] Still, Aristotle recorded that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile, an observation repeated by Pliny the Elder in his Historia Naturalis.

The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the autumn to wintering grounds in warmer regions to the south.

These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water.

[32] Bar-headed geese Anser indicus have been recorded by GPS flying at up to 6,540 m (21,460 ft) while crossing the Himalayas, at the same time engaging in the highest rates of climb to altitude for any bird.

[47] In short-lived species that migrate alone, such as the Eurasian blackcap Sylvia atricapilla or the yellow-billed cuckoo Coccyzus americanus, first-year migrants follow a genetically determined route that is alterable with selective breeding.

[48][49] Many migration routes of long-distance migratory birds are circuitous due to evolutionary history: the breeding range of Northern wheatears Oenanthe oenanthe has expanded to cover the entire Northern Hemisphere, but the species still migrates up to 14,500 km to reach ancestral wintering grounds in sub-Saharan Africa rather than establish new wintering grounds closer to breeding areas.

[51] [52] Great snipes make non-stop flights of 4,000–7,000 km, lasting 60–90 h, during which they change their average cruising heights from 2,000 m (above sea level) at night to around 4,000 m during daytime.

[57][58] Some bar-tailed godwits Limosa lapponica baueri have the longest known non-stop flight of any migrant, flying 11,000 km from Alaska to their New Zealand non-breeding areas.

[60] One Arctic tern, ringed (banded) as a chick on the Farne Islands in Northumberland off the British east coast, reached Melbourne, Australia in just three months from fledging, a sea journey of over 22,000 km (12,000 nmi), while another also from the Farne Islands with a light level geolocator tag 'G82' covered a staggering 96,000 km (52,000 nmi) in just 10 months from the end of one breeding season to the start of the next, travelling not just the length of the Atlantic Ocean and the width of the Indian Ocean, but also half way across the South Pacific to the boundary between the Ross and Amundsen Seas before returning back west along the Antarctic coast and back up the Atlantic.

[63] The most pelagic species, mainly in the 'tubenose' order Procellariiformes, are great wanderers, and the albatrosses of the southern oceans may circle the globe as they ride the "Roaring Forties" outside the breeding season.

Many are among the longest-distance migrants; sooty shearwaters Puffinus griseus nesting on the Falkland Islands migrate 14,000 km (7,600 nmi) between the breeding colony and the North Atlantic Ocean off Norway.

Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land, and these birds cannot maintain active flight for long distances.

The Batumi bottleneck in the Caucasus is one of the heaviest migratory funnels on earth, created when hundreds of thousands of soaring birds avoid flying over the Black Sea surface and across high mountains.

[74] Many bird species in arid regions across southern Australia are nomadic; they follow water and food supply around the country in an irregular pattern, unrelated to season but related to rainfall.

Bohemian waxwings Bombycilla garrulus well show this unpredictable variation in annual numbers, with five major arrivals in Britain during the nineteenth century, but 18 between the years 1937 and 2000.

In the period before migration, many birds display higher activity or Zugunruhe (German: migratory restlessness), first described by Johann Friedrich Naumann in 1795, as well as physiological changes such as increased fat deposition.

Satellite tracking of day migrating raptors such as ospreys and honey buzzards has shown that older individuals are better at making corrections for wind drift.

Because birds migrate between northern and southern regions, the magnetic field strengths at different latitudes let it interpret the radical pair mechanism more accurately and let it know when it has reached its destination.

[89] There is a neural connection between the eye and "Cluster N", the part of the forebrain that is active during migrational orientation, suggesting that birds may actually be able to see the magnetic field of the Earth.

Reverse migration, where the genetic programming of young birds fails to work properly, can lead to rarities turning up as vagrants thousands of kilometres out of range.

After a trial with Canada geese Branta canadensis, microlight aircraft were used in the US to teach safe migration routes to reintroduced whooping cranes Grus americana.

[101] Theoretical analyses show that detours that increase flight distance by up to 20% will often be adaptive on aerodynamic grounds – a bird that loads itself with food to cross a long barrier flies less efficiently.

[124] Stable isotopes of hydrogen, oxygen, carbon, nitrogen, and sulphur can establish avian migratory connectivity between wintering sites and breeding grounds.

[125][126] An approach to identify migration intensity makes use of upward pointing microphones to record the nocturnal contact calls of flocks flying overhead.

[127] An older technique developed by George Lowery and others to quantify migration involves observing the face of the full moon with a telescope and counting the silhouettes of flocks of birds as they fly at night.

[141] Unfortunately, many historic stopover sites have been destroyed or drastically reduced due to human agricultural development, leading to an increased risk of bird extinction, especially in the face of climate change.

[147] For example, in California, legislation changes have made it illegal for farmers to burn excess rice straw, so instead they have begun flooding their fields during the winter.

[148] Similar practices are now taking place across the nation, with the Mississippi Alluvial Valley being a primary area of interest due to its agricultural use and its importance for migration.

[146] This practice requires extremely low investment on behalf of the farmers, and researchers believe that mutually beneficial approaches such as this are key to wildlife conservation moving forward.