Bird migration is a phenomenon that has fascinated and perplexed humans for thousands of years. Seasonal migrations, where birds fly huge distances between breeding and wintering grounds, seem almost miraculous. How do birds know where to go, and how do they navigate such long journeys?
The mystery of bird migration has only been solved relatively recently through decades of patient observation, experimentation, and technological advances. By piecing together clues from careful observations, bird banding, radar studies, and other ingenious experiments, scientists were finally able to map migration routes, understand navigation mechanisms, and learn the triggers for migration.
Early Observations and Theories
For most of human history, seasonal disappearances and reappearances of birds were shrouded in mystery and myth. Various fanciful theories emerged to explain these phenomena, for example:
- Birds hibernated in mud at the bottom of lakes and rivers during the winter months.
- Birds flew to the moon or planets during winter.
- Swallows buried themselves in riverbanks and lake bottoms, re-emerging in spring.
Aristotle recorded some of the earliest scientific observations of migration in the 4th century BC. He noted that birds disappeared in the winter and reappeared in the summer, and that some species only appeared in summer. He theorized that birds migrated south to escape the cold and avoid food shortages. Other ancient scholars like Pliny the Elder and Averroes made similar proposals, but could not fully account for navigation over immense distances.
As early as 1703, an English schoolteacher named James Fennel observed migratory flock formations and directions. He collected reports of migrations from a network of observers and speculated that birds used celestial navigation based on sun or star positions. In 1810, a German farmer named Johann Andreas Naumann conducted more systematic surveys and concluded that birds migrate to find favorable food and climate.
Early Bird Banding Studies
A major breakthrough came in 1803, when a European stilt was found in Germany with an African band attached. This proved conclusively that at least some birds migrated between Africa and Europe. More discoveries of banded birds slowly confirmed that migration was widespread.
In 1899, a Danish schoolteacher named Hans Christian Cornelius Mortensen launched the first large-scale, systematic bird-banding program. He put aluminum bands on the legs of over 165,000 birds of more than 155 species. When banded birds were recovered, the date and location revealed key migration data. Mortensen demonstrated that storks and other European birds migrated south to African wintering grounds.
Other researchers soon adopted bird-banding programs. Frederick Lincoln began banding birds in North America in 1902. By the 1920s and 1930s, scientists like Frederick C. Lincoln, Leon Cole, and Jack Miner had tracked waterfowl migrations across North America using recoveries of banded birds. In Europe, Scottish ornithologist George M. Sutton tracked migration routes through Eurasia.
As banding data accumulated, patterns began to emerge. The speed and regularity of migrations was revealed. Many species followed defined flyways over land or ocean. Band recoveries documented the incredible distances some birds traveled between breeding and wintering grounds. Some examples:
- Arctic terns migrate from the Arctic to the Antarctic and back each year, covering over 80,000 km round-trip.
- Bar-tailed godwits fly nonstop over the Pacific from Alaska to New Zealand, a distance of 11,000 km.
- Blackpoll warblers provide one of the longest overwater migrations, flying nonstop from northeastern North America to Venezuela, a 4,000 km journey over the Atlantic.
Uncovering Navigation Mechanisms
Bird banding revealed migration routes, but still left the question: how do birds navigate so accurately over such enormous distances? This puzzle was tackled with a variety of experimental approaches.
In the 1940s, Austrian scientist Gustav Kramer hypothesized that birds orient themselves using the sun’s position. He performed orientation cage experiments that provided the first proof that birds possess “true navigation” based on the sun. Birds were able to orient themselves correctly in cages under shifting conditions and even when Kramer attempted to deceive them with mirrors.
In the late 1950s, British scientists William T. Keeton and Peter S. Brown conducted orientation experiments with savannah sparrows that demonstrated the ability to navigate using the stars. When Keeton raised birds under artificial starry skies, the birds would orient incorrectly based on the fake star patterns.
Other experiments revealed birds’ abilities to sense the Earth’s magnetic field as a navigational aid. In the 1960s, William Griffin showed conclusively that birds can detect magnetic fields. Steven Emlen created magnetic “disorientation” cages in the 1970s and showed that interfering with the magnetic sense impaired navigation ability.
Studies by Stephen Emlen, Franz and Eleonora Sauer, and Wolfgang and Roswitha Wiltschko demonstrated that birds possess an innate magnetic compass. The magnetic sense likely provides both latitude and directional information. The magnetic compass is used together with celestial cues and an “internal map” of the environs.
Radar Studies Track Movements
In the mid 1900s, radar provided another revolutionary tool to study bird migrations invisible to the naked eye. Pioneering radar ornithologists like British ecologist David Lack used radar to prove that many songbirds undertake nocturnal migration.
Using surveillance radars, researchers could track altitude, speed, and directions of migratory flocks over a wide area. Distinctive radar signatures enabled coarse identification of species composition. Weather surveillance radar networks track migration patterns across continents.
Collaborative effort like the European ENRAM network have monitored bird migration traffic rates, volumes, trends, and responses to climate change. Doppler weather radar can even estimate bird densities aloft. Miniaturized tracking radars follow individual migrants to study flight mechanics.
Unraveling Triggers and Timing
What cues tell birds when to migrate? Migration timing follows an annual schedule synchronized to seasonal changes. Day length is considered the primary trigger, while supplementary factors fine-tune timing.
In the 1950s, Erwin Stresemann demonstrated migratory restlessness triggered by seasonal hormonal cycles. Later physiological studies found fat deposition rates linked photoperiod. In 1970, Eberhard Gwinner showed day length alone induces migratory condition in caged birds.
Secondary factors like weather and food availability adjust exact timing. Warm temperatures, good food supply, and favorable winds accelerate migration, while cold weather and lack of food delays movement. Individual health and genetics also affect timing.
Tracking by Satellite
Miniaturized tracking technology has revolutionized migration research since the 1990s. Tags weighing < 1 gram can record light level, altitude, location, flight speed and transmit data by satellite.
The doyen of satellite tracking, Wolfgang Fiedler, has tracked European songbirds like white storks and lesser spotted eagles over Africa. His research details flight performance, navigation efficiency, effects of wind, and mortality risks.
Martin Wikelski’s ICARUS Initiative uses nanotags to track individual migrants globally via the International Space Station. Satellite tracking is unraveling mysteries like routes taken over oceans and how migrants orient in unfamiliar territory.
Conclusions
Over many centuries, the persistent quest to unravel bird migration has progressed through phases:
- Ancient myths and speculation
- Careful observation of seasonal patterns
- Banding studies revealing origins, routes, distances
- Experiments probing navigation mechanisms
- Radar unveiling nocturnal movements
- Hormonal and genetic insights into timing
- Satellite tracking of individual journeys
Each development has built on previous knowledge in a cumulative process. We now have detailed data on migration routes, navigation systems, and annual timing for many species. Yet new technologies continue to provide insights into these epic annual journeys. Bird migration remains a marvel, but a marvel that science has progressively illuminated through ingenuity and determination.
