Every fact in the three migration lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/sΒ² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: how animals navigate is still being researched; the sources describe several cues (sun, stars, magnetic field, landmarks, smell) that are likely combined, and the role of smell in pigeons is controversial [2]. Distances are rounded and differ between individuals and populations: some humpback whale populations travel about 5,000 miles, others less [5]. The godwit figures are for one tracked bird in 2007 [7]; the kilometre values are our conversions of the miles given in the sources (1 mile β 1.6 km). The shadow stick, globe and magnet activities model what animals use; they do not show how the animals' senses work.
All sources
[1] The basics: how, why, and where of bird migration
Cornell Lab of Ornithology (All About Birds) Β· Reliability: High
Type: University ornithology laboratory (education article)
Migration is any regular, large-scale movement of animals. Birds that nest in the north move there in spring to use a few months of abundant insects and plants, plenty of nesting space and long summer days for raising young; escaping cold is not a main driver. Changes in day length are the main cue, with temperature and food; hormones change and caged birds show migratory restlessness (Zugunruhe). Most birds migrate at night. More than 300 North American species are long-distance migrants. Arctic Terns fly more than 50,000 km (31,000 miles) a year. Before migrating, birds store energy as fat. Birds navigate with the sun, the stars, the Earth's magnetic field, landmarks and even smell.
[2] The basics: how birds navigate when they migrate
Cornell Lab of Ornithology (All About Birds) Β· Reliability: High
Type: University ornithology laboratory (education article)
Gustav Kramer (1951) showed that starlings shift their migratory direction when the apparent position of the sun is moved with mirrors; the sun compass is linked to the bird's internal clock. Stephen Emlen (1967) showed that young Indigo Buntings learn north and south from the rotation of the stars around a central point such as the North Star; birds raised without star rotation oriented randomly. Robin experiments in the early 1960s showed that changing the direction of the magnetic field changed the birds' direction; birds seem to sense the axis and dip angle of the field but not which end is north, and may estimate latitude from field strength. Night migrants may use polarised light at sunset; day migrants follow landmarks such as mountain ranges, rivers and lakes; a theory that pigeons use smells is controversial.
British Trust for Ornithology (BTO) Β· Reliability: High
Type: National bird research charity
Migration is typically driven by the availability of resources such as food, mates or nest sites. Warblers, Swifts and Swallows come to Britain to breed after wintering in Africa. Many birds seen in the UK in passage and winter periods breed in the far north. Fat is the most energy-dense fuel; small songbirds (passerines) can be up to 50% body fat just before their first big migratory flight. Much was learned from large-scale ringing schemes and, more recently, tracking devices.
Monarchs cannot survive the cold winters of northern climates. The eastern population overwinters in oyamel fir forests at about 2,400β3,600 m in the same 11 to 12 mountain areas of the States of Mexico and MichoacΓ‘n, from October to late March; the western population overwinters on the California coast. Some fly up to 3,000 miles, about 50 to 100 miles a day, and the trip can take up to two months. Navigation seems to combine the earth's magnetic pull and the position of the sun. The generation that flies south makes the full trip; it takes 3 to 4 generations to reach the northern United States and Canada again.
Humpback whales have one of the longest migrations of any mammal: some populations swim about 5,000 miles from tropical breeding grounds to colder, more productive feeding grounds. The trip from Alaska to HawaiΚ»i, about 3,000 miles, can take as few as 28 days. Feeding grounds are in cold, productive waters, where they feed and build up blubber in the warmer months; calving is in shallow, warm waters.
[6] How do salmon know where their home is when they return from the ocean?
U.S. Geological Survey Β· Reliability: High
Type: National geological and biological survey (FAQ)
Scientists believe salmon navigate in the ocean using the earth's magnetic field like a compass. Once they reach their home river, they use smell to find their home stream; they build their smell memory as young fish migrating to the ocean.
[7] Flight of the Kuaka: a godwit makes the longest nonstop flight ever recorded
Cornell Lab of Ornithology (All About Birds) Β· Reliability: High
Type: University ornithology laboratory (article)
In 2007 a Bar-tailed Godwit known as E7 flew about 7,250 miles nonstop from Alaska to New Zealand in about 8 days (29 August to 7 September), at an average speed of nearly 35 miles per hour, with no food or water. It carried a surgically implanted satellite transmitter of about 25 g that sent signals to satellites; a rule of thumb is that a transmitter should not weigh more than 3% of the bird. Before migration a godwit's weight can more than double with fat, which gives more than twice the energy of carbohydrate or protein.
[8] Crossing the ultimate ecological barrier: evidence for an 11,000-km-long non-stop flight from Alaska to New Zealand
U.S. Geological Survey (publication record, The Condor, 2005) Β· Reliability: High
Type: Peer-reviewed research paper (government science agency record)
A 2005 study argued that Bar-tailed Godwits fly about 11,000 km nonstop across the Pacific from Alaska to New Zealand, based on re-sightings of marked birds, the timing of migration, records along the route, extreme fat loads and flight simulation models.
National Audubon Society Β· Reliability: Medium-high
Type: Bird conservation society (field guide)
The Arctic Tern breeds on coasts and tundra in the far north and migrates from the high Arctic to the Antarctic; some Arctic Terns may migrate farther than any other birds.