What Is Migration?
Migration is the regular, seasonal movement of animals from one area to another and back again. Animals migrate to:
- Escape cold winters and find warmer climates
- Follow food sources (e.g. grass, fish, insects) that are only available seasonally
- Reach better breeding grounds or safer nesting sites
Migration is different from simply moving — it is a predictable, cyclical behaviour tied to seasons.
Who Migrates?
Birds — About 1 800 species of birds migrate. The Arctic tern holds the record: it migrates from the Arctic to the Antarctic and back each year — a round trip of up to 90 000 km.
Fish — Salmon hatch in freshwater rivers, migrate to the ocean as adults, and return to the exact river they were born in to spawn (lay eggs). They can travel hundreds of kilometres upstream, jumping waterfalls.
Mammals — Wildebeest follow rainfall and fresh grass in a circular migration of about 3 000 km around the Serengeti in East Africa, forming herds of over a million animals.
Insects — Monarch butterflies migrate up to 4 000 km from Canada and the USA to forests in Central Mexico each autumn. Remarkably, no individual butterfly completes the full cycle — it takes 3–4 generations.
How do animals navigate? Different species use different methods: birds use the position of the sun and stars, Earth's magnetic field, and familiar landmarks. Monarch butterflies use a solar compass calibrated with an internal clock. Salmon use smell — they can detect the exact chemical signature of their birth river.
Why Is Migration at Risk?
Human activities threaten migration in several ways:
- Habitat loss — nesting and resting sites along migration routes are destroyed
- Climate change — animals arrive at their destination before or after food is available
- Light pollution — birds navigate by stars; artificial light confuses them
- Barriers — roads, fences, and dams block traditional land migration routes
Migration is costly — animals burn enormous amounts of energy travelling long distances. Animals prepare by eating extra food (hyperphagia) to build fat reserves. Bar-tailed godwits fly 11 000 km non-stop from Alaska to New Zealand, losing half their body weight during the flight.
Real world: Swallows leaving gardens in autumn
Every autumn, swallows that nested in barns and under roof eaves throughout Europe and North America disappear. They migrate to sub-Saharan Africa to spend the winter where insects are plentiful, then return in spring to the same nesting spots. Farmers and gardeners have noticed swallows leaving for thousands of years — it is one of the most visible signs of seasonal migration.
Real world: Salmon in rivers
When salmon swimming upstream in rivers are shown on nature documentaries or spotted by anglers and bears, they are in the middle of migration. These fish were born in that river, spent years in the ocean, and are now returning — guided by the smell of the water — to spawn in the exact spot where they hatched.
Real world: Bird feeders emptying in spring
Many people who keep garden bird feeders notice that certain species like fieldfares and redwings stop visiting in spring. These are migratory birds that winter in warmer areas and then fly north to breed. Putting up feeders and monitoring which birds visit is a simple way to observe migration patterns.
Where do you see this in real life? Keep a record of which birds visit your garden through the year — some will only appear in winter (like fieldfares) or summer (like swallows), reflecting their migration patterns. You can also track monarch butterfly migration online through citizen science projects.