What is Bird Migration? A Complete Guide to Nature’s Great Journeys

Many birds commonly seen across the UK are actually migrants, spending part of the year in distant parts of the world. Some less frequent visitors arrive during the winter months, migrating here from colder northern regions. So, what is bird migration, why do birds undertake these incredible journeys, and which species travel the longest distances?

Basics of Bird Migration: Why Birds Travel

Bird migration is the seasonal movement between breeding and overwintering areas, primarily driven by the need for food and suitable breeding conditions. Most migration occurs in the Northern Hemisphere, with birds moving south away from colder breeding grounds as winter approaches and returning north in spring.

Food availability is the main motivator for migration. During colder months, insects and other food sources become scarce, so birds travel to areas where resources are more abundant. Additionally, longer daylight hours in northern summers provide ample time for birds to feed their young, making these regions ideal breeding grounds despite the need to move elsewhere during winter.

This journey is physically demanding and stressful, exposing migrating birds to exhaustion and predators. Some predators, such as the Eleonora’s Falcon, have adapted their own life cycles to exploit migration peaks, timing their breeding seasons to coincide with the passage of migrating birds. Interestingly, not all birds migrate far: partial migration is common where some individuals of a species migrate while others remain, such as 44% of non-passerines and 32% of passerines in Australia exhibiting this behaviour.

Techniques Birds Use During Migration

Migrating birds employ various strategies to conserve energy and manage risks. For example, geese flying in a distinctive V-formation benefit from reduced air resistance, saving 12-20% of energy compared to solitary flight. Some species, like Red Knots, even gain speed by flying in flocks.

Altitude also plays a role; certain species fly at remarkable heights to take advantage of favourable wind currents. Bar-headed Geese have been documented flying over the Himalayas at altitudes up to 6,540 metres, while remains of other migrating birds have been found at 5,000 metres near Mount Everest.

Long-Distance Migrants and Their Epic Routes

One iconic British migrant is the barn swallow, a small, colourful bird with a majestic migration story. UK barn swallows have been tracked travelling all the way to South Africa, demonstrating the incredible distances these little birds cover.

Migration routes are often learned as juveniles, following their parents and navigating using geographical landmarks such as coastlines and mountain ranges. Among the longest non-stop flights is the Bar-tailed Godwit's impressive journey from Alaska to New Zealand, covering 11,000 kilometres while carrying over half its body weight in fat.

The overall long-distance migration champion is the Arctic Tern, which annually travels between Arctic and Antarctic regions—a minimum of 19,000 kilometres each way. One ringed tern from the Farne Islands in Northumberland was recorded flying 22,000 kilometres to Australia in three months, only half its total yearly travel.

Raptor Migration: Using Thermals to Soar

Birds of prey, or raptors, along with storks, often migrate using thermal air columns, rising on warm air to conserve energy. However, thermals only form over land, compelling these birds to choose the narrowest crossing points over water, such as at Gibraltar or the Bosporus. The Batumi bottleneck in Georgia is a well-known migration corridor, with thousands of birds funnelled through this passage, minimizing energy loss to between 10-20% of their body weight.

Night Migration: Travel Under Cover of Darkness

Many smaller birds, such as hummingbirds, flycatchers, and warblers, migrate at night. This reduces risks of overheating and predation, plus allows daytime for foraging and rest. However, since resting during flight is impossible, many species have adapted by modifying sleep patterns to endure continuous travel without deep sleep.

How Do Birds Know When to Migrate?

Birds are thought to use environmental cues such as day length and temperature changes to time their migrations. Some species migrate shorter distances within their range when seasons change, like the Merlin or Skylark, known as short-distance migrants. Others, like the Common Chaffinch, rarely move more than a few kilometres, classifying as residents.

Bird migration remains one of nature's most remarkable displays of instinct and endurance, linking diverse habitats across continents and supporting biodiversity worldwide.

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