Bird migration is a complex behavior that involves innate genetic programming and learned experiences. Birds use a variety of navigational aids, including the sun, stars, Earth’s magnetic field, and landmarks, to find their way during their often long and arduous journeys.
But why has this behavioral pattern developed in the first place? Let’s find out!
Why Do Birds Migrate?
Birds migrate for several reasons, all linked to their survival in one way or another. One primary motivation is food. In regions with cold winters, food sources like insects become scarce. Birds move to warmer areas where food remains available throughout the year.
For example, Barn Swallows feed exclusively on flying insects that become scarce or nonexistent in winter, prompting them to migrate to warmer areas.
Migration also helps birds escape harsh weather. By traveling to milder climates or even to lower elevations, they can better regulate their body temperature and conserve energy, which is particularly important for smaller birds that are more susceptible to cold.
Another significant factor driving migration is the need for suitable breeding habitats. Birds often travel to specific locations that offer optimal conditions for raising their young, such as ample food, ideal temperatures, and safe nesting sites.
Related: How high do birds fly?
It might seem strange for birds that winter in the tropics to migrate north in spring. However, the tropical ancestors of these birds likely dispersed northward because the seasonal abundance of insect food and longer days allowed them to raise more offspring. This advantage led to higher reproductive success compared to their tropical relatives.
As their breeding zones shifted northward during periods of glacial retreat, these birds continued to migrate back to the tropics during winter when food became scarce in the north. All in all, migrating allows birds to continuously find environments that meet their needs throughout the year.
Timeframes
Birds start migrating based on an internal biological calendar that has evolved over generations. This calendar is influenced by environmental cues like changes in day length and temperature. Changing weather may provide additional cues.
One of the primary triggers for migration is the change in day length. As days shorten in late summer and early fall, hormonal changes cause birds to experience “migratory restlessness”, prompting them to gather in flocks, eat more, and prepare for their journey south. Conversely, lengthening days in late winter and early spring signal birds to return north.
Food availability also plays a crucial role in migration timing. Birds migrate north in spring to take advantage of the abundance of emerging insects and budding plants, essential for feeding themselves and their young. As food sources become scarce in the fall, birds are prompted to head south again.
Most birds navigate by instinct, relying on environmental cues such as the position of the sun, stars, magnetic fields, and landmarks. Once a bird successfully completes its first migration, it often repeats the same pattern throughout its life.
However, for some species like swans, geese, and cranes, young birds learn migration routes and timing by following their parents, ensuring they travel at the right time and follow the correct path.
The Birds That Stay Behind
Some birds stay for the winter because they have adapted to survive in colder climates and can find sufficient food sources even when temperatures drop. In addition, by staying in their territory year-round, these birds can maintain control over their nesting sites and feeding grounds, giving them a competitive advantage when the breeding season begins in spring.
Birds that stay for the winter have physiological and behavioral adaptations that allow them to withstand cold temperatures. These adaptations include fluffing up their feathers to trap heat and some species even grow extra down feathers! Some species may even enter a state of torpor, a state of hibernation, to lower their metabolic rate.
Resident birds often have a more varied diet compared to insectivorous migratory birds. They may feed on seeds, nuts, berries, and even small animals, preferring those with a higher energy content.
For example, birds like chickadees, nuthatches, and woodpeckers feed on seeds and nuts they find in trees and shrubs. Some woodpeckers, like the Acorn Woodpecker, cache food in troves for later consumption. Certain birds, such as robins and waxwings, feed on berries and other fruits that persist on plants through the winter.
In areas where humans put out bird feeders, many birds take advantage of the readily available food. Those feeders, stocked with seeds, suet, and other bird food, can be a crucial resource during harsh weather. Some birds have also adapted to urban and suburban environments where they can find food in garbage, at feeders, or by foraging around human dwellings.
Notable Migrators
Here are some notable North American bird species known for their impressive migrations:
- Arctic Tern – migrates up to 44,000 miles annually and has the longest migration of any bird, traveling from its breeding ground in the Arctic to its wintering grounds in the Antarctic.
- Rufous Hummingbird – migrates up to 3,900 miles one way from its breeding grounds in the Pacific Northwest and Alaska to its wintering grounds in Mexico, covering one of the longest migration distances relative to its body size. They navigate through a wide range of altitudes, from sea level to up to 12,000 feet, and have exceptional memory and navigation skills despite their small size, often returning to the same feeders and flowers year after year along their migratory route.
- Swainson’s Hawk – the roundtrip between North America and Argentina can cover more than 12,000 miles. They move in large flocks of thousands of individuals.
- American Golden Plover – migrates up to 20,000 miles each year between the Canadian Arctic and the southern parts of South America. The species is remarkable for its non-stop flights over the open ocean, reaching flight speeds of up to 80 mph.
- Red Knot – migrates up to 18,000 between Arctic North America and the coastal United States and Central America in a year. They are reliant on specific stopover sites, such as Delaware Bay, where they feed on horseshoe crab eggs.
- Blackpoll Warbler – migrates around 12,000 miles roundtrip each year between the northeastern United States and South America. These small birds hold the record for the longest overwater flight for a songbird, flying 1,800 miles non-stop for 3 days over the Atlantic Ocean.
Frequently Asked Questions
How do birds know where to go?
Birds know where to go during migration by relying on their instincts and experience and following magnetic fields, celestial cues, and landmarks.
Do birds have specific flyways?
Birds have specific flyways—established migration routes that they follow annually, often characterized by predictable stopover sites.
Which birds cross the longest distances during migration?
The Arctic Tern crosses the longest distances during migration, traveling up to 44,000 miles annually between its breeding grounds in the Arctic and its wintering grounds in the Antarctic.
Do woodpeckers migrate?
Some woodpecker species migrate, but many do not. For example, the Yellow-bellied Sapsucker is a migratory woodpecker, traveling between its breeding grounds in North America and its wintering grounds in the southern United States, Central America, and the Caribbean, while other species like the Downy Woodpecker are typically resident and do not migrate.
Do owls migrate?
Some owl species migrate, while others do not. For example, the Snowy Owl is known for its irruptive migrations, moving south from the Arctic in response to prey availability, whereas species like the Great Horned Owl are generally resident and do not migrate.
Do cardinals migrate?
Northern Cardinals do not migrate; they are year-round residents throughout their range in North and Central America.
