Types of Migration
The Marvel of Bird Migration
Imagine being able to ‘cheat’ the seasons and live in an everlasting summer, or at least avoid the worst of the winter chill. Well, many birds do just that. The power of flight enables avians to move great distances to avoid food shortages in the colder months, and as much as forty percent of the world’s birds are considered regular migrants.
However, migration is a complex set of behaviors rather than a simple action, and birds have developed many different strategies. From altitudinal migrants that rise and fall with the seasons to nomads that go wherever they must to find food, birds don’t always follow the classic north/south migration path.
In this guide, we’ll discuss some of the major types of bird migration. Read along to learn where birds go and how they cope with changing seasons and food availability.
Northern Pintails (pictured) migrate at night traveling at speeds of nearly 50 miles an hour
Understanding Bird Migration
Definition and Importance
Migration is the movement of birds from one area to another, usually on a seasonal basis, and from areas with low food availability to high food availability.
This typically means heading north in the spring to nest in the higher latitude part of their range and then returning south in the fall to overwinter at lower latitudes. However, there are some other important migratory strategies that we’ll delve into later in this guide.
The Value of Migration
Migration doesn’t only benefit the birds themselves but also all the plants and animals that interact with them in their natural habitats. As vital components of healthy ecosystems, migratory birds contribute by dispersing seeds, pollinating plants, and occupying various levels on the food chain as both predators and prey.
Continue reading to learn about four major migration strategies.
Pictured: Snow Geese in-flight. Seasonal migration is the most common form of bird movement
Different Types of Migration
Seasonal Migration
Seasonal migration is the most common form of bird movement. It involves short (just a few miles) to very long (thousands of miles) distance movements that see birds travel across varied landscapes, often moving between different countries, continents, and even hemispheres.
Purpose
As the seasons change and the days grow shorter, plants and insects enter a phase of dormancy, and the scenery may change to dead and dry vegetation, bare shrubs and trees, and, in some areas, a blanket of snow and ice. This halt in primary production forces many birds to move away to warmer regions with longer days and richer food supplies.
Site fidelity
Seasonal migrations can be somewhat flexible (as you’ll learn a little later in this guide), but many species are highly predictable in their movements. Some birds return to precisely the same nesting and overwintering sites year after year.
Notable Examples of Seasonal Migrants
A notable example of a seasonal migrant is the Ruby-throated Hummingbird (pictured)
Altitudinal Migration
Altitudinal migration is a form of seasonal migration where birds move between high and low elevations rather than between high and low latitudes.
Worldwide, over 1,200 birds are thought to be altitudinal migrants, representing over 10% of all species and including over 20% of North American birds. Examples occur everywhere from the Arctic to the tropics, including species from many families.
Purpose
As a general rule, daytime temperatures decrease with altitude due to an effect known as lapse rate. While conditions may be ideal for feeding and breeding at high altitudes in summer, birds may be forced to move to lower elevations for the coldest months of fall and winter.
Notable Examples of Altitudinal Migrants
- Mountain Quail
- Northern Saw-whet Owl
- American Dipper
- Western Bluebird
- Skylark
- Brewer’s Blackbird
- House Finch
- Snow Bunting
The Western Bluebird (pictured) is an altitudinal migrant
Partial Migration
Some bird species are partially migratory, with some of the population remaining in the same area all year and the rest making seasonal migrations. This type of migration is typical in species with extensive distributions covering areas suitable for both seasonal and year-round occupation.
Purpose
Like other migration behaviors, limited food resources are the primary cause of partial migrations. However, changes in food availability can vary from year to year depending on factors like rainfall and temperature, so individuals or populations may vary their movements somewhat.
Sometimes, individuals from the same local population stay put while others migrate. The following factors could cause this behavior:
- Some of the population are fit enough to survive the winter, or alternatively, some of the population are fit enough to migrate.
- Dominant individuals defend the limited resources in winter, forcing others to disperse.
- Part of the population is genetically ‘wired’ to migrate.
Notable Examples of Partial Migrants
- Mallard
- Great Blue Heron
- Bald Eagle
- Killdeer
- European Starlings
- Song Thrush
- Lapwing
- American Robin
- European Robin
Some bird species are partially migratory such as the American Bald Eagle (pictured)
Irruptive Migration
Irruptions are irregular migrations that see large numbers of birds disperse from overpopulated or resource-depleted areas. During irruption years, birdwatchers may spot unusually high numbers of birds that are typically rare or absent from their region.
Purpose
Food resources aren’t always predictable from one year to the next, so birds don’t necessarily ‘yo-yo’ between north and south or high and low elevations.
Some adaptable birds take a more nomadic approach and go wherever food sources are most available. Erratic rainfall patterns in arid areas and failed fruit and nut crops cause local irruptions of birds in some years.
Notable Examples of Irruptive Migrants:
- Black-capped Chickadee
- Waxwing
- Redpoll
- Crossbill
- Evening Grosbeak
- Pine Siskin
- Great Spotted Woodpecker
- Snowy Owl
- Rough-legged Hawk
A notable example of an 'Irruptive Migrant' is the Rough-legged Hawk (pictured)
Challenges and Adaptations
Migration Challenges
Migration is a taxing and dangerous strategy that many bird species rely on for survival and reproduction. Read on to learn about some of the challenges migrating birds face and how they are adapted to overcome these hurdles.
Fuel
Flying long distances requires an extraordinary amount of energy, so some birds fuel up in advance of their journey by storing large amounts of body fat. They may also make frequent stops to feed along the way, sometimes lasting weeks at a time.
Navigation
Traveling across countries and continents to arrive at the same breeding sites year after year might sound miraculous, but birds are perfectly equipped for the job. They find their way using some remarkable techniques, including seeing the Earth’s magnetic field, navigating by the stars, and remembering familiar landmarks.
Weather
Strong winds and heavy rain are a challenge for migratory birds that can cause them to veer far off course. Bad weather during an ocean crossing can also prove fatal if birds have to fight against powerful winds.
Pictured: A Snowy Owl taking-off during the winter
Predation
Predation is a constant threat to small and medium-sized birds, although few predators can catch them in flight. They are most at risk from aerial hunting birds of prey like Falcons, although they face many other enemies at stopover sites. Migrating at night is a strategy that helps many birds avoid predation.
Hunting
Many birds visit the same migration stopover sites year after year, which puts them at high risk in countries where hunting is not regulated.
Collisions
Birds that migrate at night are particularly vulnerable to collisions with human infrastructure like skyscrapers and towers. Artificially lit buildings are thought to confuse migrating birds that navigate by starlight.
Climate change
Accelerated climate change is a new threat that may affect migratory birds by altering their habitats and the timing of seasonal changes like the budding of deciduous plants and the emergence of insects. Some birds are adjusting their migratory paths and timing to compensate, but the net results are yet to be seen.
Small and medium-sized birds are most at risk from aerial hunting birds of prey like the Peregrine Falcon (pictured)
Conservation Considerations
Sadly, migratory birds are in decline worldwide due to habitat loss and other factors. Protecting them is a complicated task due to logistical and political challenges, but also because of the effective size of the areas these birds use.
Many migrants rely on suitable habitats all along their migratory path, unlike resident birds that can spend their entire lives in a single protected area. Human population growth, increasing urbanization, and agricultural and industrial development are major challenges that threaten their future.
The Common Redpoll is an Irruptive Migrant. They seemingly explode in numbers during irruption years
Summary
From Arctic Terns that bounce between the northern and southern hemispheres to Redpolls that seemingly explode in numbers during irruption years, each migratory species has developed its own solution to the problem of food shortages.
Spare a thought for migrating birds and the challenges they must overcome, and wish them well on their next journey!
Recap
- Birds migrate primarily to avoid food shortages.
- Migrations may be highly consistent or irregular.
- Most migratory birds in the northern hemisphere migrate north to south in the fall/autumn and return north in the spring.
- Altitudinal migrants move between high and low elevations as the seasons change.
- Migratory birds are well adapted to navigate and travel, although habitat loss is a major threat to their future.