What is bird migration? - Why do some birds migrate at night?
Migration is one of the most important components of many bird species' annual life cycles. Learn more about bird migration and how it occurs!
Bird migration brings the hope and excitement of a new season of birds for birdwatchers and wildlife lovers across North America. However, migration is no simple task for these birds. It requires serious time, effort, and risk by the birds undertaking it.
What is bird migration?
Bird migration is the regular, seasonal movement of individual birds away from and back to the breeding grounds. This is different from dispersal, which is the movement of an individual bird from one breeding site to another. There are five basic categories of avian residency and migration:
A resident species is one that remains in the same region or location year-round.
A nomadic species is a bird species that moves irregularly, following no set pattern, usually in response to environmental variations such as food resources.
Partial migration is a pattern that varies across the population; some individuals within a population or species migrate, and others are residents.
Obligate migration is a pattern in which individuals follow the same sequence annually; the bird migrates to the same location on the same schedule each year.
A facultative migration is a pattern that can have multiple variations in the distance and timing as a result of response to environmental conditions.
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Why do birds migrate?
All birds migrate as a result of their adaptation to the variations that occur in their environment. What does that mean? Some birds migrate as a result of food unavailability. Some birds migrate due to weather impacts. Others move solely based on their biological clocks. Birds use these biological clocks to help them monitor both the time of day AND the time of year. Birds have the ability to sense the time of day, even when light cues, like the position of the sun, are unavailable. Birds can reset and synchronize these biological clocks using the photoperiod, which is the relative lengths of light and dark periods of the day. These explanations may not fully capture all reasons that birds migrate, but this covers the greatest number of reasons that birds migrate.
How many birds migrate?
Imagine you could see all the skies of North America at night during fall migration. Your eyes would be filled with not only stars but BILLIONS of birds moving, chipping, struggling, and striving to reach new grounds for the winter. Yes, BILLIONS! Between four and six billion birds move from north to south in the fall and two to four billion move north again in the spring. Why the difference in numbers between the seasons? Not all birds survive the arduous journey of migration. This rigorous trip claims the life of billions of birds each year. While many bird mortalities related to migration have natural causes, likely a greater number of bird losses are related to human-induced threats such as outdoor cats, windows, artificial lights, and habitat loss. These numbers can be staggering, but they do not provide all the details of the number of birds that migrate. We still need to consider the diversity of bird migration!
The three billion plus birds that migrate represent over 350 species of our regularly occurring terrestrial birds in the US and Canada. This does not include pelagic/oceanic species that migrate in the waters near the continent, but only the birds that are found regularly within the bounds of the lands we reside. These numbers represent a truly marvelous spectacle; with an abundance of diversity and sky-filling numbers, migration is awe-inspiring.
How do birds navigate during migration?
Birds orient themselves using a variety of intricate methods, including magnetic fields, the sun, stars, and smell! Many species of birds are able to sense the magnetic fields of the earth using specialized cells in the upper part of the bill containing magnetite. However, this is not the only magnetic sensing ability in birds. Several experiments focused on the abilities of birds to "see" magnetic fields have shown that birds can "see" direction based on a reaction between the magnetic fields and a special pigment in the avian eye. Birds are also able to use the sun to aid in orientation. While birds do not wear watches, they do need to be aware of the "time" to make proper use of the sun's position for orienteering. Birds also use the stars to aid in their migration navigation. However, it is not memorization of constellations but a determination of the location of the poles based upon the way that the stars "move" around the poles.
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Do all birds migrate at night?
In North America, 80% of migratory birds migrate at night. The majority of these nocturnal migrants are smaller species such as warblers, sparrows, wrens, and thrushes. Larger migratory birds are typically found migrating during the day when they can use thermal columns, or columns of rising air created by solar radiation, for lift and soaring. Globally, there has not been enough study to capture the true number of migratory bird species. There are estimates of 1800 long-distance migrants in the world, but this number does not capture all migratory methods.
Why do some birds migrate at night?
Nocturnal migration allows for safer passage and easier navigation for many birds. Flying in nighttime skies is usually accompanied by less wind, fewer predators, and access to stars for orienteering to breeding or wintering grounds.
Quite possibly, the most important factor for a migratory bird to make the decision to take flight is wind speed and direction. A tailwind helps a bird reduce energy output. A headwind can keep a bird grounded until conditions are favorable.
While there are predators out at night, typically, most birds will not experience them while flying at altitude. Until the birds land, the night sky can be safe for their migratory pathways until they experience human-created issues like skyglow, windows, and skyscrapers.
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How to help migratory birds
Migration is possibly the most grueling part of a bird's annual life cycle. Between natural predators, introduced predators, invasive competition, shrinking habitat, light pollution, chemical pollution, windows, skyscrapers, and others, birds face a gauntlet to survive a twice-annual migration. Because many of the dangers migratory birds face are the fault of humans, there are small, incremental steps we can take in our own backyards to help migratory birds:
These small steps are actions anyone can take at their own home. They require minimal costs, minimal time, and the smallest levels of effort. Yet, if we love birds, why aren't we all taking them?
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