Birds migrate twice a year, traveling thousands of miles between their breeding grounds and their wintering grounds. But how do they know when it’s time to head back north again for breeding season? Here we’ll explore the cues birds use to determine when to migrate.
Changes in Day Length
One of the most important factors controlling migration is change in day length. As days start to get longer in late winter and early spring, birds detect the increase in daylight which stimulates them to migrate back north and begin breeding activities. This response is triggered by hormones that are activated by the longer days.
Birds have special photoreceptors in their eyes that allow them to detect small, incremental changes in day length over the seasons. Even a difference of only a few minutes of light is enough to stimulate migratory activity. This gives birds an advantage over other animals that may not pick up on these subtle light changes.
Different species rely on day length cues to different degrees. For some, like American Robins, increased day length alone is enough to trigger migration. Other species, like Black-capped Chickadees, require additional cues as well.
Changing Temperatures
Increasing temperatures are another sign to birds that spring is on its way. As the northern hemisphere warms in spring, temperatures provide an additional signal, beyond day length, that northerly migration should commence.
Some birds appear to be particularly sensitive to temperature shifts. Tree Swallows, for example, arrive on their breeding grounds earlier in years with warmer spring temperatures. Temperature likely interacts with day length, reinforcing photoperiodic cues that it’s time to migrate north.
Availability of Food
The availability of food resources is another factor in migration timing for some bird species. Insects, seeds, fruits and other foods necessary for breeding and raising chicks become available as spring blooms. The presence of these essential foods provides confirmation to birds that conditions are suitable for nesting in their summer habitat.
Purple Martins, for instance, time their migration to coincide with the emergence of flying insects that they feed on. If food is scarce for a prolonged time, some birds may delay nesting or produce fewer eggs.
Genetics and Experience
Migration timing is also influenced by genetics and previous experience. Many migratory behaviors are inherited, with timing pre-programmed by the bird’s genes. Individuals from northern versus southern populations often have genetically encoded differences in when they migrate.
Previous experience also plays a role. Older birds that have made the journey before likely remember environmental cues and conditions from prior trips. Their past migrations help guide their current travels. Young birds on their first migration rely more on innate genetic programs until they gain experience.
How Birds Detect Migration Cues
But how exactly do birds pick up on signals like changes in day length? And how do external cues get translated into migratory activity?
Birds have a variety of specialized systems and capabilities that allow them to register and respond to migration triggers:
- Photoreceptors in their eyes detect subtle light fluctuations
- Pineal glands in their brains register day/night cycles
- Hormones like corticosterone influence migration and navigation
- Fat stores fuel long flights and provide feedback on timing
- Circadian rhythms entrain birds to environmental signals
- Genetic programs influence timing on a population level
Together, these systems allow birds to accurately monitor conditions, prime their physiology, and know when it’s time to head north.
Is Climate Change Affecting Migration Timing?
There is evidence that climate change is impacting when birds migrate. With global temperatures on the rise, many species are now arriving on their breeding grounds earlier as spring is advancing.
One study found that over a 50 year period, the spring migration period advanced by over 10 days for certain bird species in the UK. Tree Swallows in North America now arrive around 15 days earlier than they did just a few decades ago.
However, some birds have not advanced their migration as much as spring itself has shifted. This means they are arriving after the peak availability of food sources like insects and plants. Mistimed migration could negatively impact breeding success and populations.
Plus, temperatures are often highly variable between years. If birds base their migration off of a warm year, they may arrive too early in a subsequently colder season. More research is needed to fully understand how climate change may disrupt the cues birds use to migrate optimally.
The Incredible Migrations Birds Undertake Each Year
Using day length, temperature, food availability and other cues, birds undertake absolutely incredible migration journeys each year. Some astounding examples include:
- Arctic Terns travel from the Arctic to the Antarctic and back – that’s over 40,000 miles round trip!
- Bar-tailed Godwits make a nonstop flight of over 7,000 miles from Alaska to New Zealand.
- Ruby-throated Hummingbirds, weighing just a few grams, migrate across the Gulf of Mexico, a 500 mile journey.
- Indian House Crows migrate in massive flocks of 200-350 million birds between India and east Africa.
These marathon migrations demonstrate the incredible abilities of birds to navigate over vast distances to find the best habitats year round.
How Does Migration Help Birds Survive?
Why do birds migrate in the first place? What are the advantages of this strenuous process? Migration helps birds optimize their breeding and survival in a number of key ways:
- Access to food – Birds migrate to take advantage of seasonal peaks in food availability
- Reproductive success – Longer days in the north allow some species to raise multiple clutches per season
- Predator avoidance – Migration removes birds from predator populations during vulnerable seasons
- Competition reduction – Migration reduces competition for limited resources
- Ideal nesting habitat – Northern breeding grounds offer excellent nesting locations
In short, migration enables bird species to take advantage of the best conditions and resources across broad geographic areas during different parts of the year.
Remaining Questions About Bird Migration
Research has shed light on many aspects of bird migration, but key questions still remain, including:
- How will climate change impact migration cues and timing in the long-term?
- What are the full genetic mechanisms controlling migratory behaviors?
- How are very young birds able to navigate during their first migration?
- Do birds have magnetic or other non-visual sensory capabilities assisting navigation?
- How do birds develop site fidelity and return to the exact same nesting sites each year?
Ongoing studies tracking bird movements and genetics will help reveal the inner workings of these impressive migratory patterns.
Conclusion
Birds have evolved to utilize environmental cues like changes in day length and temperature to determine when to migrate north and south each year. Their complex physiological systems allow them to detect and respond to seasonal shifts in light, food availability and weather across thousands of miles. While climate change may disrupt some migration patterns, birds demonstrate remarkable adaptations that ensure continued survival of their awe-inspiring annual journeys.