Each year, billions of birds migrate to and from the United States. Globally, these creatures travel thousands of miles to reach their seasonal habitats. Some, like the Arctic (Sterna paradisaea), accumulate remarkable mileage throughout their lives and famously return to their birthplace.
But how do birds navigate during these extensive migrations?
Birds wield a variety of sensory adaptations that aid in their navigation—some are well understood, while others remain a mystery to us.
“We know that birds utilize various cues to navigate their migratory paths,” Miriam Reedvogel, the director of the German Institute of Ornithology, shared with Live Science.
Vision and smell are fundamental cues for navigation. Birds often remember familiar landmarks, such as rivers and mountain ranges, from previous journeys. In contrast, seabirds, which travel over open waters, may lean more on their sense of smell. In a study, scientists found that when they blocked the nasal passage of Scopoli’s Shearwaters (Calonectris diomedea), the birds navigated fine over land but struggled over water.
Birds also utilize the sun and stars for orientation. During the day, they can use a Sun Compass. By integrating the sun’s position in the sky with their internal circadian rhythm, birds can navigate effectively. However, artificial light can disrupt this navigation, highlighting the sun compass’s critical role.
Many birds migrate at night, meaning the sun’s position becomes irrelevant. During these nocturnal flights, birds depend on the stars for navigation. They use a Star Compass by learning the constellations that guide them near the celestial pole, indicated by Polaris (the North Star).
Related: Why Aren’t All the Birds Flying?
Magnetic Field
But what happens when clouds obscure the sky, and birds cannot spot the sun, stars, or landmarks? That’s when their remarkable sense of magnetic reception comes into play. This ability enables birds to detect Earth’s magnetic field, generated by the molten metals in the planet’s core. It may sound like something out of science fiction, but research indicates that disturbances in magnetic fields can greatly affect birds’ navigational abilities. For instance, one study found that altering the magnetic field around pigeons disoriented them and affected their ability to return home.
While we know birds can sense magnetic fields, the exact mechanisms are still unclear. Peter Hore, a chemistry professor at Oxford University, believes birds may rely on chemical reactions that depend on the direction and strength of Earth’s magnetic field. While several theories exist, Hore posits that a molecule called cryptochrome within the birds’ retinas could be integral.
Lab experiments have shown that isolated cryptochrome reacts to magnetic fields when exposed to blue light. However, researchers still do not know how sensitive cryptochrome is to the minute variations found in Earth’s magnetic field.
“I know very little about the specifics of how this compass functions,” Hore mentioned. “We don’t even have a clear idea how many cryptochrome molecules reside in a bird’s retina.”
Additional studies suggest that the birds’ beaks may have magnetic receptors that interact with magnetite, an iron-based mineral situated at their beaks’ tips. These receptors connect to the brain via key neural pathways, implying they might offer another way for birds to gauge magnetic field strength.
Besides magnetic reception, birds can discern direction by detecting polarized light. This type of light features waves that vibrate in a specific aligned direction. Sunlight scatters through the Earth’s atmosphere, leading to a predictable polarization pattern. Birds can sense these patterns using specialized retinal cells, which inform them of the sun’s location even on overcast days.
Putting the Pieces Together
Birds employ various sensory methods depending on the situation, similar to how humans might navigate using sight in daylight but rely on touch in dimly lit rooms at night.
“Birds likely integrate multiple navigational cues, and we are confident that different indicators hold varying significance throughout their journeys,” Reedvogel noted. Hore corroborated this, whereas magnetic reception is less effective during thunderstorms or high solar activity, which can disrupt the Earth’s magnetic field.
Ultimately, these navigational strategies are also rooted in the genetic drives of migrating birds. They inherit the instinct to migrate from their parents, and the distance and direction of their travels are largely genetically determined. Researchers like Reedvogel are examining which specific genes impact migration and their functions.
Both scientists emphasize that understanding these systems is crucial for future bird conservation efforts. Relocating bird species has become a major focus within wildlife conservation, yet results have been inconsistent; in 45% of studies, it was found that relocated birds returned to their original locations. Analysis has revealed that human interventions to relocate birds have often been unsuccessful.
“Efforts to move these birds have not yielded good results,” Hore stated. “They are such adept navigators that, if displaced, they simply return home.”
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Source: www.livescience.com