Bird navigation using magnetic fields

In Girl in the Glass, Declan explains that he can sense the Epistoliths. When he’s told this is impossible, he replies, “Why not? Birds sense the Earth’s electromagnetic field and use it for navigation. Perhaps the Epistoliths interact with me in a similar fashion.”

As a biologist, Declan is aware that birds navigate long distances using the Earth's magnetic field, although the mechanism of action wouldn’t be understood until the 1960s. Birds have specialized proteins in their eyes, cytochromes, that are sensitive to magnetic fields. This allows them to "see" the magnetic field, possibly as light and dark patterns, even when weather conditions limit their visual cues.

When a photon of blue light hits a cryptochrome molecule, it reacts to create particles sensitive to magnetic fields, influencing the signal sent to the bird's brain. This helps them navigate using the Earth's magnetic field like a compass to find north-south directions. Birds may also use the variations in the magnetic field's intensity to create a magnetic map of their environment. 

Birds also rely on visual cues, such as using the sun's position and the polarized light patterns to determine direction. At night, they use the stars for navigation, and during the daytime, familiar landmarks like coastlines, mountains, and rivers can also help birds navigate. However, when weather conditions are poor, birds can still travel using the Earth’s magnetic field. Declan might be a few years ahead on his recognition of this concept, but he’s a very perceptive, scholarly biologist.