Certain animals and insects can orient themselves to magnetic north through a sense known as magnetoreception (also known as magnetoception). This sense aids animals and birds as they navigate, sometimes migrating over thousands of miles.
For decades, researchers on theory for this ability in birds was iron-rich cells in beaks, although this theory was disproved a few years ago. More recently, a theory proposes that Cry4, a light-sensitive protein found in the eye’s retina, may be responsible for magnetoreception.
Previous research has looked into Cry4 and the most recent publication looked at gene expression of Cry1, Cry2 and Cry4 in the retina, muscle and brain of zebra finches to see how levels varied over the course of a day. The researchers hypothesized that retinal cryptochromes needed for magnetoreception would be measurable at constant level since birds use orientation not just for migration but also for daily spatial tasks.
While Cry1 and Cry2 show variation over the circadian rhythm, Cry4 levels remained constant, leading researchers to conclude that cry4 was the “most likely candidate magnetoreceptor of the light-dependent magnetic compass in birds.”
Pinzon-Rodriguez, A., Bensch, S., & Muheim, R. (2018). Expression patterns of cryptochrome genes in avian retina suggest involvement of Cry4 in light-dependent magnetoreception. Journal of The Royal Society Interface, 15(140), 20180058.