About 60 years ago, it was discovered that birds can use the Earth’s magnetic field for finding directions (the magnetic compass sense). Since then, the magnetic compass has been demonstrated in many species, mainly by behavioural responses of birds (shift in oriented activity in round arenas) to a changed magnetic field. However, it is still poorly understood how exactly the magnetic sense is working and where magnetoreceptors reside in the bird’s body. Recent studies have suggested that a bird’s magnetic compass is a part of vision. Simplified, birds seem to ‘see’ magnetic field lines as visual patterns, perhaps superimposed on their normal visual perception of the world. Chemical reactions involving cryptochromes (CRYs) − a family of light-sensitive proteins − are assumed to play a crucial role in it.
It has been reported that the ability of birds to sense magnetic field depends on the presence of light with specific properties (e.g., only under specific wavelengths). To explain that, biophysicists proposed a Radical Pair Hypothesis (RPH). It suggests that CRYs form radicals (molecule with unpaired electrons) which are sensitive to the direction of ambient magnetic fields so that different alignment of radicals leads to different products in magnetoreceptors (supposedly, in photoreceptors where CRYs are found) depending on the direction of the magnetic field.
Image courtesy of DigitasLBi.
Though the RPH is still hard to conceive, there is a growing body of experimental support for it. One line of evidence comes from studies reporting that electromagnetic (EM) noise (0–10 MHz) disturbs the magnetic compass sense in captive birds. Biophysics explains that by the resonance effect−specific radio frequencies may disturb unpaired electrons in the CRYs’ radicals. Though RPH demands further investigations, the disturbing effect has been clearly shown by a few studies. Therefore, a question arises how human-induced and ubiquitous electromagnetic noise can affect birds’ magnetic sense in the wild? To address it, I proposed a set of experiments using homing pigeons. Trained pigeons return from tens of km and use both the sun (under clear sky) and magnetic (under total overcast) compass senses to keep directions. I will train young pigeons to home when they are deployed with miniature GPS trackers and micro-generators attached as little ‘backpacks’. These devices are now small and light enough to be carried by pigeons. GPS trackers will document birds’ trajectories whereas micro-generators will act as sources of controlled and harmless broadband radio frequencies (RF) disturbances.
By processing tracking data, I will analyse parameters of homing performance against absence/presence of RF disturbances with different characteristics, weather conditions and some other relevant factors. If RF disturbances would indeed affect birds’ magnetic compass sense (e.g. increase flight direction scatter), I will measure electromagnetic noise near some relevant anthropogenic sources (e.g. AM radio towers). Combining these measurements with the results of pigeons’ flights, I will build a computer model visualising how far the disturbing effect on the avian magnetic compass can propagate from anthropogenic sources in the wild. This project will be the first one testing if anthropogenic EM noise disrupts magnetic compass in freely living birds.
