Depleted liver macrophages left pigeon navigation intact in sun but impaired it under clouds

Homing pigeons whose liver macrophages had been depleted navigated a 19-kilometer trained route without difficulty under sunny skies but struggled to find their way home when skies were overcast. @ScienceMagazine reported that the birds normally orient by the Sun's position in clear weather and switch to a magnetic sense when clouds obscure the Sun.

Pigeons with intact liver macrophages completed the route under overcast conditions without issue.

Researchers knocked out the macrophages by injecting clodronate liposomes. The treated pigeons retained normal navigation on sunny days. Under cloudy skies, however, they showed clear impairment. The outcome indicates that liver macrophages participate in the magnetic sense used by the birds during overcast flights.