Loss of a putative magnetoreceptor in tyrant flycatchers
Migratory birds possess remarkable accuracy in orientation and navigation, which involves various compass systems including the magnetic compass. Identifying the primary magnetosensor remains a fundamental open question. Cryptochromes (Cry) have been shown to be magnetically sensitive, specifically Cry4 shows enhanced magnetic sensitivity in a migratory songbird compared to resident species.  Here, we investigate cryptochromes and their potential involvement in magnetoreception in a phylogenetic framework, integrating molecular evolutionary analyses with protein dynamics modeling. We base our analysis on 363 bird genomes and associate different selection regimes with migratory behaviour. We show that Cry4 is characterised by strong positive selection and high variability, typical characteristics of sensor proteins. We identify key sites that likely facilitated the evolution of a highly optimised sensory protein for night time compass orientation in songbirds. Additionally, we show that Cry4 was lost in hummingbirds, parrots and Tyranni (Suboscines) and thus identified a natural comparative gene knockout, which can be used to test the function of Cry4 in birds. In contrast, the other two cryptochromes present in birds Cry1 and Cry2, were highly conserved in all species, indicating basal, non-sensory functions. Our results support a specialization or functional differentiation in terms of the discussed magnetosensory functionality of Cry4 in songbirds.