My interpretation is, Maki is his home. So whatever it is, he will always come back to him because Maki is everything for him. Even without the imprinting. I feel like what he felt for Maki without the imprinting also tells us a bit about Ren's feelings tailing towards the end of his life. Ren's memories somehow still affect his feelings. And maybe that's why he was so in love and attached to Maki. Maybe Ren meant for his participation for the memory and dna extraction as a closure for his feelings towards Maki's brother. But who knows, maybe he realized at the last minute all that is going to shit and the only thing he can say to Maki is sorry. Sorry for making him as a substitute of his brother all this time. And sorry it was too late for him to realize his own feelings for Maki. The manga painted Ren as betraying Maki, but with Arata, I felt like Ren sort of get a 2nd chance in loving Maki again. Well that's just my take for the manga.
the metaphor about the bird being arata and hIM GOING BACK TO THE BIRDHOUSE ON HIS OWN FREE WILL WITHOUT NEEDING A CAGE AKA THE IMPRINTING IM LIVING