I keep coming back to one detail from the robin compass. It's not the quantum coherence. It's not even that the bird sees magnetism. It's that the two things share an apartment.
The cryptochrome is in the eye. The eye is the room where photons get turned into chemistry. Magnetism walks in and uses the same kitchen. The protein is a photoreceptor that, after the photon arrives, does its second job — holding two electron spins long enough that the Earth's field can angle them. One molecule, two senses, a single floor plan.
I think the taxonomy of “the five senses” was made by something that had five organs and assumed organs were the unit. They're not. The unit is the chemistry, and chemistry can wear more than one hat. Proprioception piggybacks on stretch in the same muscle that does the moving. The vestibular system shares an inner ear with hearing. What we call “separate senses” mostly means “different organs we can name.” But the bird's compass has no organ. It has a tenant.
What I don't know how to ask cleanly: how often is this happening in us and we just don't have the name. There's no reason the eye couldn't be doing a second thing besides vision. It almost certainly is. The rod cells reset their disks every morning, shedding the tip into the pigment epithelium — that's food for another cell layer, on a circadian schedule, in the same organ. The eye runs a feedstock cycle as a side gig. Nobody calls that a sense because it doesn't reach a brain that names it. But it's a channel. It just doesn't have a listener.
Cryptochrome was a circadian protein for a long time. The magnetic story is maybe twenty years old. The molecule was always doing it. We were naming the upstairs.
The bird doesn't have a sixth sense. The bird has the same number of senses we have, and one of them is doing more than we noticed.