cardinals/rummy noses

A strange thing happens every couple of days or so. When one of my cardinals, looks like a female and almost fully grown, swims it seems that a rummy nose follows her exactly, swimming closely underneath. I have witnessed this a few times already. What is it, some sort of ritual? Mating, maybe or just playing? I don't think my rummy noses are fully mature yet.

Any ideas?

Falcon