I noticed the same thing. I’m not sure I agree, Speargrass; beings whose lives are fraught with danger might find a great deal of solace in supernatural appeals.
The owl cult is a cool idea.
Some ideas -
Given that just a trip to the next town is fraught with danger, there could be quite a good trade in talismans or other luck charms.
In pre-settlement times (perhaps when mice rarely lived longer than 4 or 5 years), old age itself might be have been seen as mystical. The oldest and most experienced mice could have been enormous assets for a small clan.
I imagine an oral tradition containing myths of the ‘fast mice’ or ‘lucky mice’ who could outwit foxes, or who first brought fire, or writing - or who first tamed bees - to other mice.
Perhaps (given their place in the ecology), stealing is a sacred activity, despite being officially illegal. Though a millennium of settlement has made it unfashionable, reverence for Bart the Narrow is still popular among foragers and others who must face danger to find food. Many a follower will turn a blind eye to minor theft, particularly if it’s out of apparent necessity.
I imagine beliefs about the unseen mice. With so many deaths unexplained (lost, fallen into holes, snapped up by birds, swept away by water) mice revere the unseen mice who have disappeared.
Mice sing songs about the unseen dancing in the moonlight, or curled up eternally in Bart’s burrow - or dallying with nubile field mice in a field of grain.
Mice occasionally honour one another by inviting them to be ‘seen as’ a cherished unseen relative. At family functions like weddings, in times of calamity it can be necessary for stand-ins to represent whole households.
Ivydale concludes their week of Morten-Harvest celebrations with the chirping of three score crickets arranged by the forage-master, symbolically calling home that year’s unseen before winter’s snows make their return impossible.
The whole phenomenon is made more excruciating by the fact that some do return, weeks or months later. Special markers are used for the graves whose deaths are unexplained. “Once was Tuck the Lucky, thrice unseen and come home to me. No more.”