The only animals I see around here are squirrels, various bird species, cats, an occasional ground hog, opossum, or raccoon. They are mainly going after the small ones.

Could also be cutworms... Go out at night with a torch and if they are the culprit, you'll find them gnawing at the stem right above soil level.

What are possible solutions? My daylilies, irises and other plants do not seem to be affected.

I've found Plankskydd to be useful for many critters, the spray on kind, not the granules.

Also Rosemary and/or peppermint oil on cottonballs near plants, using old medicine bottles or the like, the protect from wetness and aid in longevity. Drill hole in plastic bottle bottom and leave cap off. Lay bottle on side. Scent can deter.

To be 100% sure: install a camera. There are endless possibilities and guessing doesn't get you anywhere.