Perhaps mud nests of a predatory wasp.

Yhose are the nests of Potters' wasps, a predatory wasp as Jean says. Caterpillars and grubs are the primary target of these particular wasps....each sealed clay pot will hold one egg and one larva as food for the developing wasp larva.

Those nests have either been opened from the inside by an emerging new wasp or from the outside by a bird who has learned what those little mud vessels hold.