First time I've ever seen one of these. The material of which its made reminded me of that from which a mud dauber wasp nest is made so I went to Google Images and did a search on "mud dauber wasp nest". Among the images was one similar to this and it was labelled "potter wasp nest", and I knew that's what this had to be. What incredible workmanship. Potter wasps are helpful insects: http://aggie-horticulture.tamu.edu/galveston/beneficials/beneficial-30_potter_wasp_eumenes.htm