Hi scavo, your glass birds are gorgeous.....and I am so pleased you showed them here and got an attribution.
Looking closely at Duncan Smith's bases, I think some of them may well be Agate slices (coloured).
I like this combination as, like your marble base it is a natural crystal formation which for me, sits well with glass and crystal sculptures.
Lovely....do you have any others to show and share with us? I would really love to see them.