This article proposes a new interpretation of part of Varro's description of his aviary, i.e. rust. 3,5,14-15. More precisely, it considers the method of feeding the birds in the cage in the tholus, which is different from the method of feeding them in the cages in the porticoes (rust. 3,5,11). In the cages in the porticoes, the birds are fed per retem, while in the cage in the tholus they receive their food sub retem. The reason for this difference is to be found in the materials chosen for building the cages and in the reasons for these choices.