It is based on the very marginal phenomenon of tool use, nest building is completely neglected. The nest building behaviour provides arguments to even speak of the 'psychology of the nest'. This implies psychological components. E. g. babies were observed playing 'to construct nests'. Stability resulting from construction is crucial for the nest's function as a protecting platform for the night.
Aesthetic qualities are absent.
These psychological aspects show clearly that the nest is intrinsically interwoven with the existence of the great apes. A definite artifact is created that suggests a high degree of identification of its maker with the produced object and its complex functions. Finally we should also ask for semantic criteria. As it seems there are no reports on semantic aspects of nest building behaviour, but recently a very interesting study has been published which shows that the Bonobos have a fairly elaborated system of traffic signs. A large group split into two parts.