Another issue is verb agreement when both subject and predicative expression are noun phrases (and differ in number or person): in English, the copula typically agrees with the syntactical subject even if it is not logically (i.e. semantically) the subject, as in "the cause of the riot is" (not "are") "these pictures of the wall". Compare Italian "la causa della rivolta sono" ("are", not "è" "is") "queste foto del muro". In instances where a syntactical subject comprises a prepositional object that is a pluralized, however, the prepositional object agrees with the predicative expression, e.g. "What kind "of birds are" those?"