From what data did Nicholls and Goldizen (2006) conclude that habitat structure affected signal production in satin bowerbirds?

A) Populations in different habitats differed in their genotypes.
B) Populations in different habitats produced different signals.
C) Males and females preferred different habitats.
D) Individuals produced different signals depending on their location within a habitat.
E) Individuals modified their habitat before producing a signal.