Did complex song and dance co-evolve with brain size in the Birds of Paradise (Aves: Paradisaeidae)?

Complex signalling behaviors, such as avian song and courtship displays,
have been associated with increases in both absolute and relative
brain size, presumably mediated by relationships between brain
size and motor coordination and learning abilities. Nonetheless,
reliably establishing these relationships is challenging due
to the difficulty of quantifying behavioral complexity. Birds of
paradise perform some of the most elaborate avian courtship
displays, involving a striking degree of complexity and motor
coordination, which makes them interesting models for investigating
the evolution of complex motor learning abilities in birds. Here,
we compiled a μCT-based brain size dataset of more than half of
the known species of birds of paradise and used multiple metrics
of display complexity to test the hypothesis that larger brains
are associated with more elaborate repertoires of motor and vocal
displays. We found no clear evidence that the complexity of
courtship display behaviors co-evolved with brain size among the
birds of paradise: this conclusion held across three kinds of
behavioral and two kinds of vocal complexity scores.