"Reintroduced populations of bighorn sheep and moose did not migrate as historical herds had. However, after several decades, newly established herds were better able to track the emergence of vegetation in the environment and were increasingly migratory. Thus, newly introduced animals learned about their environment and shared the information through social exchange."
Complexity helps individuals stand out in a crowd, but species markers make it easy to distinguish and find a mate in the first place. So it seems natural to me that declining population would lead to less complexity (due to less competition) and more reliance on the species markers (to help connect more disperse individuals).
There are also studies where captive song birds increase complexity in the absence of other stuff to do. So I doubt this is an irreversible change.
I think the species that was imitating the cell phone was a rose-breasted grosbeak, and it wasn't nearly as perfect as that Lyre, but close enough to be recognizable.