Anagoya university study found that birds start singing in the spring when the days are what?

The Nagoya University study headed by Takashi Yoshimura revealed that birds start singing in spring when days are longer. Their behavior was linked to a hormone called Thyrotrophin which is triggered during longer days. The process results in the expansion of the male testes in birds, which make them sing.

Cultural convergence is the theory that two cultures will be more and more like each other a very notable example of this is around the u.s.-mexico border, where 

Ihave no idea. about. that