Final answer:
Bird populations may become separate species due to reproductive isolation caused by differences in mating calls, which prevents them from interbreeding and leads to speciation over time.
Step-by-step explanation:
Bird populations that do not interbreed because they cannot recognize each other's mating calls may evolve into separate species due to reproductive isolation. Reproductive isolation can occur due to several mechanisms, including behavioral differences like varying mating calls. Over time, this isolation prevents the exchange of genetic material between the two populations and subject to different environmental pressures and natural selection, may lead to speciation. This is called allopatric speciation when it occurs because of geographical separation and sympatric speciation when it occurs within the same geographical area due to other reproductive barriers.