The idea is that modern cities are so noisey that urban birds have to try to sing over city noise. This could mean that these birds become less attractive to females, this is becuase by trying to sing louder birds have less control over the sound they produce. The song may lose quality and become rougher sounding.
According to Dr Sue Anne Zollinger, of the University of St Andrews
The difference between urban and rural birdsong is becoming so great that the two groups could now be unable to communicate, leading to inbreeding and a weakened gene pool