[originaltext]　　The rising level of noise in urban areas may be preventing some

　　The rising level of noise in urban areas may be preventing some species of songbirds from setting up home in developed areas, a study has concluded. (19) Canadian researchers found that noisy surroundings masked the lower frequencies of bird songs, affecting the way some species communicated. (21) Unable to hear all elements of a song, females could perceive singing males as ill-suited mates, they added. There has been a growing interest in preserving or increasing the biodiversity of songbirds in urban areas. This leads to the investigation on whether there was a link between bird diversity and noise levels. Dr. Proppe is a professor of Calvin College. His team surveyed species at 113 sites in natural areas within the city of Edmonton. (20) They found that the number of species at each location tended to be lower when noise levels were higher. The decrease in species richness was one of the study’s major findings. The study also focused on seven species that did inhabit the area to see if their abundance was affected as the urban noise increased. What they found is that three of the species did have lower abundance in locations that were noisier. (21) Dr. Proppe suggested that females may perceive the song as abnormal if they could not hear the lower frequencies. If the adults stop pairing and mating, then the number of offspring would decrease as a result. Over a period of time, this could have a potential impact on the abundance of the species.
　　19. How does the urban noise affect the communication of birds?
　　20. What did Dr. Proppe’s team find?
　　21. Why do some birds stop pairing and mating in some urban areas?