Scientists and the curious,
Scientists Mark McDonald of WhaleAcoustics in Bellvue, Colo., John Hildebrand of Scripps Oceanography, and Sarah Mesnick of NOAA Fisheries Southwest Fisheries Science Center, haven been studying data regarding the songs of blue whales from around the world, and noticed a pattern. The pitch, that is the audio frequency of the songs, has been steadily curbing downward.
After examining a number of possible reasons for the change in pitch, the scientists think it may be because there's been a slight increase in the numbers of blue whales, once commercial whaling was banned. With fewer whales, the whales were also spread farther apart, and the higher pitched songs would have traveled farther (despite the seeming contradiction in physics, this is because when they sing at a higher pitch, they also sing more loudly). In other words, the decrease may be a return to a more "traditional" style of singing.
The scientists published their research in the journal Endangered Species Research, but you can read about it in The Telegraph, and Science Daily. And thanks to National Geographic, here's a video of blue whales, complete with singing.