Biologists have also recorded whale songs and fragments of song themes in the colder, northern waters where whales feed in summer. The separate stocks mingle in the feeding grounds, and it is possible that the new season's song is developed and passed among males while feeding. Prior to this discovery, scientists had few theories about how whales managed to all learn the same song. According to one theory, whales broadcast the song over a deep-ocean thermocline, a layer of water of a temperature particularly suited to transmit the low frequencies of whale sounds. The likelihood of the song being learned in summer and fall while mingling opens up additional avenues for exploring whale behavior.
Some scientists now speculate that males sing to trigger the estrous cycle in females.
We may never learn the actual purpose of whale song. There is abundant evidence suggesting that male humpbacks use song as a warning to other males, in effect to stake out an acoustic spatial territory. On the other hand, song may be a competitive display, and only the best and most powerful singers will attract sexually receptive females. Songbirds also sing in identifiable, repetitive themes that vary according to the season and usually peak at breeding time. Geographically distinct bird populations also have different dialects.