While almost all birds vocalize, only about half of them have musicality. They include wrens, thrushes, warblers, vireos, mimics, finches, and others, commonly called songbirds that are assigned to Passeri (Oscines), a suborder of the perching birds order (Passeriformes).
Songs vary from the simple whistle of the Black-capped Chickadee to the complex aria of the Wood Thrush. Some birds have a single song like the chickadee, while others have hundreds. Recordings of one particular Brown Thrasher revealed more than 2,000 distinct songs.
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Many species sing different songs in sequences that can be repeated perfectly. In addition to the virtuoso performances of adults, young songbirds must learn their songs — sometimes when barely out of the nest — from tutors. How is all of this possible?
The answer lies in the cleverly wired avian brain, of which three main functions relate to song learning and singing. It receives sensory information, processes the information, and sends motor impulses to a particular site to cause an effect, such as muscle contraction.
Pathways to birdsong
To better understand how the brain coordinates singing and song learning, take a look at the accompanying diagram (below). Two major motor pathways that carry messages from the brain — one red and one blue — are involved with singing. Surgical procedures have demonstrated that the red pathway controls singing, and the blue pathway coordinates song learning.
Within the pathways are enlarged structures called nuclei that consist of specialized nerve cells that control, integrate, process, or store information related to songs and singing. While this discussion will remain general, the nuclei are labeled for those who might be familiar with them, even though I won’t discuss them specifically.
The red pathway consists of two prominent nuclei (HVC and RA) and a connecting pathway to the cranial nerve (nXII) that innervates the sound production organ, the syrinx. HVC is the control center. The blue pathway also connects HVC to RA, but through a longer, more convoluted path that includes area X, DLM, and LMAN. A sensory path (green), associated with hearing, communicates field L with the motor nuclei HVC and RA (auditory information is likely stored in other areas as well).
Now consider a recently fledged male that needs to learn his songs. During this early period in his life, his brain is especially receptive to learning songs, and he learns from his father or other nearby adult males (tutors). The songs the youngster hears are brought from the inner ear to a hearing center in the brain (like field L) where they are stored in a precise way, like templates, for future reference. If a youngster cannot hear, he will not form a template and will not develop appropriate adult songs.
Soon after the youngster has filed away the songs from tutors, he begins to vocalize with brief notes and syllables. Over time, these little bits of songs are pieced together as subsongs, and later they’re modified to form the typical adult songs of the species.
For song learning and specific matching to occur, the youngster must be able to hear his own singing. While the exact mechanisms have not been worked out, the likely scenario is that as the youngster hears his songs, the audio information goes to a hearing center and then to HVC. When the young bird sings again, the information passes through the red pathway as usual and also through the blue pathway.
Somehow, in the maze of the blue pathway, the sound of the youngster’s song is compared with the tutor’s song, which is stored as a template. When this information arrives at RA, it modifies the “song producing message” coming down from HVC so the new song produced in the syrinx will sound more like the template. This pattern is repeated until the youngster’s song matches the template perfectly, and the song is then said to be crystallized.
It is also known that sex hormones play a role in brain-song interactions. A song cannot be crystallized or properly stored, for example, without an elevated level of testosterone.
While scientists are still debating whether mammalian brains can regenerate nerve cells, birdbrains can definitely do so. Information on the new crystallized songs is stored in newly formed cells of the HVC. The new cells seem to code for specific syllables or phrases, and many cells must fire in precise order to produce an entire song.
Some birds learn songs only in their first year, while others are able to learn in later periods of their life. It is believed that some of the later learning follows the pattern described, and some will involve only a modification of existing songs.
The beauty, complexity, and precise repeatability of songs for many songbirds borders on the unbelievable, as does the exactness of song learning by juveniles. All of this happens in a magnificent brain that is slow to give up its secrets and unmatched as the source of amazing avian behaviors.
This article from Eldon Greij’s column “Amazing Birds” appeared in the May/June 2019 issue of BirdWatching.