Blackbird singing quietly in the undergrowth

I heard very quiet birdsong today. Thought it came from nearby, and found the source: a blackbird! Apparently, young birds will sing quietly to themselves to practice singing. This is called a subsong. The singing stopped when we made eye-contact, I imagine the blackbird suddenly felt self conscious.

Academic mentions of subsong seems to go back to this Thorpe and Pilcher (1958) paper, but I couldn’t find a pdf for it.

Papers:

• Thorpe, W. H., & Pilcher, P. M. (1958). The nature and characteristics of subsong. British birds, 51(12), 509-14.
• Dabelsteen, T., McGREGOR, P. K., Lampe, H. M., Langmore, N. E., & Holland, J. O. (1998). Quiet song in song birds: an overlooked phenomenon. Bioacoustics, 9(2), 89-105.