Why and How?
Q. What causes robins to sing in the spring?
A. Singing, like many other behaviors (including migration) is governed primarily by changing day length (photoperiod, or amount of daylight).
days get longer, a bird's physiology changes, especially hormone levels.
Hormone levels stimulate appropriate changes in physiology and behavior
to prepare the bird for breeding. If you captured a robin in December
and placed it into a room and turned the lights on earlier and earlier
each day, you would stimulate the robin to enter breeding condition.
Q. How do baby birds learn their songs?
A. During the Critical Period, a young robin hears the song of its parents while in the nest and imprints on it. Then there is an 8-month Silent Period. This is followed by a Subsong Period, where the young robin starts emitting sounds. At first they are unstructured notes and, in a few weeks, the sounds become more like the species-specific song. With continued practice, the song becomes more crystallized, and eventually perfected.