The social factor in song learning in american sparrows  (Page 7/9)

In 2001, Nordby et al. note that there is something logically wrong with the strict 50-day sensitive period. They reasoned that since it is advantageous for songbirds to share songs with neighbors (for reasons explained in previous sections), song learning should continue into the first fall and winter, when juvenile songbirds have a better idea of who their neighbors will be the following spring. Nordby et al. performed their own experiments and found that song sparrows can learn songs as late as 147 days old. In fact, half of the chicks in their experiment learned most of the songs in their repertoire from the tutor that they were exposed to only during their first fall. Songs learned during this time were also very accurate. This kind of learning (learning after the supposed sensitive period) is called de novo late learning. In most studies, tutors during the early sensitive period are conveniently called “early” tutors, while tutors for de novo late learning are called “late” tutors.

Burt et al. (2007), however, are unable to show that juveniles actually learn more from tutors they are exposed to later in life, a finding that appears to contradict Nordby et al. (2001)’s argument for the importance of learning from late tutors. The group performed an experiment where juvenile song sparrows are exposed to two song tutors only during their natal spring and summer (when they are less than 50 days old) and to two different song tutors only during their first winter (when they are at least 200 days old). In their experimental setup, the two late tutors each had similar songs to one of the two early tutors; one of the late tutors interacted with the juveniles, while the other one did not. However, while the authors found that juveniles learn more from the early tutors, the late tutors still have a large effect on song learning. Juveniles learned more songs from the late tutor and its early counterpart if they interacted with the late tutor (Burt et al. 2007).

Whether songbirds learn whole songs when they are young and then modify and trim their song storage once they meet their neighbors (selective attrition) or learn whole songs throughout the first year of their life (de novo late learning) is still under debate. Most researchers agree, however, that there is a sensitive period for song learning and it varies with each individual bird (Baptista&Petrinovich 1986; Nelson&Marler 1994). Also, songbirds become less and less able to acquire songs as they age; even within the so-called sensitive period, sensitivity to song learning decreases with time (Baptista&Petrinovich 1986; Baptista et al. 1993). This decrease in the ability to acquire songs with age supports theories on the adaptive function of song learning. Song sparrows often return to their same territory year after year to nest (Nice 1941). Therefore, it is likely that as they age, birds come across the same neighbors every year. So once they are old, it is no longer adaptive to learn songs from neighbors because they already share songs with them.