Fig. 3.

The number of [³H]thymidine-labeled neurons per day of treatment as a function of bird age at the time of injection (leftand middle). Birds were killed 4 months after [³H]thymidine injections. Eachtriangle represents one bird. Both hearing-intact birds and unilaterally deafened birds showed a decline in the total number of new HVC neurons. This was also true for adult-formed HVC-RA projection neurons (middle) identified by double labeling with [³H]thymidine and Fluoro-Gold. At 1 month after cell birth dating with BrdU, the number of new neurons double labeled with BrdU and Hu per day of BrdU injections also declined with bird age (right). These results indicate that the production or survival of new neurons while migrating or shortly after arrival in the HVC decreases with bird age.