Accelerated redevelopment of vocal skills is preceded by lasting reorganization of the song motor circuitry

Complex motor skills take considerable time and practice to learn. Without continued practice the level of skill performance quickly degrades, posing a problem for the timely utilization of skilled motor behaviors. Here we quantified the recurring development of vocal motor skills and the accompanying changes in synaptic connectivity in the brain of a songbird, while manipulating skill performance by consecutively administrating and withdrawing testosterone. We demonstrate that a songbird with prior singing experience can significantly accelerate the re-acquisition of vocal performance. We further demonstrate that an increase in vocal performance is accompanied by a pronounced synaptic pruning in the forebrain vocal motor area HVC, a reduction that is not reversed when birds stop singing. These results provide evidence that lasting synaptic changes in the motor circuitry are associated with the savings of motor skills, enabling a rapid recovery of motor performance under environmental time constraints.


Sample-size estimation
 You should state whether an appropriate sample size was computed when the study was being designed  You should state the statistical method of sample size computation and any required assumptions  If no explicit power analysis was used, you should describe how you decided what sample (replicate) size (number) to use Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission: Replicates  You should report how often each experiment was performed  You should include a definition of biological versus technical replication  The data obtained should be provided and sufficient information should be provided to indicate the number of independent biological and/or technical replicates  If you encountered any outliers, you should describe how these were handled  Criteria for exclusion/inclusion of data should be clearly stated  High-throughput sequence data should be uploaded before submission, with a private link for reviewers provided (these are available from both GEO and ArrayExpress) Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission:

P24:
No sample sizes were computed before the experiments, as no data was available on the possible effects of our hormone treatment assay on song acquisition speed and associated modifications in dendritic spine densities prior to this study.
eLife Sciences Publications, Ltd is a limited liability non-profit non-stock corporation incorporated in the State of Delaware, USA, with company number 5030732, and is registered in the UK with company number FC030576 and branch number BR015634 at the address 1st Floor, 24 Hills Road, Cambridge CB2 1JP | August 2014 2 P24: A technical replication was defined as a repeated measurement of the same individual and biological replications constitute the number of sampled individuals (n).
The number of independent biological and technical replicates are listed for each experiment in the Materials and Methods section (P19 -Song recordings and analysis; P22 -Spine and dendrite analysis). The number of biological replicates (n) is further provided in the legend of each corresponding figure.
P19: Recorded song data was computationally filtered before analysis to remove nonsong related cage noises from the data. P24: No outliers or further data were excluded from analysis. Statistical reporting  Statistical analysis methods should be described and justified  Raw data should be presented in figures whenever informative to do so (typically when N per group is less than 10)  For each experiment, you should identify the statistical tests used, exact values of N, definitions of center, methods of multiple test correction, and dispersion and precision measures (e.g., mean, median, SD, SEM, confidence intervals; and, for the major substantive results, a measure of effect size (e.g., Pearson's r, Cohen's d)  Report exact p-values wherever possible alongside the summary statistics and 95% confidence intervals. These should be reported for all key questions and not only when the p-value is less than 0.05.
Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission: (For large datasets, or papers with a very large number of statistical tests, you may upload a single table file with tests, Ns, etc., with reference to sections in the manuscript.)

Group allocation
 Indicate how samples were allocated into experimental groups (in the case of clinical studies, please specify allocation to treatment method); if randomization was used, please also state if restricted randomization was applied  Indicate if masking was used during group allocation, data collection and/or data analysis Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission: Additional data files ("source data")  We encourage you to upload relevant additional data files, such as numerical data that are represented as a graph in a figure, or as a summary table

P24:
The statistical tests that are used to determine group differences in each experiment are reported in the Materials and Methods section.
For each comparison statistical tests, N and P values are provided in the legends of the corresponding figures and in relevant places in the Results section.
The raw data used for analysis is provided as source data.

P24:
All measurement values in the text are given as means ± SEM, unless stated otherwise.

P19:
For the behavioral experiments, individuals were randomly selected from our colony, constituting a single group that was repeatedly measured. Group allocation for statistical analysis was determined by the different treatment stages of each individual.

P22-23:
For spine and dendrite quantifications, individuals were randomly allocated to the different experimental groups, and data was analyzed by an experimenter that was blind to the experimental condition of the animals.  Where provided, these should be in the most useful format, and they can be uploaded as "Source data" files linked to a main figure or table  Include model definition files including the full list of parameters used  Include code used for data analysis (e.g., R, MatLab)  Avoid stating that data files are "available upon request" Please indicate the figures or tables for which source data files have been provided: