of black-capped chickadees suggest a selective mechanisms for neuronal survival in direct correlation with seasonal food caching behavior. Developmentally, progenitor cells called radial glial cells are thought to mitigate newly born neurons to their destinations. Radial glial cells extend processes from their soma in the avian ventricular zone to the parenchyma of the adult forebrain. These New neurons have been observed as early as 3 days after thymidine administration in the HVC and as early as 7 days before reaching the hippocampus. Avian migration of new neurons are analogous to mammalian species, providing a future direction in exploring neurogenesis in mammalian species and beyond. However, captivity has been shown to reduce hippocampal volumes when compared to wild counterparts. Reduced neurogenesis in captive birds may be caused by stress, lack of exercise, diminished social interaction, and limited caching opportunities. Tracking neurogenesis The creation of new functional neurons can be measured in several ways, summarized in the following sections. DNA labelling Labelled DNA can trace dividing cell's lineage, and determine the location of its daughter cells. A nucleic acid analog is inserted into the genome of a neuron-generating cell (such as a glial cell or neural stem cell). Thymine analogs (3H) thymidine and BrdU are commonly used DNA labels, and are used for radiolabelling and immunohistochemistry respectively. Fate determination via neuronal lineage markers DNA labeling can be used in conjunction with neuronal lineage markers to determine the fate of new functional brain cells. First, incorporated labeled nucleotides are used to detect the populations of newly divided daughter cells. Specific cell types are then determined with unique differences in their expression of proteins, which can be used as antigens in an immunoassay. For example, NeuN/Fox3 and GFAP are antigens commonly used to detect neurons, glia, and ependymal cells. Ki67 is the most commonly used antigen to detect cell proliferation. Some antigens can be used to measure specific stem cell stages. For example, stem cells requires the sox2 gene to maintain pluripotency and is used to detect enduring concentrations of stem cells in CNS tissue. The protein nestin is an intermediate filament, which is essential for the radial growth of axons, and is therefore used to detect the formation of new synapses. Cre-Lox recombination Some genetic tracing studies utilize cre-lox recombination to bind a promoter to a reporter gene, such as lacZ or GFP gene. This method can be used for long term quantification