['DNA barcode differences within animal species are usually much less than differences among species, making it generally straightforward to match unknowns to a reference library. Here we aim to better understand the evolutionary mechanisms underlying this usual "barcode gap" pattern. We employ avian barcode libraries to test a central prediction of neutral theory, namely, intraspecific variation equals 2 Nµ, where N is population size and µ is mutations per site per generation. Birds are uniquely suited for this task: they have the best-known species limits, are well represented in barcode libraries, and, most critically, are the only large group with documented census population sizes. In addition, we ask if mitochondrial molecular clock measurements conform to neutral theory prediction of clock rate equals µ.', 'Intraspecific COI barcode variation was uniformly low regardless of census population size (n\u200a=\u200a142 species in 15 families). Apparent outliers reflected lumping of reproductively isolated populations or hybrid lineages. Re-analysis of a published survey of cytochrome b variation in diverse birds (n\u200a=\u200a93 species in 39 families) further confirmed uniformly low intraspecific variation. Hybridization/gene flow among species/populations was the main limitation to DNA barcode identification.'] Does dNA barcoding work in practice but not in ( neutral ) theory?