Sensitivity to track initialization error is quantified as a function of clutter density for the nearest neighbor assignment function (NNAF). Two statistical distributions of track initialization error are derived under idealized hypothesis representing excellent and poor performance of the NNAF in clutter. A track initialization error region is obtained by constraining the mean initialization error (under the excellent NNAF performance hypothesis) to have significance level (alpha) (under the poor NNAF performance hypothesis). The initialization sensitivity region is derived for general nonlinear state equations and for measurement equations with additive Gaussian noise. Examples with constant velocity linear models are given. There exists a critical clutter density at which the initialization error region is the empty set. Thus, even perfect track initialization is a poor initialization in clutter whose density exceeds critical density. An explicit expression for the critical clutter density is derived.