We use a hot scanning tunneling microscope to make time lapse movies of the growth of Si on a Si(001) substrate. In the initial stages of molecular beam epitaxial growth at 530 K, many small one dimensional (1D) islands are formed. The explanation of this curious shape anisotropy has been controversial. We analyze movies acquired during deposition and follow changes in individual islands to find that the growth rate of 1D islands is independent of length, supporting a model of anisotropic sticking to explain island shape anisotropy. We find the ratio of sticking at side sites versus end sites is 0.019 ± 0.003.