This study provides evidence of morphological differences in support of the current phylogenetic division of the owls into two familes (Strigidae, Tytonidae), with respect to the digital tendon locking mechanism (TLM). This mechanism consists of modified surfaces on the flexor tendons in the digits, which, when engaged form a lock that holds the digits in a flexed position. Although species-specific differences in TLM structure were apparent, consistent inter-family differences occurred in the degree of expression of the TLM, its location within the digit, and in the structure and arrangement of the locking elements. Phylogenetic differences also occurred in the location of the associated digital ligaments. The TLM of the Strigidae was found to resemble that of the Falconiformes (Falconidae and Accipitridae). However, the Tytonidae possess a more distinctive pattern, characterised by a digit 1 TLM located at the opposite end of the digit and bearing reversed plicae. The biomechanics of the mechanism are considered, along with the functional implications of the varying structures in an attempt to understand the adaptiveness, or otherwise, of this-little known structure within an avian order.