We developed a simple behavioral indicator of prey patch conditions in diving animals. This indicator is a proportion of observed patch residence time to the "standard" residence time (POS). The standard residence time was defined as an optimal patch residence time maximizing the proportion of bottom time to the duration of dive cycle. We quantified the relationship between the POS and the prey richness index calculated from the number and size of prey-like objects on recorded still images from an animal-borne digital camera logger, and found significantly positive correlation between them. We found a similar positive correlation between the POS and a behavioral index (index of patch quality) developed previously in diving animals. These suggest that the POS is indeed a behavioral index of prey patch condition. Although this behavioral indicator was developed and tested in the context of foraging ecology of diving animals, this indicator can be applied to other central place foraging animals.