Discrete animal-mounted sensors and tags have a wide range of potential applications for researching wild animals and their environments. These "biologgers" could be used to monitor the animal's location, metabolic output, or used as environmental monitoring sentinels. Such applications are made possible by recent decreases in the size, mass, and power consumption of modern microelectronics. Despite these performance increases, bio-logging systems need to generate power in-situ for extended deployments, and piezoelectric vibrational energy harvesters are a potential power source. In the past these devices have not been generally characterized, resulting in a lack of clarity as to their capabilities. In this dissertation, the design and limits of piezoelectric energy harvesting devices are explored. Given a set of design goals, these devices require careful consideration of the piezoelectric element thickness in order to maximize power output. Additionally, harvested power can be related to the material strength, providing a limit for the technology that spans device mass and the frequency of the harvested vibration. As efficiency is an important parameter for any device harvesting energy from an animal, this work explores how it is defined for these devices, proposes an alternative definition, and shows how conventional definitions can over predict efficiency. With an understanding of the capabilities of piezoelectric energy harvesting, the case for its viability as a power source for avian bio-logging applications is then presented. A method of estimating the harvestable energy available from a variety of bird species is shown to predict sufficient power available for the electronics typically used in bio-loggers. These results led to testing of piezoelectric devices on pigeons in flight. The results of the pigeon flight experiments show significant power production, with respect to bio-logger power requirements, and limited effects on the acceleration of the bird.