The device has a sensor that periodically measures and records the ambient light level. Following retrieval of the device, the light level records are converted into latitudes and longitudes, which allows the geographic position of the animal to be estimated twice daily.The instrument is also configured to detect and record the history of time spent in water against time spent flying or on land. For seabirds, this can provide time-budget data related to activity and feeding behaviour. Small size, a low profile and robust packaging are crucial for effective deployments on flying birds and diving animals. Long battery life is often essential (e.g. after fledging, juvenile wandering albatrosses Diomedea exulans usually remain at sea for at least five to seven years). Low unit and operation costs are also very desirable, together with simplicity of operation. Dimensions of the instrument are 22◊19◊12 mm. Weight is 9 g in air and 4 g in water. The device is pressure tested to 800 m depth of water,and can log data for up to eight years. The total cost of components is less than $70. The principles, design specification and circuit description of the device are presented here, together with a representative migration track, obtained during field tests on grey-headed albatrosses Thalassarche chrysostoma, to illustrate performance.