A voyage to Africa by Mr Swift

A male common swift Apus apus was equipped with a light logger on August 5, 2010, and again captured in his nest 298 days later. The data stored in the light logger enables analysis of the fascinating travel it made in this time period. The state of the art algorithm for geolocation based on light loggers consists in computing first sunrise and sunset from the logged data, which are then converted to midday (gives longitude) and day length (gives latitude). This approach has singularities at the spring and fall equinoxes, and gives a bias for fast day transitions in the east-west direction. We derive a flexible particle filter solution, where sunset and sunrise are processed in separately measurement updates, and where the motion model has two modes, one for migration and one for stationary long time visits, which are designed to fit the flying pattern of the swift. This approach circumvents the aforementioned problems with singularity and bias, and provides realistic confidence bounds on the geolocation as well as an estimate of the migration mode.