Fig. 1

Sketch of solar geolocation principles and our method of analysis. A solar geolocator (shown in black on bird’s back) records light levels and times (example raw data in lower panel). When the animal moves, its position is unobserved (hidden), but it can be estimated by the pattern of light changes measured at twilight. We combine a physical (observation) model about how light levels change with position and time with some basic knowledge of the patterns of movement between twilights (movement model) along with all previous and subsequent positions in a hidden Markov model framework. Then, using the particle filter, we arrive at the most likely position and movement for each twilight