Accounting for leg-tucking behaviour
Puffins tend to tuck their legs into their plumage while resting on the water, usually just one at a time, but sometimes both (supporting information figure S1, pers. obs.), leaving the logger dry despite the puffin being on the water surface (figure 1). This means it can be difficult to distinguish between flight and rest from a geolocator immersion signal alone (Fayet et al. 2016). We developed a new method that identified and accounted for leg-tucking using concurrent light signals. This works on the basis that light levels recorded by the logger will be lower than expected for a given solar angle if the logger is obscured (supporting information), i.e. tucked into plumage (figure 1). Solar angles were calculated based on astronomical formulae using date, time, and location. These lower-than-expected light fixes were classified as tucking, and were appended to concurrent immersion data using a time series merge implemented using the xts package (Ryan & Ulrich 2020) to account for missing or delayed points in either data stream. The immersion data points associated with these fixes were adjusted to 100% wet, to reflect that they were assumed to be resting on water despite the logger reading fully or partially dry. This correction could only be applied to data occurring during daylight hours (solar angle > -6 degrees) and not during the hours of darkness.