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.