Identifying moult periods in dual-equipped birds
We then assumed the corrected proportion of time spent dry per day was time in flight (propflight) and calculated the 5-day rolling average of this (propflight-5). Only values with a predicted solar angle of > -6 were used for this analysis, as some anomalous sustained dry periods were retained in nocturnal data points. Though these periods could in fact be due to sustained flight occurring at night, it is more likely due to puffins tucking both legs at once (Robertson et al. 2012, pers. obs.), or visits to the nest burrow approaching the breeding season. The tucking correction applied earlier will not have captured this on either leg’s logger, as the expected light level at these low solar angles (< -6 degrees) is zero. An inferred flightless period of minimum 30 consecutive days was identified as a persistently low set of values of propflight-5, identified using an incrementing threshold. Put simply, days with propflight-5 below this threshold value were defined as flightless. This value was iteratively increased from 0 by 0.0002 increments until a minimum sequence of 30 consecutive days were defined as flightless. This minimum duration was defined according to previous estimates of this species (Harris et al. 2010) and observations of captive puffins (D. Dial, National Aquarium USA, pers. comms). Moult was therefore defined as a continuous period of at least 30 days, during which little to no flight was inferred to have occurred by the processed immersion data. The same process was repeated, omitting any initially identified moult, to explore the possibility of a second flightless moult.