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.