DOI: 10.5061/dryad.rbnzs7h78
title: Use of sea ice by arctic terns Sterna paradisaea in Antarctica and impacts of climate change

description: Arctic Terns spend their breeding and non-breeding seasons in polar
 environments at opposite ends of the world. The sensitivity of polar
 regions to climate change makes it essential to understand the ecology of
 Arctic Terns but the remoteness of the Antarctic presents a considerable
 challenge. One solution is to use ‘biologgers’ to monitor remotely their
 behaviour and distribution in the Antarctic. Data from birds tagged with
 light-level global location sensors (geolocators) in 2015 and 2017 showed
 that a third of their annual cycle was spent amongst Antarctic sea ice.
 After reaching the East Antarctic in the austral spring, they gradually
 moved west, foraging in fragmented ice zones of the Antarctic coastline,
 leaving in the austral autumn for their return northward migration via the
 Atlantic. Changes in patterns of movement between phases of 24-h daylight
 and diel day/night conditions were likely linked to the annual moult, and
 stable isotope analyses suggest that krill (Euphausia species) was an
 important component of their diet. There were marked differences in
 movement behaviour between Arctic Terns tagged in 2015 compared to 2017
 that may relate to unusual changes in sea-ice extent. The Arctic Tern may
 be unique amongst seabirds that utilise the Antarctic environment in
 summer in being able to move widely without nesting constraints, and may
 present a means of characterising the effects of climate change on species
 dependent for foraging on Antarctic sea-ice and krill.
description: Geolocator light-level data (maximum light-levels in 5 minute
 intervals referenced to GMT with the internal clock) represent 47
 wintering profiles of geolocator deployments on Arctic Terns
 <i>Sterna paradisaea</i> from the Farne Islands (latitude
 55.617; longitude -1.656), Northumberland in 2015 and 2017. The 23
 geolocators from the 2015 cohort had been programmed to record either over
 the full light-level range (13 geolocators) or in ‘clipped’ mode where
 light-level data were sufficient to identify dawn and dusk thresholds
 accurately but temperature was not recorded. In the full light-range
 recording mode (used for all geolocators in 2017), minimum temperatures
 were recorded every 4 h. The deployment site was the calibration location.
 Four .csv files are provided for each geolocator deployment for
 geolocators programmed in full-light-level mode, and three .csv files for
 geolocator deployments where the geolocator was operating in clipped mode
 (no temperature data). For each .csv file, the first three characters
 (consisting of ‘G’ followed by a two-digit number) indicate the geolocator
 deployment, and subsequent characters indicate the file
 contents: _calibration_period_lux_data.csv: raw
 date/time and light-level data for the calibration periods at the
 calibration site; _lux_data_antarctic.csv: raw
 date/time and light-level data for the Antarctic between 16 October and 12
 April next year; _meta_data.csv: metadata for the raw
 light-level and temperature data, including geolocator type and operating
 characteristics; _min-temp.csv: date/time and minimum
 temperature data (degrees Celsius) between 1 October and 15
 April. Raw light-level data can be interpreted using
 appropriate software (R version 3.3.1 and the R package <i>FLightR
 </i>were used<i> </i>in the paper to estimate position,
 applying use/stay criteria for land of 50 and 10 km, respectively).
 Twilight thresholds are necessary to estimate day/night length and the
 time of midnight and noon for the determination of latitude and longitude,
 respectively, but this is not possible in 24-h daylight when light levels
 are not reduced below the twilight thresholds. For birds in 24-h daylight,
 longitude can be estimated from light-level data by curve-fitting to
 estimate the time of midnight, represented by minimum daily light levels.
 For light-level data files, the columns are date/time and light levels
 (units). The files for one deployment, G77, differ in that in addition to
 raw date/time and light-level data, additional columns (twilight, interp
 and excluded) indicate dawn/dusk threshold assignments output from the
 ‘preprocessLight’ function of the R package BAStag program used for
 initial data processing.