DOI: 10.5061/dryad.xwdbrv1n9
title: Data from: Migration of black terns Chlidonias niger and common terns Sterna hirundo between South Sweden and the Atlantic coast of Africa

description: Light-level geolocators were used to record the annual migration cycles of
 black terns Chlidonias niger (9 inds, 11 journeys) and common terns Sterna
 hirundo (7 inds, 11 journeys) breeding in southernmost Sweden. The black
 terns used two different winter (Oct – Mar) regions along the Atlantic
 coast of Africa, either north of the equator between Senegal and Liberia
 (3 inds) or south of the equator between Gabon and northern Namibia (5
 inds). All the common terns travelled to winter quarters south of the
 equator, mainly along the coasts of Namibia and South Africa. One juvenile
 common tern was tracked during the first twenty months of its life. This
 bird spent its first winter at South Africa, after which it migrated north
 of the equator to over-summer as one-year old non-breeder in tropical
 waters off Ghana, after which it returned to South Africa for its second
 winter. This record demonstrates that one-year old terns may undertake
 extensive intra-African migration to distant over-summering areas.
 Comparing geolocator results from Swedish and Dutch black tern populations
 indicate that they have similar migration habits, with a possible tendency
 of relatively more individuals migrating south of the equator in the more
 northerly Swedish population (leap-frog migration). Comparing geolocator
 and ringing results among common tern populations indicates a fascinating
 and complex pattern of scale-dependent geographic segregation and
 intermixing along the coasts of Africa.  
description: We used geolocators manufactured by Migrate Technology Ltd
 (Intigeo series; Fox 2021) and British Antarctic Survey (BAS). The
 attached geolocators weighed 1.5-2.0 g (including glue, string and/or
 plastic ring), which corresponds to ≤ 3% of the body mass of the black
 tern (average body mass = 65g) and &lt; 2% of the body mass of the
 common tern (average body mass = 137 g). During 2014 –
 2021 a total of 69 black terns were equipped with geolocators at two
 breeding areas in the southernmost province of Sweden (Scania):
 Kristianstads Vattenrike (northeastern Scania), and the lakes Krankesjön
 and Börringesjön (southwest Scania), using a leg-loop harness. Out of
 these 15 birds were recaptured and the loggers removed. However, since 6
 of the loggers were lost in the mail, this data sets includes data from 9
 black terns. 66 common terns were equipped with
 geolocators 2009 – 2015 in two different areas of southernmost Sweden: at
 the coastal archipelago of northeastern Scania and at the Lake Krankesjön.
 22 of the loggers were attached on a color ring on the leg of the bird,
 and 44 loggers were attached using a leg-loop harness. A total of 8
 loggers with useful data were retrieved. Two of the loggers were from the
 same bird with repeated journeys (1,5 + 1 year).  One retrieved logger was
 from a juvenile bird that got a logger just before it fledged at Lake
 Krankesjön on 27 Jul 2013. It was found dead at Fisherhaven (34.355⁰S,
 19.118⁰E) in the western Cape Province of South Africa on 25 Jan 2018.
 Useful data were obtained from the first 20 months (1,5 year) of the
 juvenile bird’s life. Hence, in total we data from six adult common terns
 and one juvenile is included in the data sets. We used
 the IntiProc software (Migrate Technology Ltd; Fox 2021) including the
 Geolight package (Lisovski & Hahn 2012) for converting light data
 into geographical positions. With the software calibration function we
 used clean light data from the periods when birds were in the known
 breeding area for calibration of the sun angle versus light threshold for
 obtaining latitude. Reliable latitude data were missing from a period of
 about a month or more around autumn and spring equinox. These invalid
 latitude data were deleted based on inspection of the annual records of
 latitude estimates for each individual. However, longitude data were
 available throughout these equinox periods and could be used for some but
 not all timing estimates of events that occurred during the equinox
 periods. Daily light and location data were inspected for each individual,
 and data from days/periods with uncertain or invalid assignments of light
 data to sunset/sunrise were omitted.
description: # Data from: Migration of black terns Chlidonias niger and common terns
 Sterna hirundo between South Sweden and the Atlantic coast of Africa
 [https://doi.org/10.5061/dryad.xwdbrv1n9](https://doi.org/10.5061/dryad.xwdbrv1n9) The submission includes 5 datasets, whereof 3 gives the raw data from the light-level geolocators used in the study. The remaining 2 datasets gives the calculated position data used for analyses in the manuscript. ## Description of the data and file structure Black_tern_raw_data.txt: Data file in plain text containing light data for the 9 black terns included in the manuscript. First column indicates species, second column indicates individual, third column indicates time stamp (YYYY-MM-DD hh:mm), fourth column indicates light level (lux). Common_tern_raw_data.txt: Data file in plain text containing light data for the 5 of the 7 common terns included in the manuscript. First column indicates species, second column indicates individual, third column indicates time stamp (YYYY-MM-DD hh:mm), fourth column indicates light level (lux). Common_tern_raw_BAS_data.txt: Data file in plain text containing light data for the 2 of the 7 common terns included in the manuscript that was equipped with a British Antarctic Survey geolocator. First column indicates species, second column indicates individual, third column indicates time stamp (YYYY-MM-DD hh:mm), fourth column indicates internal clock value, fifth column indicates light level value. Black_tern_used_data.txt: Data file in plain text containing the position data used for the 9 black terns included in the manuscript. First column indicates species, second column indicates individual, third column indicates time stamp (YYYY-MM-DD hh:mm), fourth column indicates latitude, fifth column indicates longitude, sixth column indicates sun elevation angle that is used for calibration of the data. Empty cells are replaced with "NA". Common_tern_used_data.txt: Data file in plain text containing the position data used for the 7 common terns included in the manuscript. First column indicates species, second column indicates individual, third column indicates time stamp (YYYY-MM-DD hh:mm), fourth column indicates latitude, fifth column indicates longitude, sixth column indicates sun elevation angle that is used for calibration of the data. Empty cells are replaced with "NA".