DOI: 10.5061/dryad.hx3ffbgdr
title: Skua and plant dispersal: Lessons from the Argentine Islands - Kyiv Peninsula region in the maritime Antarctic

description: Birds are one of the most likely dispersal vectors for plants in
 Antarctica. We studied the nesting behavior of south polar skua
 (Catharacta maccormicki) and brown skua (Catharacta antarctica lonnbergi)
 to assess their potential role in ornithochory in the Argentine Islands -
 Kyiv Peninsula region. Nest samples were collected during 2009-2020 years
 in the Argentine Islands - Kyiv Peninsula region including all islands and
 coasts of the Graham Land from the Lemaire Channel to the islands of
 Berthelot Islands from north to south and extending from west to east from
 the Roca Islands, Cruls Islands, Rasmussen Point to the coast. We found
 that skuas utilize different nest building materials, including
 bryophytes, vascular plants (hairgrass Deschampsia antarctica), and
 lichens. In south polar skua nests, mosses and lichens dominate in the
 nest material; in brown skuas Deschampsia antarctica and mosses dominate.
 Both bird species likely collect nest components from nearby vegetation
 formations (&lt;1 m distant). We conclude that C. maccormicki and C.
 antarctica lonnbergi are not selective in their choice of plant species,
 simply using the materials that dominate near the nest. Therefore, both
 species carry these materials from nearby sites, and only occasionally
 bring them from distant places.  In conclusion, for both species
 we did not find any evidence to support their involvement in long-distance
 ornithochory (stomatochory) in the region.
description: Our fieldwork
 was done in the maritime Antarctic, in the Argentine Islands-Kyiv
 Peninsula region. The study area included all islands and coastlines of
 Graham Coast from Lemaire Channel to Berthelot Islands on the north-south
 direction with Roca Islands, Cruls Island, Rasmussen Point on the
 west-east direction side. Field surveys and sampling were done during
 seven seasons between 2009 and 2020. The
 name of each scientific season includes two years (for example 2009/2010),
 because the main scientific work is carried out from December to April.
 In every season
 the majority of samples from south polar
 skua nests were collected on Galindez Island
 where
 the Ukrainian Antarctic station “Academic Vernadsky” is
 located. We estimate that the island holds at
 most 30 south skua nests per season. Depending on the ice situation in
 different years the sampling was also carried out on other islands in the
 Argentine Islands archipelago and nearby sites.
 The
 number of
 south
 polar skua and brown skua nests
 examined varied between years. The selection of nests
 collected depended on the ice conditions
 during the season. On
 Galindez Island,
 samples of all south polar
 skua nests were collected
 in each study year, while on the other sites collecting depended on island availability in a given year. The brown
 skua is quite
 rare in the region, as it is the southern edge of its nesting area;
 therefore all known and
 available nests were examined
 every year.
 The
 nests of the birds were
 observed late in the nesting
 season, during
 a short period to reduce
 birds disturbance. GPS
 coordinates and pictures of each nest were
 taken. During four seasons 2013/2014,
 2015/2016, 2018/2019, 2019/2020, plant formations in which each nest was located were
 registered. All nests were analyzed for the
 percentage composition by different nesting
 materials.
 In the field, we quantified the nest composition
 (percentage of the
 total fraction
 of
 bryophytes, hairgrass
 <em>D.
 antarctica</em>,
 lichens, feathers, limpet
 shells, bones and
 algae).
 Additionally, in
 the seasons 2013/2014, 2015/2016, 2018/2019, 2019/2020
 samples of material of
 approximately 10 × 10 cm were collected from all
 or from the
 majority of nests. In these
 samples, moss composition was
 determined in detail. Moss and lichen species
 were identified
 based on standard taxonomic guides
 (Øvstedal and Smith 2001, Ochyra et al. 2008). 
  Statistical analysys For statistical analysis, the average
 values of the defined classes of materials of the south polar skua nests
 for each season were compared. We calculated the test value as
 c<sup>2</sup> = (observed value – expected
 value)<sup>2</sup>/ (expected value). Results of these
 pairwise comparisons are expressed in relative units (Table
 2). The value of the criterion statistics
 according to the criterion of median (Mood median test) was determined for
 each pair of distribution of nest contents by years (for the limit a =
 0.05 of the χ<sup>2 </sup>distribution, when comparing the two
 distributions, the limit value is 3.84) (Pollard 1982, Corder and
 Foreman 2014).  To
 assess how much the
 contents of different components of nests located on different moss banks
 persist or differ over time, we formed a series of pairwise
 spatial
 differences in the modules for data sets by
 percentages of mosses, lichens, and feathers. The other components of the
 nest were not included in the analysis, as their values were
 mostly
 zero.
 A series of pairwise spatial
 differences for the nests between the 2013/2014 and 2015/2016 seasons were
 compared by the regression technique by CurveExpert 1.3 software. Test
 value was calculated by formula F<sub>1,
 n-2</sub>=t<sup>2</sup><sub>n-2</sub>=(n-2)R2/(1-R2), where n=52, for the upper limit a=0.05 of the F-distribution point value is 4.08 for n = 52.