Movement Patterns of Brown Skuas During the Non-Breeding Period: Internal vs. Environmental Drivers J. Krietsch , M. Kopp , S. Lisovski , J. Esefeld , M. Stelter & H.-U. Peter 1 1 1 2 1 1 Polar & Bird Ecology Group, Institute of Ecology, Friedrich Schiller University, Jena, Germany 2 Centre for Integrative Ecology, Deakin University, Geelong, Australia Introduction 70°W Pelagic seabirds spend most of their life, often except the breeding period, on the open ocean far off the continental coasts. The non-breeding period is an important part of the ecology of their entire life history and has huge effects on survival and selection of the species. Therefore it is essential to understand the biological mechanisim’s behind the distribution of migratory birds during this period. 50°W Rio de la Plata Estuary The aim of this on-going analysis is to investigate in detail the spatiotemporal distribution of Brown Skuas (Fig. 5) from King George Island (KGI) and tries to detect the drivers of their movement patterns. We want to find out 40°S a) whether the birds are mainly driven by environmental factors (like food availability) or b) by internal factors (like gender or individually conserved migration routes). Probability Density 60°S Therefore we use remote sensed data indicating food availability and tracking data (light-level geolocation) over consecutive years. 0.99 0.95 0.9 0.8 0.7 Fig. 2: Probability density distribution of 28 tracked Brown Skua during the wintering periods 2007-2010 with a total of 47 migration cycles. Grey line - Patagonian Shelf Break 50°W 70°W Fig 5: Study species: The Brown Skua (Catharacta antarctica lonnbergi) a highly opportunistic predator and scavenger. 50°W 1 1 70°W 1 Probability Density Breeding Site - KGI 60°S 0.99 0.95 0.9 0.8 0.7 log NPP Fig. 3: Probability density distribution of one Brown Skua (# 030210) with a complex movement pattern during the non-breeding period in 2007. Grey line – Patagonian Shelf Break Fig. 1: Mean net primary production (NPP) of 2007 in mg C/m²/day during the non-breeding period of the Brown Skua. Grey line - Patagonian Shelf Break Methods 70°W We equipped 32 Brown Skua individuals on KGI, Maritime Antarctic (Fig. 1) with light-level geolocators (Type MK5 & MK9) during 4 breeding seasons (2007-2010). Blood samples were taken to sex the birds genetically. The R package ‘BAStag’ was used to determine twilight times (sunrise, sunset) from raw light recordings over time. Movement tracks were estimated using the R package ‘SGAT’(2). The implemented Estelle model uses the threshold based positioning approach and a Markov Chain Monte Carlo method with a metropolis sampler to derive probability distributions of positions under the following assumptions: • distribution of the birds travel speed • error distribution of twilight times (accuracy of detected twilights) • zenith angle (defines sunrise & sunset) • zenith angle associated to twilight times(3) • land mask (locations at sea are far more likely than locations on land) If water temperature was recorded by the geolocator: • Sea surface temperature mask (high probability in the range of the recorded water temperature) Contact Johannes Krietsch Polar- & Bird Ecology Group Institute of Ecology Friedrich-Schiller-University Jena Dornburger Str. 159, 07743 Jena, Germany 0 6 5 7 7 8 8 log mean NPP during wintering time 2007 (in mgC/m**²/day) Mean NPP in 2007 (in mg C/m²/day) Fig. 6: Trend between the mean net primary production (NPP) of 2007 (see Fig. 1) and the mean probability density distribution of # 030210 (see Fig. 3) in 2007. Preliminary Results 50°W We obtained 47 migration cycles of 28 Brown Skuas (21 ♀ and 7 ♂). During the austral winter birds were widely distributed over the Patagonian Shelf and around the shelf break (Fig. 2). In contrast to previous studies (4), Brown Skuas from KGI spent most time over costal and shallow waters. The highest density distribution was located in the confluence zone of the Falkland Current and the Brazil Current, and reaches the Rio de la Plata Delta. Rio de la Plata Estuary (1) [email protected] 0.5 0.5 40°S Falkland Current 60°S Time Spend 40°S Rio de la Plata Estuary Brazil Current 0 Rio de la Plata Estuary Probability Density Almost all individuals covered the same wintering sites (Fig. 4) during consecutive years. We found no differences between the distribution of females and males. 40°S Individual 60°S # 108272 # 127377 # 108294 # 138530 Fig. 4: Comparison of selected probability density distributions (quantile 0.95) of four individuals over two successive years. Solid line – 2007, Dashed line – 2008, Grey line – Patagonian Shelf Break Geolocators of 14 individuals recorded water temperature (every 10 minutes) allowing for a more accurate estimation of positions (Fig. 3) and can be used for further more detailed spatiotemporal analysis. Preliminary results reveal that the NPP distribution coincide (Fig. 6) with the probability density distribution of the skua population. In conclusion, our preliminary results suggest, that the distribution of our study population during the non-breeding period is partly conserved on the individual level and overall driven by food availability at sea. References BAStag: https://github.com/SWotherspoon/BAStag SGAT: https://github.com/SWotherspoon/SGAT (3) Lisovski, S., Hewson, C.M., et al. (2012): Geolocation by light: accuracy and precision affected by environmental factors. Methods in Ecology and Evolution 3, 603-612. (4) Phillips, R.A., Catry, P., et al. (2007): Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes. Marine Ecology Progress Series 345, 281-291. (1) (2) Partly funded by the DFG (Pe 454-16)