CONICET | Buscador de Institutos y Recursos Humanos

Migration is a well know behavior in birds (Newton 2008) and may involve complex processes and mechanisms. For instance, populations may show partial migration, with some individuals migrating to winter quarters while others remaining in or near the breeding colony year-round (Dingle 1996). In some particular cases, members within a given population may follow distinct migration strategies, which is known as differential migration, occurring between sexes, age groups, or morphs within a population (Ketterson and Nolan 1983). These different migration behaviors are usually considered to be a strategic response to spatially seasonal variability in the environment, allowing some individuals in the population to avoid unfavorable conditions (Cresswell et al. 2011).Seabirds are in general migratory or dispersive (Burger and Gochfeld 1996). Gulls (Laridae), in particular, show different patterns of migration, with trips ranging from hundreds of kilometers to long distance journeys of thousands of kilometers (Hatch et al. 2011, Klaassen et al. 2012, Bustnes et al. 2013). The understanding of bird movements can be assessed through ringing studies, in fact the re-sighting of permanent filed readable marking have been used traditionally in migratory studies. This low cost methodology has the advantage of allowing the study of large numbers of individuals (Bairlein 2003), and the use of numbered plastic color rings may even allow the identification of individuals without the need of recapture. However, over the past three decades, the use of electronic devices to monitor the meso- and large-scale movements of birds has greatly enhanced our understanding of migration patterns, moving from knowing few refueling and wintering spots in the migration route, to currently visualizing complex migration behavior and use of foraging and resting areas along the way. Moreover, the size of devices has importantly decreased over time, thus allowing the deployment of devices in smaller species and for longer periods of time (Burger and Shaffer 2008) . The Olrog?s gull Larus atlanticus is an endemic species of the southwestern Atlantic coast only breeding in Argentina (Yorio et al. 2013). The species has been listed by the IUCN as Near Threatened (BirdLife International 2019) and included in Appendix I of the Convention on the Conservation of Migratory Species of Wild Animals (CMS 2018). At a local scale the species is listed as Vulnerable in Argentina and Endangered in Uruguay (Azpiroz and Caballero-Sadi 2017, MAyDS and AA 2017). Main threats included pollution, habitat degradation and interactions with sport fishing activities. Its breeding range is mostly restricted to southern Buenos Aires province, with nesting sites varying between 7 and 12 depending of the year, and a global breeding population of less than 8,000 pairs (Yorio et al. 2013). During the non-breeding season, individuals of different age classes migrate north, being sighted in coastal areas of northern Buenos Aires province (Favero et al. 2001), Uruguay (Escalante 1970) and southern Brazil (Pacheco et al. 2009, Gava Just et al. 2018). Little is known about its wintering movements. In Mar Chiquita Lagoon (Buenos Aires province), a regularly reported wintering area for the species in Argentina (Isacch et al. 2016), individuals tracked with GPS and VHF radio-transmitters were reported to use a restricted area in the vicinity of the mouth of the lagoon (Berón et al. 2007, Ravasi et al. 2018). However no information on wider-range movements connecting breeding and non-breeding areas is yet available. In this context our study was designed to analyse the migration of Olrog?s gulls considering differences in sex and age, by following individuals from the population breeding at the Bahía San Blas Protected Area (Buenos Aires province, Argentina) instrumented with tracking loggers, and complementing this information with sightings of breeding adults and fledglings ringed in this breeding area.

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