| Summary: | Abstract Background Skuas and jaegers (Charadriiformes: Stercorariidae) are seabirds breeding at moderate to high latitudes and some perform extensive post-breeding transequatorial migrations. Most species overwinter and perform significant portions of their migratory flyways along the Pacific coast of South America, but scant information is available on their at-sea ecology in this waters. Our aims in this study were to determine: 1) the timing of occurrence and fluctuations in abundance of skua and jaeger species, 2) their spatial distribution within the coastal zone and 3) at-sea behavior of birds, including flock size and interactions with other seabird species. Results Between July 2006 and October 2013, we conducted at-sea bird counts at Valparaiso Bay (33°S) in central Chile and confirmed the occurrence of Chilean skuas (Stercorarius chilensis), Brown skuas (S. antarcticus), and Parasitic jaegers (S. parasiticus). Parasitic jaegers are regular austral summer visitors (November to March), whereas Brown skuas occur in the area only in winter (July to October). Chilean skuas were regularly recorded year-round in the area with higher abundances between late winter and early spring (August to October). Brown and Chilean skuas where observed comparatively offshore, whereas Jaegers presented a more coastal distribution, probably associated to host presence. Chilean skuas kleptoparasitized similar-sized (shearwaters and fulmars) and larger seabird species (boobies), whereas jaegers chased only smaller coastal birds (gulls and terns). Brown skuas engaged in no kleptoparasitic behaviors. All three species were observed mostly as solitary individuals. Conclusions Skuas and jaegers showed in general a marked seasonality in their occurrence and abundance (only Chilean skua occurs year-round) and use this area as a commuting and stopover zone within their extensive migratory flyway along the southeastern Pacific.
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