Tyto Tagus: Barn Owl post-fledging dispersal in the Tagus Valley (Portugal)

Abstract

The Tagus Estuary lowland farmland receives a notable concentration of Barn Owls during the post-fledging dispersal period, with an abundance of more than 15 owls/km along some dirt roads in an area known as Ponta da Erva. The TytoTagus project aimed at (1) determining the origin of the barn owls found in the Tagus Estuary by visual colour ring recaptures via car transects and (2) determining juvenile owl dispersal patterns in the Tagus Valley by radio-tracking. From 165 Barn Owls ringed as nestlings in 2007-2012, 6.2% were visually recaptured from two areas near the estuary: 14 from Benavente – Vila Franca (<15 km) and two from Coruche (45-60 km). In order to understand the factors which may be relevant to Barn Owls dispersing towards the estuary, 41 juveniles were tagged with VHF transmitters (29 in Benavente – Vila Franca and 12 in Coruche). During emancipation contact was lost with 16 owls and 13 were found dead. One tag failed when the owl was still in the nest and 12 owls were tracked during dispersal. Dispersal consisted of a succession of temporary roosts, alternating dispersal movements with stop phases in which individuals remained in an area and used one single or several nearby roosts. Distance to the nest clearly corresponded to approximation to the estuary in six owls. In the other cases, owls remained in Coruche or came from nearby nests and were roosting in a northern area of the estuary (despite increased distance to Ponta da Erva, south floodplain). Although the distance between roosts and hunting areas was generally < 3 km, juveniles hunting in the estuary may have roosts at least 10 km away. Juvenile Barn Owls mainly used trees as roosting sites (e.g. trees along roadsides and riparian galleries) and forest patches contiguous to open agricultural areas (mixed stands of cork oak and pine, montados and pine forests).