TytoTagus Project: Common Barn Owl post-fledging dispersal and survival in the Tagus Valley, Portugal

Abstract

The Tagus Estuary hosts a high concentration of juvenile Common Barn Owls (Tyto alba) during the post-fledging dispersal period with more than 15 owls/km detected along roads in the south floodplain of Vila Franca de Xira (SF). The Tyto Tagus project examined the origin of these birds with six re-sightings of 136 colour-ringed nestlings (2006–2008) and subsequently (2009-2012) with 41 VHF radio-marked juveniles from three areas: 16 in Benavente, 13 in the SF and 12 in Coruche. Five re-sightings were of colour-ringed juveniles from nests in Benavente (<15 km from the SF) and one from a nest from Coruche (45–60 km from the SF). One tag failed while the owl was still near the nest, contact was lost with 19 radio-marked owls (15 during fledging), 19 owls were found dead (13 during fledging), and the battery was used up for two owls. One juvenile was found dead immediately after leaving the nest, but 11 others were tracked during dispersal, in which they used a succession of temporary settlements with single or several roosts, alternating with longer movements. Six of these owls moved towards SF. Others remained in Coruche or roosted in a northern area of the floodplain near their nests. The distance between roosts and hunting areas was generally <3 km, but some juveniles hunting in the SF had roosts >11 km away. Juvenile Common Barn Owls mainly roosted in trees along roadsides and riparian areas but also in forest patches (i.e., in mixed stands of cork oak (Quercus suber) and pine (Pinus spp.), montados and pine forests) adjacent to open agricultural areas. Between 7.3% and 43.9% of the owls survived the post-fledging dispersal period. Future studies should assess the impact of road mortality near the SF and consider the use of new technologies to track juveniles until they nest.