Lourenço, P.M., Alonso, H., Alves, J.A., Carvalho, A.T., Catry, T., Costa, H., Costa, J.S., Dias, M.P., Encarnação, V., Fernandes, P., Leal, A.I., Martins, R., Moniz, F., Pardal, S., Rocha, A. & Santos, C.D. (2018) Monitoring waterbird populations in the Tejo estuary, Portugal: report for the decade 2007-2016. Airo, 25, 3-31.
The Tejo estuary is a key site for wintering and migratory waterbirds both at the national and international levels. Here we report the main findings of an ongoing monitoring programme of waterbirds in the main high tide roosts of the estuary. A decade of monthly counts (between 2007 and 2016) revealed peaks in waterbird abundance occurring between August and February, with monthly averages of over 28000 birds. Overall, our data highlight the importance of this wetland during winter and autumn migration, with lower but also relevant numbers during spring migration. Despite some variation over the decade, we found some consistency in the relative importance of the several roosts, with the same five sites harbouring over 80% of all counted birds across years and waterbird groups. Three of these roosts (Samouco, Vasa Sacos and Ribeira das Enguias) are located within national protected areas, while the other two (Corroios and Alhos Vedros/Moita) have no legal protection. We also attempt to evaluate the relative importance of remote saltmarshes in the north-eastern part of the estuary, which were not part of the present monitoring effort but can harbour important numbers of several waterbird species. In the monitored roosts, we observed different population trends for different waterbird species, ranging from strong increases (in several wildfowl species, two shorebirds and great cormorant) to strong decreases (in four shorebird species). In several cases, local population trends were dissimilar to the known trends across the flyway, but this comparison may be hindered by a temporal mismatch between our data and available flyway-wide data. Nevertheless, local trends for dunlin, redshank and ruddy turnstone may be of particular concern as these species show decreases in the Tejo estuary, in contrast with the flyway trend, suggesting that they may be facing problems locally.