Shorebird Studies in the Tagus Estuary

Why studying shorebirds?

Shorebirds are conspicuous elements of virtually all estuaries during the winter. Many species of shorebirds feed on the macro-invertebrates in the intertidal sediments of estuaries during the low tide, and seek refuge in adjoining areas for resting and maintenance activities during the high tide. Hence, wintering shorebirds have the potential to act as integrative indicators of the quality of estuarine areas, reflecting not only the quality of the sediment flats, on which they depend for feeding, but also that of the surrounding habitats, which they seek as refuge. Estuaries and their adjoining habitats, are currently under threat by an ever increasing human presence and disturbance. As a consequence, several long-term monitoring programs have revealed sustained population declines of many shorebirds species in Europe and elsewhere.

Some estuaries are also routinely used by very large numbers of actively migrating shorebirds. Each year, the fate of hundred of thousands migrants depends on the existence of a network of good quality stopover sites along their migratory flyway, which they use to rebuild their body condition between the successive legs of their migratory trajectories.

It is increasingly important to be able to monitor and maintain the quality of estuaries to ensure the conservation of shorebird populations.

Background: what is known and ... what is unknown

Many north European estuaries have been the focus of long-term research and observation programmes, and they now possess a wealth of data on the ecosystems composition, structure and functioning. This knowledge has enabled an in-depth understanding of the main factors influencing the individual condition, the behaviour and the distribution of wintering shorebirds, and is now of great use to predict the impact of human activities and habitat degradation (as those resulted from climate change, for example) on the survival of shorebirds
. This situation contrasts with that observed in most southern European wetlands, where both baseline information and advanced research are scant.

The Tagus estuary (southern Portugal) is located on a migratory pathway known as “East Atlantic Flyway”, and supports a large number of shorebirds during the winter and in migration. The wintering shorebirds of the Tagus estuary have been the focus of recent research (see Projects) but many important ecological questions are still unresolved. The factors determining the spatio-temporal variation in the distribution of birds in the feeding area are only partially understood. Also, the scales at which these factors operate are now beginning to be unveiled. Further insights into these issues depend, on one hand, on long-term data about the distribution and abundance of prey and about their recruitment schedule, and on the other hand, on further correlative research on foraging strategies of birds.

The lack of knowledge is particularly acute in relation to numbers and behaviour of actively migrating birds in the Tagus estuary. The Tagus estuary is believed to act as a important stopover site, particularly during the northward migration. However, the magnitude of the migrating populations, the residence period of birds, the preferred foraging sites for refuelling, the rates of weight increase and many other parameters of the birds’ migratory ecology are virtually unknown. This lack of knowledge prevents an adequate assessment of the quality of the Tagus estuary as a stopover site.