Pred-Tagis
Birds as predators of invertebrates in intertidal habitats: role in estuarine environments and scale-dependent processes
2003-2006
Project Summary
This project was centered on the foraging ecology of wintering shorebirds, mainly by analysing their role as consumers of invertebrates in estuarine ecosystems. Its main purpose was to identify key factors determining the distribution and abundance of shorebirds, to clarify the mechanisms through which they operate, and the magnitude and "shape" of their influence.
The work was carried out in the Tagus estuary, the largest intertidal area in Iberia. Internationally significant numbers of shorebirds use the estuary, both as a wintering area and as a stop-over during migration. The ecological conditions found in the Tagus estuary are typical of those found in most south European estuaries.
The project provided detailed data on the temporal variability in the abundance of mudflat invertebrates, poorly known in southern estuaries. The lack of such data had limited the possibility of interpreting, for example, the variation in shorebird distribution and abundance between years.
We examined the factors determining the surface availability of invertebrates, which in southern Europe are likely to function differently from the north, where most research on this topic has been carried out. In fact, the species composition of the southern invertebrate communities is different, and the climatic conditions to which they are subjected during the winter are much milder.
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This project also quantified the contribution of nocturnal feeding to the energy budget of shorebirds. This was done by examining the amount of time allocated to nocturnal feeding, the changes in the feeding behaviour, and the corresponding consequences on feeding efficiency. This research topic took particular attention to the effects of public illumination. The levels of public illumination were shown to exert a significant influence in the selection of feeding areas by shorebirds, which will benefit from enhanced visual capabilities and thus increase their feeding success in strongly illuminated areas.
We have further examined the processes involved in feeding site selection by measuring the response of shorebird to situations of increased and decreased food availability. By using areas of different size, we were able to analyse the effect of the scale of manipulation on the magnitude of the response. Finally, we started to test if shorebirds deplete invertebrate populations. Exclosure experiments were set up in intertidal areas, enabling a direct measurement of change in the abundance of invertebrates exposed to different levels of predation.
Funding
Fundação para a Ciência e Tecnologia (FCT). Portugal.