Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

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Determinants of bird habitat use in TIDE estuaries

1. Summary

The distribution of waterbirds in estuarine habitats and the identification of the main factors affecting bird habitat use have been investigated within the TIDE project. A methodological approach has been proposed for this type of study (see TIDE Tool “Guidelines on bird habitat analysis methodology” ) combining high water bird count data with the characterization of environmental conditions (including natural habitat areas, water quality parameter and indicators of anthropogenic disturbance) in multivariate analysis (community distribution models) and species-habitat regression models in order to identify a series of habitat requirements for different bird species.

Three TIDE estuaries, the Elbe (D), Weser (D), and Humber (UK), were used as case studies as they share similar broad characteristics (e.g. they have a strong tidal influence, important port areas) but also present a different distribution of the pressures and habitats along the estuarine continuum which might affect the bird habitat use in different ways, leading to different potential outcomes.

The estuarine hydrogeomorphological characteristics indirectly affect the distributions of higher predators within estuarine areas as they determine the extent of intertidal mudflats and marsh habitats. In particular, intertidal mudflats are important for waders and marsh for wildfowl and as refuge areas for fishes. Although TIDE only quantified the value of habitats available within the estuary at a small spatial scale (i.e. within an average area of 6km2 around the roosting sites), the obtained results suggested that habitat availability on a wider spatial scale (i.e. around the estuary) can also increase the numbers of birds roosting in certain estuarine areas by providing additional feeding grounds that can be used by birds. This effect has been observed, for example, with waders in the polyhaline zones of the Elbe, due to the presence of extensive mudflats in adjacent marine areas, or with wildfowl in the oligohaline zone of the Humber, due to the presence of adjacent inland habitats.

Larger estuarine habitats appear to support greater bird densities compared to smaller habitats, especially for generalist feeders (i.e. species that are able to take advantage from a wider range of food prey, such as Dunlin and Redshank). This may be due to the higher diversity of resources associated to wider habitats benefiting the aggregation of these generalist feeders. In turn, this is less evident for specialist feeders, such as Bar-tailed Godwit, which are more likely to depend on the distribution of specific prey, a factor that might be more relevant at a smaller spatial scale (i.e. within a mudflat) hence resulting in a contrasting relationship with the total intertidal habitat area.

Lower bird abundances are generally observed in areas where natural estuarine habitats are smaller, this reduced habitat availability being the result of the natural variability in the estuarine morphology (e.g. narrower mudflats present in the freshwater zone compared to the estuarine meso- and polyhaline zones) or the presence of anthropogenic developments and land-claim (e.g. smaller mudflat areas in the mesohaline zone of the Humber or in the freshwater and oligohaline zone of the Elbe). Hence, the availability of natural estuarine habitats mainly determines the density of waders and wildfowl, especially in the Weser and Humber.

Water quality characteristics such as the salinity gradient, nutrient levels and organic enrichment are also important in affecting species distribution, a feature particularly evident in the Elbe. The effect of the salinity gradient was predominant in the Elbe, especially for bird densities as a whole, but particularly for Dunlin (both increasing with the increasing salinity), although this effect is more likely related to other factors that are correlated with the salinity gradient in the estuary rather than to an effect of salinity itself. These factors include the distribution and availability along the estuarine gradient of feeding habitats (both within the estuary and in adjacent areas, e.g. extensive mudflats in the Wadden Sea) and food resources (as indicated by longitudinal changes in benthic invertebrate communities), as well as the lower degree of anthropogenic disturbance favouring bird use of the outer sands / remote islands located in the polyhaline zone of the Elbe.

It is acknowledged that the findings are based on limited data and require an assumption of an association to be made between high water distribution and usage (the data used in the analysis) and low water foraging distribution being in the same general area. However, if these assumptions are valid, then the findings have important implications for estuary managers, in that the data indicate that larger mudflat areas have a greater carrying capacity for waterbirds per ha than smaller mudflat areas. This has implications for habitat loss and mitigation/compensation measures, in that a development within an extensive intertidal mudflat will not only have a direct impact through habitat loss, but a potential additive effect through fragmentation of mudflat area. Furthermore, in terms of compensation for such losses, the provision of new habitat, for instance through managed realignment, needs to positioned so that it is contiguous to adjacent habitat, otherwise again a fragmentation effect may occur. In both scenarios (e.g. potential fragmentation and reduced carrying capacity through habitat loss and compensation), it may be necessary to provide ‘over compensation’ in the form of a greater offset provision ratio.

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