6. Bird assemblages distribution and relationship with environmental variables
The distribution of bird assemblages within the studied TIDE estuaries and its relationship with the environmental variables described in Chapter 3 was investigated by applying multivariate analysis to the data. This analysis allowed the identification of the main environmental gradients affecting the distribution of waders and wildfowl communities across the estuarine areas (units or sectors). A temporal component was also included in the analysis in order to account for possible changes in the spatial distribution of species over different periods of time (measured as 5-year periods) as a response to possible changes in the habitat availability and quality over the periods. Further information on the data treatment, the analysis and its limitations (due to data availability), and detailed results are provided in Appendix 3.
A high similarity was observed between bird species within the wader and wildfowl groups in their distribution within the three studied estuaries, particularly when considering the most abundant species (Dunlin, Golden Plover, Lapwing, Oystercatcher, Curlew for waders; Shelduck, Wigeon, Mallard, Teal for wildfowl) (Appendix 3). For wildfowl in particular, species having similar modes in the use of the estuarine habitat (as indicated by the functional groups described in Chapter 4) showed similar distribution in each of the studied estuaries, although some differences were observed between estuaries. For example, the estuarine feeder species are widely distributed across all the estuarine zones in the Elbe, whereas they show high densities in the polyhaline and mesohaline areas of the Weser and in the oligohaline and mesohaline areas of the Humber. A lower similarity in the spatial distribution within each estuarine system was observed between the wader species sharing similar habitat use (Appendix 3), although this is likely dependent on how functional groups were defined for waders. In contrast to wildfowl, for which functional groups allowed clear discrimination of different habitat preferences (e.g. freshwater and sea ducks) at the estuarine scale, a higher overlapping of the broad habitat preferences occurred between the functional groups defined for waders (e.g. specialist or generalist feeders, both of them feeding on mudflats; or species feeding or roosting on mudflats), thus leading to a lower agreement between the species distribution at the estuary scale and their allocation to the same functional group.
The multivariate analysis applied to the bird data (separately for waders and wildfowl and for each estuary) also highlighted a general predominance of the spatial variability in bird density distribution in the studied areas. Although certain variability in the species density occurred across the different periods covered by the data, the differences in the species distribution were higher among the different sectors/units located along the banks of each estuary (Appendix 3). Below, the results on the species distribution within the estuarine areas and their relationships with the environmental gradients in them are provided by estuary.
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