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

5. General characteristics of bird assemblages in TIDE estuaries

In total, forty species (19 waders and 21 wildfowl) were included in the analysed datasets, although the number of species varied between estuaries (
Table 2 ). Species were allocated to functional groups (guilds) in order to highlight general patterns in the functioning of wader and wildfowl community. In particular, wader species were allocated to the following 4 guild categories:
  • Generalist feeder species predominantly feeding on mudflat (Mud F);
  • Specialist feeder species predominantly feeding on mudflat, preying on larger/specific prey (F specialist);
  • Species predominantly roosting on mudflat (Mud R);
  • Species showing a loose association with mudflat (Mud).
The guild categories identified for wildfowl species, in turn, are as follows:
  • Estuarine feeder species, spending most of their life in estuaries (Est F);
  • Species showing a loose association with marsh (Marsh); (in the Humber, these are usually feral expanding geese populations, mostly breeding in the upper estuarine zone);
  • Species grazing on mudflats on Zostera/Enteromorpha (Mud Grazer);
  • Species roosting on mudflats but feeding mostly inland (Mud R/ F inland);
  • Fish eating duck/diver (Subtidal);
  • Freshwater duck (FW duck);
  • Sea duck (mostly marine) (Sea duck).
Overall, the Elbe estuary shows a high importance (in terms of average annual number of birds), with higher counts generally observed along the north bank (SH) of the estuary, particularly for waders (Table 2 , Appendix 1). The Humber estuary also proves to be an important site (in relative quantitative terms) particularly for waders.

Dunlin, a small wader commonly feeding on benthic prey in the estuarine mudflats, dominates the wader assemblages in the Weser and Elbe estuaries (where it accounts for 24% to 50% of the total average maximum annual count), this species being abundant and frequent also in the Humber (accounting for 20% of the total count) (Table 2 , Appendix 2). Other abundant wader species relying on estuarine mudflats for feeding are the Oystercatcher, Curlew, Bar-tailed Godwit, Knot and Grey Plover. Also Lapwing and Golden Plover, two wader species using estuarine mudflats mostly for roosting, are highly abundant, particularly in the Humber estuary (where Golden Plover dominates the wader assemblage in quantitative terms overall) but also in the Elbe (particularly in the southern bank, NDS) (Table 2 , Appendix 2).

As regards wildfowl, estuarine feeder species such as Shelduck, Wigeon and Mallard show the highest abundance overall, particularly in the Humber (accounting for 75% of the total counts on average), Weser (48%) and in the northern bank of the Elbe (57%), with Shelduck being particularly important in this latter area in terms of both relative and absolute abundance (Table 2 , Appendix 2). Teal is also relatively abundant in particular in the Humber estuary, with 12% of the wildfowl total count accounted for by this species. Goose species commonly associated to marsh habitats are also abundant in these assemblages, with Barnacle Goose particularly represented in the Elbe estuary (where it accounts for between 36% and 54% of the wildfowl total counts) and European White-fronted Goose in the Weser (17% of the wildfowl total count) (Table 2 , Appendix 2).

When considering the broad scale spatial distribution of bird assemblages within the estuaries (in terms of differences between the estuarine salinity zones), an increase in the total density of waders and wildfowl is observed generally towards the outer estuary (Figure 2 ). The species most represented in these outer zones are Dunlin, Oystercatcher, Curlew and Knot among the waders, and Shelduck, Mallard, Wigeon and Barnacle Goose for the wildfowl. Particularly high densities are observed in the polyhaline zone of the southern bank of the Elbe estuary (Niedersachsen area), where bird data regard mainly the outer sands / remote islands. However, it should be noted that counting units in this zone are generally of very small area (max. 0.2 km2) compared to those in the other zones of the same estuary (with a minimum area between 2 and 8 km2) and that very high counts have been recorded in these areas, thus leading to the very high species density observed, particularly around the island of Scharhöm compared to the other areas.

The oligohaline zone in the Humber estuary also seems to support dense wader and wildfowl assemblages. Wader total density in particular shows higher values in this zone compared to similar zones in the other estuaries, with the abundance of the roosting species Golden Plover and Lapwing being mainly responsible for this result (Figure 2 ). It is of note that the Humber estuary is also the only site where a decrease in the total wildfowl density is observed towards the outer areas, mainly due to the higher density of Teal, Mallard and Wigeon in the oligohaline and mesohaline zones. A relatively low total density of wildfowl can be observed in the Weser in the polyhaline zone compared to the other salinity zones in the same estuary (despite the increase of Shelduck density with the salinity gradient) and to what is observed in the same zone in the Elbe (Figure 2 ).

General characteristics of bird assemblages in TIDE estuaries

The wader and wildfowl assemblages in the studied TIDE estuaries included a total of 19and 21 species respectively. Wader assemblages are numerically dominated by species using estuarine mudflats for feeding, like Dunlin, Oystercatcher, Curlew and Knot, but also species roosting on mudflats, like Lapwing and Golden Plover, are locally abundant. Wildfowl assemblages are dominated by duck species (Shelduck, Wigeon, Mallard and Teal being the most numerous), with also goose species being locally highly abundant (e.g. Barnacle Goose in the Elbe). In general, higher densities of wader and wildfowl species feeding on mudflats are found in the outer part of the studied estuaries (polyhaline zone), this pattern being particularly marked when considering the southern bank of the Elbe. However, in the Weser and especially in the Humber, the oligohaline zone appears to be important as well in supporting dense populations of waders roosting on mudflats (Lapwing and Golden Plover) as well as high wildfowl numbers, including Teal, Wigeon and Mallard.

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