Assemblages of waterbirds

Size of waterbird SPAs

 

During the period of this review, The Wash held the largest number of waterbirds (400,273), with the Ribble and Alt (301,449), Morecambe Bay (210,668), Humber Flats and Marshes (152,817), Upper Solway Flats and Marshes (133,222), the Blackwater Estuary (109,815) and Foulness (107,468) all holding more than 100,000 waterbirds. It is not possible to sum the peak mean counts for each site given in Table 6.104.1 owing to the double counting that would occur, due to movement between sites within and between winters. However, the average of January counts for these sites between 1993 and 1997 is 2,084,272. To this total can be added the January totals of waterbirds wintering at a further 57 SPAs that hold qualifying species of wintering waterbirds but at population levels below 20,000. The addition of these totals indicates that the UK SPA network holds a minimum 2,186,443 waterbirds in January. In terms of assessing total numbers of waterfowl that gain conservation benefits from the UK SPA network, it should be noted that this total is an underestimate for four reasons:
 
  • the totals used on sites holding <20,000 waterbirds relate to just the totals of qualifying species. To this should be added other 'non-qualifying' waterbirds occurring on those sites;
  • the totals do not account adequately for those non-breeding waterbird species where peak occurrence on the network is in months other than January (e.g. Tufted Duck where peak national occurrence is in December). The most extreme example of this is species that occur on the SPA network during migration periods, en route to/from Arctic breeding grounds and wintering areas further south in Europe or Africa (e.g. populations of waders such as East Greenland Dunlin Calidris alpina arctica and North Siberian/West and South African Knot Calidris canutus canutus);
  • since the January counts are 'snapshots' they makes no allowance for turnover of birds in migratory periods. This can be very considerable as birds pass through the UK, but is technically very difficult to assess (Frederiksen et al. 2001); and
  • periods of extremely cold weather, especially further east in Europe, displace large numbers of waterbirds to seek milder areas in Britain and Ireland (Ridgill & Fox 1990). The winters included in this review were generally mild, although there were significant periods of severe cold in 1995/96 and 1996/97.

 

Making allowance for these factors, it can be conservatively estimated that about 3,000,000 non-breeding waterbirds directly benefit from the general wetland habitat conservation provisions of the UK SPA network. This is a significant proportion of total East Atlantic Flyway populations of waders and North-west European populations of waterbirds.