The little egret is increasing in number in Wales, according to the report
The importance of wetlands in Wales for the survival of migratory birds is highlighted in an annual study.
Both the Dee and Severn estuaries feature in the top 20 sites for water birds in the UK, says RSPB Cymru.
The Dee estuary has more than 129,000 birds on average and "internationally important" numbers of seven species.
The report, produced by several conservation groups, also suggests "startling changes" in the fortunes of some of the birds visiting Wales.
Katie-Jo Luxton, RSPB Cymru's director, said: "Two aspects of this long-running study really stand out. Firstly, bird populations change over time, understanding this is critical to developing future conservation strategies.
"Secondly, Wales' wetlands, and particularly those on the coast, are of enormous importance to the survival of a great number of water birds, which migrate from many other countries.
"The protection and wise management of our wetlands is a priority to ensure they continue to provide a lifeline to bird populations in the face of climate change, a view recognised by the convention on wetlands of international importance."
The Dee estuary was the most important site for water birds in Wales and seventh on the UK list, featuring species including pintail, redshank and oystercatcher.
The Severn estuary was the second most important site in Wales, also with internationally important numbers of seven species, and a bird population of more than 69,000.
Other key sites in Wales include Carmarthen Bay, Burry Inlet and Cleddau estuary.
TOP WATER BIRD SITES IN WALES
Dee Estuary: 129,271 birds; seven species of international importance
Severn Estuary: 69,482 birds; seven species of international importance
Carmarthen Bay: 48,086 birds; two species of international importance
Burry Inlet: 47,507 birds; four species of international importance
Cleddau Estuary: 24,905 birds; no species of international importance
Dyfi Estuary: 12,279 birds; no species of international importance
Source: Waterbirds in the UK 2007/08
The report highlighted changes in the fortunes of the thousands of water birds from the Arctic and northern Europe that spent the winter in Wales.
Richard Hearn, head of species monitoring at the Wildfowl and Wetlands Trust, said: "Where declines of these migratory water birds have been noted, we must guard against assuming these birds are wintering further east in parts of Europe previously too cold.
"Climate-related changes in distribution have the potential to mask more serious conservation problems, so it is more important than ever that we get the complete picture by working with partners abroad to monitor these birds across their flyways."
The three birds faring the worst, compared with a decade ago, included the Greenland white-fronted goose, pochard and dunlin.
The species recording the greatest increases over the period included the little egret and black-tailed godwit.
Some declining species, such as the ringed plover and bar-tailed godwit, appear to be shifting away from wintering in Wales, preferring to spend the winter in continental Europe, according to the report.
The Waterbirds in the UK 2007/08 survey is produced by The British Trust for Ornithology, the Wildfowl and Wetlands Trust, the RSPB and the Joint Nature Conservation Committee.