The Ramsar Convention defines ‘waterfowl’ as species of birds that are “ecologically dependent upon wetlands” and has defined “waterbird” as being synonymous with “waterfowl” for the purposes of the application of the Convention. However, in the second edition of Waterfowl Population Estimates, ‘waterfowl’ were defined more precisely as all species of the families Gaviidae, Podicipedidae, Pelecanidae, Phalacrocoracidae, Anhingidae, Ardeidae, Balaenicipitidae, Scopidae, Ciconiidae, Threskiornithidae, Phoenicopteridae, Anhimidae, Anatidae, Pedionomidae, Gruidae, Aramidae, Rallidae, Heliornithidae, Eurypygidae, Jacanidae, Rostratulidae, Dromadidae, Haematopodidae, Ibidorhynchidae, Recurvirostridae, Burhinidae, Glareolidae, Charadriidae, Scolopacidae, Thinocoridae, Laridae, Sternidae and Rynchopidae. Only a minority of wetland bird populations are excluded by this approach. Conversely, the inclusion of whole families resulted in the waterfowl list containing a few non-wetland species such as some seabirds and stone curlews. These rather minor anomalies were thought to be outweighed by the convenience of a whole-taxon approach to the definition of ‘waterfowl’ and, in particular, considering the complications that would arise from applying the definition rigidly to every species.

Red-breasted Merganser Red-breasted Merganser, by John Anderson

The 3rd, 4th and 5th editions of Waterbird Population Estimates considers the same families of birds as were covered in the earlier editions. However, the term ‘waterbird’ implies a broader meaning than the strict definition of ‘waterfowl’ given in the second edition, and more in keeping with the Ramsar definition of ‘waterfowl’, i.e. birds that are ecologically dependent on wetlands. Many participants in the International Waterbird Census, coordinated by Wetlands International, already submit counts of wetland birds additional to the families listed above, and it has been proposed that future editions of Waterbird Population Estimates should include population estimates for these, wherever possible.

One of the most logical expansions would be to include additional families of birds traditionally regarded as seabirds. Many of the species of ‘waterbirds’ currently included in Waterbird Population Estimates series are strictly marine species that would equally merit the name ‘seabird’, notably many species of cormorants (Phalacrocoracidae), gulls (Laridae) and terns (Sternidae), while many of the ‘seabirds’, currently excluded, might equally be termed ‘waterbirds’, as they make extensive use of shallow, inshore waters. Of the seabird groups, perhaps only the four families of Procellariiformes (Diomedeidae, Procellariidae, Hydrobatidae and Pelecanoididae) do not include any species that can be regarded as waterbirds. A majority of species in these families are exclusively pelagic away from the breeding sites, rarely straying into inshore waters except when storm driven. At least some of the species in the other ‘seabird’ families (Spheniscidae, Phaethontidae, Sulidae, Fregatidae, Stercorariidae and Alcidae) make use of shallow, inshore waters, and could therefore be considered ‘waterbirds’ appropriate for inclusion in Waterbird Population Estimates. It has therefore been proposed that, for the sake of consistency, future editions of Waterbird Population Estimates should include at least these groups of seabirds. A proposal was also made in the 4th edition of the AEWA Conservation Status Review to include migratory seabird populations, and these populations have since been included in subsequent publications of the CSR.

The first four editions of Waterbird Population Estimates were restricted to native populations of waterbirds occurring in a natural, wild state, and did not include those populations of waterbirds that have been introduced outside their natural range, either deliberately or accidentally, by humans. This approach has been retained in the 5th edition. However, it is recognised that some artificially introduced populations of waterbirds can have a negative impact on native populations of other species. The accidental introduction of the North American Ruddy Duck Oxyura jamaicensis into the wild in Europe and the threat which this is now posing to the already Globally Threatened White-headed Duck Oxyura leucocephala has been well documented. It has therefore been proposed that future editions of Waterbird Population Estimates will include established populations of non-native waterbirds, so that their status can be monitored more closely. Established populations of non-native species could be defined as those populations that have been self-supporting in the wild state for at least 10–15 generations, to exclude those frequent but unsuccessful breeding attempts by recent escapes from captivity. All participants in the International Waterbird Census are encouraged to submit counts of non-native waterbirds, and contributors to the next edition of Waterbird Population Estimates will be requested to provide estimates for these populations. For such populations, however, 1% thresholds will not be published, since the Ramsar Convention has indicated (Resolution VII 11) that such non-native species should not be used as part of a supporting case for classification of a wetland of international importance.