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The aerial and ground methods of counting birds in a coastal area during different ice conditions were compared. Ice coverage of water was an important factor affecting the results of the two methods. When the water was ice-free, more birds were counted from the ground, whereas during ice conditions, higher numbers were obtained from the air. The first group of waterbirds with the smallest difference between the two methods (average 6%) contained seven species: Mute Swan  Cygnus olor , Whooper Swan  Cygnus cygnus , Greater Scaup  Aythya marila , Tufted Duck  Aythya fuligula , Common Goldeneye  Bucephala clangula , Smew  Mergellus albellus  and Goosander  Mergus merganser ; these were treated as the core group. The second group with a moderate difference (average 20%) included another six species: Mallard  Anas platyrhynchos , Eurasian Wigeon  Mareca penelope , Common Pochard  Aythya ferina , Great Crested Grebe  Podiceps cristatus  and Eurasian Coot  Fulica atra . The third group with a large difference (average 85%) included five species, all of the Anatini tribe: Gadwall  Mareca strepera , Northern Pintail  Anas acuta , Northern Shoveler  Spatula clypeata , Eurasian Teal  Anas crecca  and Garganey  Spatula querquedula . During ice conditions, smaller numbers of most species were counted from the ground. The exception here was Mallard, more of which were counted from the ground, but the difference between two methods was relatively small in this species (7.5%). Under ice-free conditions, both methods can be used interchangeably for the most numerous birds occupying open water (core group) without any significant impact on the results. When water areas are frozen over, air counts are preferable as the results are more reliable. The cost analysis shows that a survey carried out by volunteer observers (reimbursement of travel expenses only) from the land is 58% cheaper, but if the observers are paid, then an aerial survey is 40% more economical.

Waterbird counts on large water bodies: comparing ground and aerial methods during different ice conditions