Abstract
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.
Highlights
Waterbirds are well-known indicators of the quality of aquatic environments
A flyway is a flight path used in bird migration (Boere & Stroud, 2006) and a flyway population is the number of individuals of a certain species included in a given flyway area
These latter ‘hot spots’ are in shallows on the open sea or in the estuaries of rivers where food, mainly mussels and fish, is plentiful (Ławicki, Guentzel & Wysocki, 2012; Marchowski et al, 2015). It happens that a significant percentage of the entire flyway population of a species gathers in a few such optimal places: for example, 14% of the entire Greater Scaup Aythya marila population
Summary
Waterbirds are well-known indicators of the quality of aquatic environments. If a particular site holds 1% or more of the flyway population of a given species, this area is said to be important for this population. Some papers on non-breeding populations compared the results of air and ground counts in Australia (Kingsford & Porter, 2009), on tidal sea coasts on the Wadden Sea in Denmark (Laursen, Frikke & Kahlet, 2008) and Germany (Scheiffarth & Becker, 2008), and in the Poyang Basin in China (Fawen, Changhao & Hongxing, 2011), but they did not take ice coverage into account This is a important factor, considering that a large proportion of waterbird species overwinter in areas around the mid-winter 0 ◦C isotherm (Van Erden & De Leeuw, 2010). It happens that a significant percentage of the entire flyway population of a species gathers in a few such optimal places: for example, 14% of the entire Greater Scaup Aythya marila population
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