|Territoriality in the common shelduck Tadorna tadorna|In: Ibis. British Ornithologists' Union/Wiley: London. ISSN 0019-1019, more
Territoriality; Tadorna tadorna (Linnaeus, 1758) [WoRMS]; ANE, British Isles, Scotland, Grampian, Ythan Estu [Marine Regions]; Brackish water
A population of Common Shelducks on the Ythan Estuary was studied intensively from December 1961 to August 1964, with special attention to territoriality and population regulation.Many of the shelducks in this population establish mutually exclusive territories; the others (usually about one third of the population) remain together in a communal flock. Both members of territorial pairs take part in territorial defence.Territory size is determined by the immediate topography of the area and by the limits of tolerance of the owner. The distribution of territories was found to be related to the distribution of Hydrobia ulvae, which is the staple item in the shelducks' diet.Territories were usually occupied by the same birds year after year. The number of territories remained constant at 71 ± 1 throughout the three years of the study.The first birds to return in the spring were not necessarily the first to take up their territories. Some territories were left unclaimed (at least for the greater part of the day) until the end of April. At no time were interlopers or neighbours ever seen trying to establish themselves in these unoccupied territories.Territories from which both birds were experimentally removed were reoccupied by birds from the surplus flock, usually within a matter of days. When females of territorial pairs were removed, the surviving males invariably deserted their territories, but when the males were removed, the females remained on their territories and eventually acquired new mates.