LENSINK R (2001) Breeding of Common Shellduck Tadorna tadorna along the river Waal; import or self supporting?. LIMOSA 74 (3): 95-102.

The Common Shelduck is a common breeding bird along the Dutch coast. In the past decades the number of in land breeding Common Shelduck has increased significantly. In this paper the question is pointed out whether this increase is due to overflow from coastal birds or due to a local increase. In the latter case the annual number of fledglings should be high enough to achieve a self-supporting population, in combination with (sub )-adult survival. In four floodplains along the river Waal data were sampled on the number of pairs, the number of chicks and the timing of the breeding season (1992-2000).
      In de past decade the numbers of breeding pairs along the rivers increased (Fig. 1). Numbers increased in March and April, and reached a maximum in May (Fig. 2) Thereafter birds without young are leaving the area, The first chicks were seen from 20 May onwards, and the median hatching date was determined in the beginning of 102 June (Fig, 3) After a severe winter the breeding process starts about two weeks later (Fig, 4), In the first two weeks adults were accompanied by on average 6,91 young, whereas only 23,9 % of the pairs had young (Tab, 1), Alter a severe winter the breeding success was lower. 8ased on subsequent observations, the daily survival of young is calculated at p = 0.9913-0,9806, This leeds to 2,8-4.6 fledglings per successful pair alter 7 weeks (07-1.1 young all pairs) The calculation of the daily survival is mainly based on the first four weeks. Survival in the last weeks is higher than in the first weeks, Therefore, the calculated daily survival is underestimated. A surviving number of 0.9-1.3 young on average seems to be a better estimate. Based on survival data of British birds, which use the same moulting and wintering areas as the Dutch birds, it is calculated that 0.8 young per year is sufficient for a stable population. It is concluded that inland breeding Common Shelducks in the Netherlands are nowadays self supporting and do not depend on overflow from coastal breeding birds. Common Shelduck Tadorna tadorna

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