LOK T, O OVERDIJK, H HORN & T PIERSMA (2009) The Spoonbill Platalea leucorodia population of the Wadden sea islands: does population growth level off?. LIMOSA 82 (3): 149-157.

After a minimum of c. 150 breeding pairs in 1968, the Dutch breeding population of Spoonbills has increased steadily. In 2008, 1907 breeding pairs were counted, of which 1179 (2358 breeding birds) on the Wadden Sea Islands. Here we investigate the population growth on the Wadden islands in more detail, and try to predict to what size the population will eventually grow. The population growth since 1985 appears to fit best to a logistic growth curve with an asymptotic population size of 2780 breeding birds. To understand the underlying processes in more detail, we developed a population model based on our data on survival probabilities and recruitment ages of colour-marked birds and on breeding success. We investigated whether these parameters depend on population size, which could explain the observed reduction in population growth. Adult survival was related to the size of the total Dutch population, and decreased from 90% to 78% when population size increased from 800 to 3600 breeding birds between 1990 and 2007. On Schier monnikoog, breeding success was negatively related to colony size. However, due to lack of data, we could not verify this observation for the other islands. Therefore, we used the mean breeding success of 1.67 young per breeding pair as a model parameter. Neither survival nor return age of subadults were density dependent: 42% of the colour-ringed juveniles were resighted in the breeding area as adult. The average age at which Spoonbills returned to the breeding area was 4.0 years, and the annual survival probability of subadult birds was 80.5%. The model predicts that the population on the Wadden Sea islands will grow to a population size of 2750 breeding adults (1375 breeding pairs), a number closely resembling the prediction of the logistic growth curve. Further research is needed to investigate whether the density dependent effect on adult survival arises in the breeding or wintering areas.

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