The Spoonbill Platalea leucorodia population of the Wadden sea islands :does population growth level off?

Na een dieptepunt van ca. 150 broedparen in 1968 is het aantal Lepelaars in Nederland gestaag gaan toenemen. In het broedseizoen van 2008 werden 1907 paren geteld, waarvan 1179 op de Waddeneilanden. Hier bekijken we de groei van de populatie op de Wadden eilanden in meer detail en proberen we te voorspellen hoe lang deze groei nog zal voortduren. We doen dat door op grond van de resultaten van een langlopend kleurringproject en waarnemingen aan het broedsucces een populatiemodel op te stellen. Het model voorspelt dat het aantal broedparen op de Wadden eilanden zal stabiliseren rond 1375, doordat de jaarlijkse overleving van volwassen Lepelaars afneemt naarmate de populatie groeit.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

Individual shifts toward safety explain age-related foraging distribution in a gregarious shorebird

Although age-related spatial segregation is ubiquitous, the underlying mechanisms are poorly understood. Here, we aim to elucidate the processes behind a previously established age-related foraging distribution of red knots (Calidris canutus canutus) in their main wintering area in West Africa (Banc d’Arguin, Mauritania). Based on 10 years of observations of 1232 uniquely color-ringed individuals of 1 to 18+ years old, we examined whether the observed age-related foraging distribution resulted from 1) spatial differences in mortality or 2) age-related shifts in habitat use. Using multistate capture–recapture modeling, we showed that with age foraging red knots moved away from the shoreline, that is, to areas with fewer surprise attacks by raptors. Considering uncertainties in the subjective gradient in predation danger with increasing distance from shore (as assessed from correlations between vigilance and distance from shore in foraging birds), we applied 2 different danger zone boundaries, at 40 m and 500 m from shore. Between years, red knots had a much higher chance to move from the dangerous nearshore area to the “safe” area beyond (71–78% and 26% for 40-m and 500-m danger zone boundary, respectively), than vice versa (4% and 14%). For neither danger zone boundary value did we find differences in annual mortality for individuals using either dangerous or safe zone, so the move away from the shore with age is attributed to individual careers rather than differential mortality. We argue that longitudinal studies like ours will reveal that ontogenetic shifts in habitat use are more common than so far acknowledged