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Data on the breeding success of several characteristic coastal breeding bird species in the Wadden Sea have been collected each year since 2015. Ten bird species considered representative of specific habitats and food groups are being monitored. The monitoring scheme on breeding success in coastal breeding birds is run as an ‘early warning system’ to follow the reproductive capacity of the bird populations in the Wadden Sea and understand the processes underlying fluctuations in populations. It is a valuable addition to the monitoring of population numbers and is carried out under a trilateral agreement with Germany and Denmark (TMAP). The data are also included in the OSPAR biodiversity indicator B3 under the Marine Strategy Framework Directive. The 2019 results indicate that little has changed in the poor breeding success observed in previous years, although there were some positive exceptions locally and for some species, partly related to special conservation measures. Sinds 2005 worden in de Waddenzee jaarlijks gegevens verzameld over het broedsucces van een aantal karakteristieke kustbroedvogels. Tien vogelsoorten worden gevolgd die in trilateraal verband representatief worden geacht voor specifieke habitats en voedselgroepen. Het reproductiemeetnet kustbroedvogels wordt uitgevoerd als een ‘early warning’-systeem om het reproducerende vermogen van de vogelpopulaties in de Waddenzee te volgen en de achterliggende processen van populatieveranderingen te doorgronden. Het fungeert als een belangrijke aanvulling op de monitoring van aantallen en aantalsveranderingen en wordt uitgevoerd in het kader van trilaterale afspraken met Duitsland en Denemarken (TMAP). Tevens worden de gegevens opgenomen in de biodiversiteitsindicator B3 van OSPAR t.b.v. de Kaderrichtlijn Marien. Uit de resultaten van 2018 blijkt dat de in eerdere jaren geschetste situatie met betrekking tot slechte broedresultaten weinig is veranderd, al zijn er lokaal en bij sommige soorten positieve uitzonderingen, mede in relatie tot speciale inrichtingsmaatregelen.

Eurasian Oystercatchers Haematopus ostralegus ostralegus are a classic example of individual feeding specialization. Feeding specialization causes morphological differentiation in Oystercatchers’ bill shapes due to varying degrees of abrasion associated with specific handling techniques for each prey species. Consequently, the Oystercatcher’s bill shape has been used as a proxy for diet specialization, as it provides a quick and easy way to assess the diet choice of this marine top-predator. However, bill shapes of Oystercatchers are categorized visually in distinct types, while it has been argued that the relevant variation is continuous. Also, it is unclear how comparable the bill-shape classification is among studies and between the sexes and how universal bill shape–diet relationships are. Here we investigate the usefulness of bill-shape types as a proxy for diet choice in Oystercatchers, using four new and two published datasets. We show that quantitative bill-morphometrics provide no evidence that bill-shape types are discrete entities. Additionally, the dimensions of the same bill-shape type differ across studies. This difference is unlikely to be caused by methodology and might reflect subtle additional feeding specialization among birds with the same bill-shape type. Moreover, we show that the tip of the bill of males is typically 7% higher than that of females with the same bill-shape type. A higher – and probably stronger – bill tip in males may explain why males had more shellfish in their diet than females with the same bill-shape type. Finally, a literature-review shows that the exact bill shape–diet relationship differs between studies and the sexes. We conclude that the interpretation of bill-shape type as a proxy for diet choice in Oystercatchers is context dependent. We propose that quantitative bill dimensions are a better proxy for feeding specialization than bill-shape types.