Oystercatchers' bill shapes as a proxy for diet specialization: More differentiation than meets the eye

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.