Growth and Abrasion of the Oystercatcher Bill in Relation To Dietary Switches

Abstract

Oystercatchers feeding in the littoral of the Wadden Sea show three different shapes of the bill tip: pointed, intermediate and blunt. Females have longer bills than males. Bill length may vary from place to place, particularly among the females. Adult males more often have blunt bills than pointed ones compared to adult females. Pointed bills of either sex are longer and have thinner tips than blunt ones. Changes in bill length of individuals ranged from -3.4 to + 2.4 mm, on average these changes were small, varying between 0.8 and 1.3 mm. Transformation of bill shape from intermediate into either blunt or pointed occurred frequently (in up to 50 % of the cases), from pointed into blunt, or the reverse less often (in up to 11.5% of the cases). Inland breeding Oystercatchers all have pointed bills. During the first three or four weeks after arrival from the wintering area (the Wadden Sea) bill length of all birds increases with about 4 to 5 mm, whereas blunt and intermediate bill tips are transformed into pointed ones and all bills wear laterally at the tip. During the next two months bill length decreases 2 to 5 mm, whereafter it remains constant for the rest of the inland stay. The rhamphotheca of the bill only grows in its proximal half. Mean growth rate was 0.4 mm per day, this being similar for free living adult birds feeding in pasture land and captive birds feeding on food pellets. Growth rate of the bill was not different when feeding either on pellets or mussels. Abrasion of the bill tip was much more regular, keeping in pace with growth, when feeding on mussels, but irregular when feeding on pellets, resulting in great changes in total bill length, because long tips broke off. Both free living and captive birds showed large interindividual variations in growth and abrasion rates. Growth rate of captive individuals varied widely but asynchronously throughout the year. Finally the correlation between bill shape, prey choice and feeding technique is discussed: a pointed bill is associated with probing for subterranean soft-bodied prey, a blunt bill with the opening, either by stabbing or hammering, of mussels or shallow buried cockles, an intermediate bill with the probing for and opening of deep buried Scrobicularia, Macoma and Mya. When conditions are stable individuals keep to one prey and one technique periods on end, hence their bill shape does not alter. When feeding conditions do change the birds are able to switch to other prey, and consequently the shape of the bill will gradually change into the new type associated with the new prey and feeding technique.