Diet dimorphism‘ within an Oystercatcher Haematopus ostralegus population– adaptive significance and effects on recent distribution dynamics

Of the 50 or so Oystercatcher pairs breeding on Skokholm in 1965 and 1966, 15% bred near the shore and 85% bred inland; 28% fed their young on limpets, and 72% on terrestrial arthropods and earthworms. ‘Terrestrial‐feeders’ were three times more successful than limpet‐feeders in fledging young, irrespective of variation in initial brood size and of hatching date but depending on differential losses to gull predation. This is because parents' vigilance is relatively low in limpet‐feeders' territories, and limpet‐fed young are on average undernourished and hence behave relatively carelessly. Since the 1930s the population of shore‐breeding gulls has increased considerably on Skokholm. A certain increase of the Oystercatcher population during this period seems to have been associated with a shift inland, until the island became saturated at the late 40s. It is proposed that the increase in shore‐breeding gulls has triggered a genetically based behavioural change in Oystercatchers, which has been responsible for the shift to inland habitats and to terrestrial food, which promotes a population increase. This interpretation for events on Skokholm may be applicable for the general trend of habitat shift and population increase of other British populations.