TERRITORY QUALITY, PARENTAL EFFORT AND REPRODUCTIVE SUCCESS OF OYSTERCATCHERS (HAEMATOPUS-OSTRALEGUS)

1. Oystercatchers that breed on the saltmarsh of the Frisian island of Schiermon-nikoog occupy two different types of territory. In 'resident' territories the nesting area and the feeding area are adjacent. In 'leapfrog' territories the nesting area and the feeding area are separated by distances of 200-500 m. 2. Between 1984 and 1989, residents fledged on average 3.5 times as many chicks per year as leapfrogs. 3. The discrepancy in fledgling production arose primarily through increased mortality of leapfrog chicks due to starvation. This was indicated by low body weights of leapfrog chicks found dead and successful enhancement of leapfrog chick growth through supplementary food. In addition, the presence of a sibling reduced the probability of fledging for leapfrog chicks, but not for resident chicks, providing further evidence that leapfrog chicks competed for a limited food supply. 4. Leapfrog parents fledged fewer chicks, not because of poor feeding conditions in their feeding territory (intake rates of leapfrogs exceeded those of residents), but because they failed to transport a sufficient amount of food to the chicks. To supply their chicks with the same amount of food as did residents, leapfrog parents should have spent c. 4000 s per low water period in transport flights: no leapfrog parent ever reached this level of effort. 5. Our data show that it is not correct to equate parental effort to the number of chicks raised, as territories of different quality require different levels of effort for successful reproduction.