Local dominance and territorial settlement of nonbreeding oystercatchers

1. We studied the mechanisms of territory acquisition in an oystercatcher (Haematopus ostralegus) breeding population on Schiermonnikoog (1983-1997), competing for low quality (leapfrog) and high quality (resident) territories. 2. Numbers of nonbreeders on the island varied from 960 to 3380 during 1984-1997, compared to ca 4620 breeders. We estimated that 50% of the nonbreeding individuals, or 27% of all adult birds (at least three years of age), were capable of breeding but failed to do so. 3. Nonbreeders were apparently active in searching breeding positions by: (a) frequent intrusions into territories and hovering ceremonies above nesting territories, evicting breeders, creating territories or filling vacancies. (b) Forming pair bonds and establishing local dominance positions on the mudflats or clubs to usurp breeders. to fill vacancies or to squeeze between territories. Depending on the location of activity, we distinguished 'aggressive club-birds,' 'hopeful residents' and 'opportunistic nonbreeders' among the nonbreeders. 4. Due to the high survival of breeders, low numbers of primary vacancies became available annually. Additional secondary vacancies became available from breeders deserting their territories for better breeding positions. The relative number of deserting individuals decreased from female leapfrogs, male leapfrogs, female residents to male residents. in the main study area leapfrog territories outnumbered resident territories 2 : I. Hence, nonbreeders had more opportunities to obtain a leapfrog: territory in both absolute and relative terms. 5. Nonbreeders filled only 46% of the resident (N = 94), and 32% of the leapfrog vacancies (N = 179). This was due to severe competition with single and paired breeders also aiming at these vacant sites. Hence, only 33% of the nonbreeders acquiring a breeding position used a vacancy (N = 236). As an alternative, paired nonbreeders squeezed between breeding territories (45%) or more rarely, single/paired nonbreeders usurped single/paired breeders (18%), or single females joined breeding pairs into polygyny (3%). 6. Hopeful residents settled on the shoreline close to their mudflat territory, usually as a resident, sometimes as an adjacent leapfrog. Aggressive club-birds with high dominance positions on the main club in the leapfrog area, settled close to this club as a leapfrog. Opportunistic nonbreeders settled in territories close to where they had intruded in previous year(s), either as a resident or as a leapfrog, A high dominance position at a given locality seems a prerequisite for getting a territory at that locality. 7. By removals of a mate during incubation, experimental breeding vacancies were created. The most interesting results were obtained from permanent removals, which resulted in clutch loss (89%) and territory loss (56% of the 'widowed' birds, N = 9). The other 44% of 'widows' got a new, mealy previously nonbreeder, mate. The experiment confirmed that: (a) nonbreeders were capable of bleeding; and (b) only cooperative pairs are able to raise a brood and defend a territory. 8. Many nonbreeders settled after severe winters, due to more vacancies arising because of increased mortality. Yet many leapfrog vacancies remained unoccupied, despite nonbreeders being available to fill these vacancies. This at first sight anomalous observation can be explained by the queue model (Ens et al., 1995), since waiting for a vacancy in a resident territory might be more beneficial than immediately accepting a low quality leapfrog territory. 9. We propose that nonbreeders face three trade offs, (a) between sampling a large area and finding many vacancies (i.e. opportunistic strategy) versus establishing a local dominance position, sampling a small area and finding fewer vacancies, but with a higher chance of successfully competing for that vacancy (i.e. hopeful resident or aggressive club-bird strategy); (b) between competing for a low quality territory with small numbers of competitors versus fighting for a high quality territory with many competitors; (c) between finding a breeding position singly, and mating a widowed breeder with breeding experience Versus pairing with a nonbreeder, thereby finding a breeding position by squeezing between breeding territories.