Site attachment of floaters predicts success in territory acquisition

In many territorial species, a fraction of all mature individuals are classified as floaters, and little is known about how these animals eventually acquire a breeding territory of their own. We observed intrusion behavior of floaters, subsequently removed breeding birds, and then observed floaters as they were filling these vacancies in an oystercatcher, Haematopus ostralegus, population. Birds familiar with the area and its inhabitants filled 80% of the experimentally created vacancies. These could be either neighbors or floaters with a former breeding history nearby, but they were mainly floaters with an intrusion record nearby. Floaters obtaining experimentally vacated territories intruded significantly nearer to this territory before removal compared with floaters not obtaining the vacancy. In general, vacancies that were not occupied by intruding floaters tended to be located in areas where less intruding floaters were seen prior to removal. We show quantitatively that only birds familiar with a site succeed in establishing a territory at that site, suggesting that local information is essential for territory acquisition. We propose that the main aim of intrusion behavior may be the collection of such information.