Dispersal decisions: common terns, Sterna hirundo, choose between colonies during prospecting

Abstract

In long-lived bird species, a deferred onset of reproduction is assumed to be linked with a so-called prospecting phase when young individuals compare potential breeding sites before they decide to settle in the home area or to emigrate. However, this has rarely been documented with empirical data because of technical difficulties in collecting sufficient data from nonbreeding individuals at several sites. In the long-lived common tern, we used a novel transponder system to identify remotely all natal prospectors visiting two colonies of different size throughout the breeding season in 2001. Males attended the colonies about twice as frequently as females. Independently of sex, the proportion of birds recorded at both colonies was much higher among individuals born at the smaller colony. In the first half of their attendance time, individuals moved twice as often between colonies as in the second half. In both sexes, prospectors clearly favouring the larger colony were much more likely to breed there in the following season than prospectors with no clear colony choice, whereas no individual favouring the smaller colony during prospecting bred subsequently at the larger colony. Among birds born at the smaller colony, a higher proportion of female than of male prospectors attended mainly the larger colony, whereas among prospectors born in the larger colony, we did not find any sexual difference in attendance patterns and most birds favoured their home colony. The study suggests that prospectors select future breeding sites and that differences in philopatry between sexes are influenced by environmental quality.