Immigrants are attracted by local pre‐breeders and recruits in a seabird colony

Abstract

Summary1. Immigration is a major demographic factor shaping population dynamics. However, due to methodological difficulties, the extent of immigration and factors affecting immigration are insufficiently studied. This is also true for seabird colonies. 2. We estimated annual immigration based on a long-term study of a colony of common terns Sterna hirundo marked with transponders, using a Bayesian integrated population model that links colony size and productivity with individual life histories. 3. Strong annual fluctuations in the number of immigrants were found. To identify whether colony-specific covariates influenced immigration, we related the number of immigrants to various proxy variables for breeding site quality, specifically colony size, productivity, number of local subadults and local recruits. Numbers of local recruits and local subadults showed strong positive correlations with number of immigrants. 4. We found that variation in immigration rate had strongly contributed to variation in colony growth rate, more so than variation in local recruitment or adult survival. 5. Collectively, results suggest that immigration strongly affects colony growth rate, that the driving force behind immigration is natal dispersal and that immigrants were attracted by local recruits.

The role of behavior in tern conservation

“…Additionally, range expansion and local population increases often result from immigration, thus via dispersal from natal or breeding colonies (Ledwón et al, 2014). In tern colonies, immigration can be the key driver of temporal variation in colony growth rate (Szostek et al, 2014b) and helps prevent inbreeding (Szczys et al, 2005;Ludwig and Becker, 2012), while philopatry may have the opposite effect (Sruoga et al, 2006).…”

“…Most measures of philopatry are probably biased toward high values because it is difficult to detect individuals who disperse (Coulson and Coulson, 2008). As in most birds, dispersal is generally female-biased (Dittmann et al, 2007;Becker et al, 2008;Draheim et al, 2010; but see Devlin et al, 2008), and male recruits therefore frequently pair with female immigrants (Becker et al, 2008;Szostek et al, 2014b).…”

“…In philopatric species, vacancies created by mortality of breeders or availability of new sites are more likely to be filled by young, inexperienced recruits rather than experienced breeders moving from other sites (Morris and Chardine, 1995;Tims et al, 2004). When movements occur, they tend to be directed toward larger colonies (Spendelow et al, 1995;Dittmann et al, 2007;Ratcliffe et al, 2008; but see Devlin et al, 2008) that are either more productive or contain more prospectors and recruits (Szostek et al, 2014b). Exchange among colonies is also more likely among colonies that are closer together (Spendelow et al, 1995;Devlin et al, 2008).…”

“…Exchange among colonies is also more likely among colonies that are closer together (Spendelow et al, 1995;Devlin et al, 2008). In some cases local populations can be maintained or grow despite low productivity or low apparent survival by immigration from other colonies that are declining or have disappeared (Szostek and Becker, 2012;Ledwón et al, 2014;Szostek et al, 2014b), but such population dynamics are unlikely to be sustainable. In other cases movement away from less successful colonies contributes to further local declines (Lebreton et al, 2003), but probably results in greater recruitment in the metapopulation as a whole.…”

Better the devil you know: common terns stay with a previous partner although pair bond duration does not affect breeding output

“…The use of transponders together with an efficient distribution of reading antennae ensured reencounter probabilities close to 1 [23] as well as an accurate identification of parents (electronic supplementary material) for the marked birds. There are partners at the colony that are not marked, which are typically immigrants [24], and some birds with incomplete breeding histories (e.g. birds transponder-tagged as adults and 13 birds ringed only).…”