Ongoing large-scale and dramatic human-induced environmental changes in climate and land use cause major challenges to the long-term survival of wildlife populations. One way for individuals to efficiently escape unfavorable local conditions is to disperse to a new habitat. Therefore, to better predict how species could respond to current environmental changes, it is essential to understand the causes and consequences of individual dispersal decisions.
Nevertheless, the consequences of dispersal in terms of reproduction and survival, in particular, remain poorly understood, in part because they depend on the balance between multiple costs and benefits, and their complex interactions. Although dispersal can indeed be beneficial by allowing individuals to escape degrading local conditions, it is also usually assumed to be costly. For instance, it may be risky in terms of survival during the travelling phase; it could also involve costs when settling in the new habitat, because dispersers have no prior knowledge of the new habitat and may be subjected to increased search costs for a suitable territory, increased predation risk and/or increased competition with local individuals. The lack of prior knowledge of the new habitat may also lead to lower foraging efficiency or maladaptation to local conditions.
Few of these costs are considered in studies investigating the consequences of dispersal even though they could, of course, be of central importance, in particular in translocation programs aiming at protecting endangered species. In a conservation context, translocation of individuals to new habitats are often used in an attempt to establish new populations, re-establish extinct populations and/or increase the size of small and declining populations of endangered species, and may be promising in the global warming context.
To explore the potential costs of dispersal in terms of the settlement and subsequent reproduction after arriving in a new habitat, we conducted a forced dispersal experiment in a wild population of a small hole-nesting passerine bird, the collared flycatcher, Ficedula albicollis. We mimicked dispersal movements by translocating individuals between woodland patches within our study area. In this species, males select and defend a breeding site (here, nest boxes provided in the study area) to attract a female.
To select a breeding site, males and females frequently visit empty nest boxes, allowing us to catch them before settlement and the start of nest building. More than 600 birds were caught this way over four years and were either translocated and released in a new patch up to a few kilometers away (mimicking natural dispersal distances in this population) or released back in the same forest patch to serve as experimental controls. We subsequently monitored the population breeding in nest boxes. We recorded settlement decisions to determine whether experimental birds successfully settled to breed within the study area and, for translocated birds, if they accepted to breed in the patch of translocation or returned to the patch of capture. We then recorded reproductive performance in terms of laying date, clutch size, number, and condition of young.
We found that translocated birds were less likely to settle and breed in the study area than control birds that had not been displaced, which may reflect a cost in terms of unfamiliarity with the local environment at the settlement stage. A lack of prior knowledge on, for example, local food sources or predation risk may affect the chances of acquiring a high-quality breeding site and/or mate. This may eventually lead to non-breeding and it may also decrease foraging efficiency and then nestling provisioning once settled, and thus lead to early breeding failure. Because individuals are caught in their nest during breeding, this would increase the likelihood of translocated birds to remain unidentified as breeders.
Here, translocation may thus have induced non-breeding or early breeding failure, which could represent a cost of the forced movement to an unfamiliar habitat. Among translocated birds, individuals were more likely to return to the patch where they had been captured before translocation when they had accumulated more prior local knowledge on this patch, i.e. when they bred or were born in the patch in previous years because they strongly benefitted from such local familiarity.
Finally, we found that translocated birds that remained in the patch where they had been released (i.e. accepted the translocation) to breed raised young in poorer condition as compared to those that returned to the patch of capture and to controls. Other breeding variables, such as laying date, clutch size, or the number of fledglings, did not differ between treatments. Again, the lower condition of young raised by those “forced dispersers”, impairing their future survival chances after fledging, could be due to the cost of breeding in an unfamiliar environment, via, for example, lower foraging efficiency and/or lower quality territory. Alternatively, it could result from lower phenotypic quality for the “forced dispersers” compared to controls and to individuals that returned to the patch of capture.
These alternative explanations are not easy to tease apart, illustrating the difficulty when investigating the causes and consequences of behaviors such as dispersal in natural populations because many factors are likely to influence the motivation to disperse and thereby the consequences of dispersal in natural conditions. Still, our experimental results indicate that translocating individuals to a new environment can have negative consequences both on their subsequent settlement decisions and breeding performance. These costs should be acknowledged in future studies to help understanding changes in the spatial distribution of populations, but also predicting the success of translocations in a conservation context while trying to minimize these costs.
Monitoring the detailed movements of individuals after displacement to get information on individuals that failed to access resources for reproduction and/or moved outside the study area would be required to fully apprehend the consequences of forced movements. Nevertheless, our study provides clear insights on the potential costs associated to settlement in a new habitat.
These findings are described in an article entitled Lower settlement following forced displacement experiment: nonbreeding as a dispersal cost in a wild bird?, recently published in the journal Animal Behaviour. This work was conducted by Marion Germain from the Université de Lyon and Uppsala University, Tomas Pärt from the Swedish University of Agricultural Sciences, and Blandine Doligez from the Université de Lyon 1.