Dispersal through fitness landscapes in a social bird: from individuals to populations

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The effect of environmental variability on natural populations is perhaps the most pressing concern facing natural scientists. The distribution of populations is affected by spatial variation in their environment and the factors driving population change have been identified in many cases. However, it is individuals rather than populations that respond to variation in the environment that they encounter in their lives, and it is the consequences of these responses for individuals' capacity to survive and reproduce (i.e. their 'fitness') that determines what happens to their population at larger spatial scales. Therefore, to understand the processes through which spatial and temporal variation affects populations, there is an urgent need to understand the impact of that fine-scale variation on the behaviour, life history and ultimately the fitness of individuals. The overall objective of this project is to fill this gap in knowledge by investigating the effect of fine-scale heterogeneity in the environment on the dispersal and evolutionary fitness of individuals in a population of social birds, and to use that understanding of individual responses to explain population-level effects. By studying a social species in which limited dispersal plays a vital role in the expression of kin-selected helping, we can also test the role of environmental heterogeneity in driving cooperation. The long-tailed tit is a small bird that exhibits a social system unique among UK bird species in which failed breeders often help close kin to raise their offspring. The behaviour, ecology and genetic composition of a population of individually marked birds living in a heterogeneous habitat has been closely monitored since 1994, providing a unique resource for this project. Direct fitness (from breeding) and indirect fitness (from helping) have been quantified for a large number of individuals using lifetime reproductive success data. Such detailed information on fitness is available for very few species, and in no other system has both direct and indirect fitness been quantified in this manner, offering us a unique opportunity to address these fundamental ecological issues.The first objective of the project is to investigate how animals move along gradients in their environment. What factors do animals respond to when deciding to disperse from one place to another? There are essentially two kind of dispersal decisions. Natal dispersal is the movement from birthplace to the place of first breeding, while breeding dispersal is the movement observed between successive breeding attempts that may occur within the same breeding season or between seasons. Using long-term data and a modeling approach we will test hypotheses concerning the factors influencing individual movements within a variable environment. The second objective is to conduct experimental manipulations of the social environment and of reproductive success to test whether there is a causal relationship between kinship and fecundity gradients and the natal and breeding dispersal decisions of individuals.The third objective is to test the hypothesis that helping behaviour is a strategy that reduces the variation in fitness among individuals. In other words, helping is a risk-averse or bet-hedging strategy that individuals adopt at times or in places when the probability of gaining fitness directly (i.e. by reproduction) is low. In addition we will investigate the consequences of variation in individual fitness for population-level phenomena, testing the hypothesis that social species experience positive density dependence due to their social interactions.Meeting these objectives will provide novel insights into the mechanisms through which individual-level responses to spatial and temporal environmental heterogeneity affect fitness and translate into population-level impacts, thereby addressing key gaps in our understanding of ecological processes.