Exploring social animal migrations : a study on long-distance avian migrants

Abstract

During the past centuries, researchers have been studying animal movements using a large variety of methods, ranging from marking individuals to applying advanced biologging devices. By now, we have gained extensive knowledge on animal movements, but, so far, most knowledge in the field of animal movement and migration comes from studying individuals in isolation. However, many animal species live in groups and group behaviour can benefit individuals, both during fine-scale movements and during long-distance migrations. The field of social animal migration has recently received more attention, but a lot remains to be learned. To uncover gaps in the research that has been done so far, I first present a systematic literature review on past studies that investigate the influence of social processes on migration decisions of animals (Chapter 2). I show that there are biases in the research that has been done. Specifically, most studies have been carried out on birds in the northern hemisphere, and on the influences of the short-term process of social information use on spatial decisions. To encourage researchers to put more emphasis on studying social influences during migration, I provide suggestions on how to design studies with high inference. Here in my thesis, I use the white stork (Ciconia ciconia; hereafter stork) as a model species to study social animal migrations. Storks are long-distance migrants that fly in groups that can be as large as several hundreds of individuals. Through years of research on storks, we have gained a deep understanding of their individual migration patterns, which makes them a good species to go beyond the individual and explore their social migratory behaviour. In addition, storks can carry high-resolution biologging devices that allow us to study their behaviour in high detail. In Chapter 3, I apply the knowledge gained in Chapter 2; I present the results of an experiment designed to quantify the effect of an altered physical and social environment during migration. I performed a delayed migration experiment that caused juvenile storks to migrate under different weather and social conditions. I show that a delayed migration has consequences ranging from a higher energy expenditure and inferior flight conditions to a higher first-year survival and a shorter migration distance in the first and second year. This chapter sparks the question whether it is still beneficial for storks to migrate long distances. In addition, the results from this chapter suggest that group composition can be crucial for the life-time development of individual migration patterns. Therefore, in Chapter 4, I examine the composition of the group in more detail. To do this, I used a feeding experiment and automated tracking technology to estimate the individual behavioural phenotype of the experimental group. I describe the group organisation of the storks in two contexts: a dominance hierarchy based on feeding interactions in a stable group and leadership during migration flights of the same individuals. I show that white storks establish a weak dominance hierarchy and exhibit a stable leader-follower group organisation during migration flights. However, group organisation does not correlate between both contexts, presumably due to different processes and individual properties playing a role in shaping group organisation in different contexts. In conclusion, to better understand animal migrations, researchers should consider the social processes that can affect migration decisions of individuals. Recent developments in biologging techniques as well as the use of experiments in the wild are promising approaches for the study of social animal migrations.