Individual consistency in bird migration

Abstract: Slight differences, which appear in the individual animals of the same species and population have been recognised and appreciated ever since Darwin. These consistent intra-specific differences in animals have been considered very important because they are often inherited, and thus, provide the basic material for evolution to act on. This applies also to behavioural characteristics, and thus, to migratory behaviour in birds too. Nevertheless, the research area of bird migration has mainly been focusing at the population and species level variation. However, as the evolutionary responses to environmental change depend on selection acting on individual differences, disentangling within- and between-individual variation becomes necessary.Every year billions of animals, from insects to whales, travel across the planet, responding to seasonally changing breeding and feeding conditions. These massive, regularly timed, two-way movements of individuals is what we traditionally call migration, a prominent aspect of the annual life history cycle of many bird species globally. It is a type of behaviour that is shaped both by the species’ environment, and its evolutionary history. All these properties make migration a suitable behaviour to study basic questions of organismal biology. As any other field in science, the study of long-distance bird migration has been considerably influenced by technological advancements. Over the course of the last decades, new analytical and technological tools have created research opportunities that allow us to observe and explore the individual differences in migratory behaviour, a perspective we currently mostly lack, due to the inherently difficult in logistical terms task to record long-distance migration. This is an old and fundamental question, asked by naturalists at least 150 years ago, but it has been impossible to address until today. The individual consistency of migratory birds has been the focus here.In our first article, using a long-term bird ringing dataset, we studied individual differences by correlating the date of marking of individual birds with the date of recapturing these birds in the same season in later years within Denmark for 15 common migrant passerine bird species. We found that individuals marked early in one year tended to be recaptured early in the same season in a following year, indicating that individuals time their seasonal migration similarly between years, and highlighting the importance of timing of migration in songbirds.In our second article, we investigated the tracks of repeated migratory journeys of a long-distance migrating raptor, the marsh harrier Circus aeruginosus, as recorded by satellite telemetry, to analyse the variation within- and between-individuals with respect to migratory routes and timing. We found that there was a stronger individual repeatability in annual migratory timing than in the geography of the routes for individual birds on repeated migratory journeys, potentially implying an endogenous control of timing, and strong environmental effects influencing route flexibility.In a follow-up study, we compared these patterns to another medium-sized raptor species migrating within the same flyway, between Sweden and West Africa, the osprey Pandion haliaetus. Unexpectedly, we found clear inter-specific contrasts with low repeatability in timing and duration, and a high repeatability in routes for ospreys, but the reverse pattern for marsh harriers. These findings suggest that the relative degree of individual consistency in space and time is not shared among all long-distance migrants, but may show variation depending on the species’ annual habits and ecology.The return of individual birds at the same location in successive years, also known as site fidelity, has long intrigued amateurs and scientists alike, as one of the most challenging “feats” of bird migration. The next article focused on site fidelity of a nocturnal solitary trans-equatorial migrant, the common cuckoo Cuculus canorus, across its’ a full annual cycle. We found that birds always returned to their breeding territories, whereas their fidelity to all other main stopover areas was much lower, as theycommonly used different localities, dispersing from tens to hundreds of kilometres, within the populations’ wider staging regions, between the years. This was the first study on annual-cycle site fidelity behaviour for a species with many stationary non-breeding annual stages in Africa, and we need similar studies of more species in order to understand how common our findings may be among terrestrial migrants.Research synthesis is the application of the scientific method to reviewing the evidence from the literature, and includes meta‐analyses and systematic reviews. Meta-analysis is a set of statistical techniques for the synthesis of the results of independent studies, and is well established now in ecology and evolutionary biology, as more open data access increase the possibility for syntheses. At the same time, as I discussed earlier, there is growing interest in individual differences in bird migration, as we gradually discover that behavioural traits, once thought to be flexible, may actually be more predictable and consistent. As a result, repeatability estimates (the most common statistical measure to quantify individual behavioural consistency) has been widely used in the literature, lending itself well for meta-analysis aiming to investigate the mean effect of individual consistency across the literature.In the last part of this thesis, I performed a meta-analysis of the published data on estimates of repeatability in migratory behavioural characters. I found a significant mean effect of repeatability in migratory behaviour, with a value close to the mean for all other animal behaviours studied so far. Moreover, I found that the most individually consistent character among bird migrants is the timing of migratory departure, particularly that in spring migration. This may imply that spring departure time can be an informative character for the study of genetic variation in avian migratory behaviour.Based on the insights provided by the findings reported in the studies that comprise this thesis, as well as their potential implications, we can also suggest some of the obvious immediate future directions that are expected to improve our knowledge on the importance of individual variation in bird migration. One step forward, for example, would be to use the existing long-term ringing datasets to further study patterns of individual consistency in relation to annual survival rates. This way we could understand better the link to the population-level changes. Another very exciting but challenging way forward will be to track juvenile birds to adulthood and death, in order to study the development of the migratory behaviour throughout the entire lifespan, and thus, investigate the relative importance of endogenous, developmental, environmental, and experience-based learning effects. Additionally, combining the existing long-term population-level observations of migratory timing, with long-term individual-based tracking within the same population, may help us understand the mechanisms involved in the widely reported climate driven advancements of the migrants’ spring arrival time (i.e. many migratory birds species breeding in the temperate zone arrive earlier due to earlier onset of spring). Finally, apart from selecting migratory departure timing as a useful trait to study genetic variation, we should prioritize our research focus on addressing the geographical dimension of individual consistency at different spatial scales, an area that we know much less about compared to that of the migratory timing.