Migratory decisions in a changing world
Birds are famous for their long and arduous migrations, often over incredible distances. Migrants face many challenges during their annual movements, but they also benefit from being able to exploit high quality feeding areas throughout the year.
Many bird species are suffering alarming population declines due to threats like climate change, habitat loss, hunting and other human pressures. In Europe, migratory species have shown particularly steep declines, perhaps because they are exposed to so many different threats on the course of their annual movements.
For some bird species, the urge to migrate is not fixed, but varies from year to year and from individual to individual. The Lesser Kestrel is a good example – most individuals migrate from their Mediterranean breeding grounds to central Africa each year to spend the winter. A small proportion, however, stay around the breeding grounds all year. Why do some birds do this? And what does it mean for their chances of survival?
We still have much to learn about how birds make migratory decisions. In particular, we have little idea about whether differences in migratory behaviour are important for the survival of species. It is possible, for example, that species with the capacity to change their migratory behaviour will be more resilient to issues like climate change.
About the Lesser Kestrel migration project
In this project, we are examining how and why Lesser Kestrels make decisions about migration. We’re hoping to discover what influences the urge to migrate, and also to find out what it means for the survival of the species as a whole. By examining the causes and consequences of differences in migration, we hope to gain a better understanding of how migratory species will cope in today’s changing world.
Lesser Kestrels breed in farmland habitats across Southern Europe, and the majority of the population perform an epic migration to spend the winter in sub-Saharan Africa. For some decades, it has been known that a small proportion of individuals fail to undertake this migration each year, instead remaining around their breeding colonies throughout the colder winter months. Our goal is to understand what provokes this behaviour, and whether it might be important in determining whether this species will persist in an uncertain future of climate change and habitat loss.
How do we follow the birds?
We are studying the Lesser Kestrels at a number of breeding sites in southern Spain. The species breeds colonially, often nesting in old farm buildings or on cliff faces. Many pairs also breed within towns, nesting in cavities in church spires or around other tall buildings.
We use a variety of techniques to track the movements of Lesser Kestrels. At each colony, we catch and ring the adults and fit them with data loggers to monitor their movements. We also take tiny feather samples from each bird to analyse their chemical composition. Subtle differences in isotope signatures in these feathers can tell us where a bird was when the feather was moulted.
A male Lesser Kestrel being weighed after ringing
What are we trying to find out?
First and foremost, we want to find out where each of the Lesser Kestrels breeding in our study area goes to spend the winter. Then, we can see whether there are certain factors that predict whether or not a bird will migrate to Africa. Is it related to age or sex? Is it related to weather conditions, or the health of the bird at the end of the breeding season? Do individual birds always follow the same strategy, or do they vary their migratory behaviour from year to year?
We also want to see whether migratory decisions have important implications for the survival of the species. Do birds that migrate to Africa start breeding at a different time to those that don’t? Do they get better or poorer nest sites? Do they raise more young?
To answer these questions, we will carefully monitor the breeding activity of each Lesser Kestrel in our study colonies.