Each year billions of birds migrate between their breeding sites in Europe and non-breeding sites in Africa. The migratory life-style allows them to make optimal use of seasonal changes in food. But how flexible is this migratory life-style when environments are rapidly changing? Can they change their timing and movement decisions to keep track of advancements in peak-food availability during breeding as a result of climate warming?
During the last 6 years, I studied the possibilities and limits of Pied flycatchers Ficedula hypoleuca to advance migration schedules. Pied flycatchers have become a model species to study phenological adaptations of long-distance migratory songbirds to climate change. Despite the great value of long-term field studies at the breeding grounds, these studies provide a limited view on the life of a migrant that spends most of the year in Africa. Various ideas were proposed to explain why birds did or didn’t arrive earlier at their breeding sites, often without actual data on their behaviour outside the breeding season.
This stimulated me to study these birds not only at a breeding site in the Netherlands, but also during the non-breeding period. By describing their migratory timing and connections, as well as ecological conditions during non-breeding, I hoped to gain insight in whether the environment is constraining these migrants in arriving and laying their eggs early at the breeding grounds.
It appeared the right time to start answering these questions since geolocators were just small enough to track these tiny birds. We really wanted to find out why some pied flycatchers arrived early at their breeding grounds and others late. Was it a difference in migration speed, non-breeding (hereafter ‘wintering’) location or departure date from Africa? And what does this tell about the potential of individuals or the population to arrive earlier at the breeding sites?
We found that individuals’ spring arrival date was reasonably consistent among years, some always being early and other being late. Yet, the geolocators data and field work at the breeding sites also showed that time schedules were also adjusted to external conditions, particularly in young birds and in autumn. The large variation in spring arrival dates in this breeding population was tightly linked to departure dates from West Africa, but not to spring migration duration. The journey of over 5 000 kilometres to the Netherlands took around two weeks, starting with a non-stop flight across the Sahara.
The most important conclusion concerns the possibilities and limits to advance spring arrival at the breeding sites. Given the short migration duration in spring, with little variation in migration speed among Dutch birds, an earlier arrival can unlikely be achieved via faster migration. However, it might be possible via adjustments in the departure date from Africa. Such population adjustments towards earlier migration schedules could be slowed down or prevented if harsh circumstances during the winter in Africa or during spring migration hinder successful and early spring arrival. Even more so, because woodland habitats in Africa are subject to large-scale changes and deforestation.
We mostly investigated adaptation of pied flycatchers to climate warming at the breeding grounds, but also wanted to study the role of wintering conditions and habitats on the spring departure decisions of birds. At that time it was still unclear where ‘our’ Dutch birds would winter, or, if they just spread out over the whole wintering range in the Guinean savannah zone south of the Sahel. Studying their departure decisions appeared very difficult, since a small sample of geolocators were deployed, while the devices had many technical failures and birds were difficult to recapture the next season. Yet, we did describe changes in arthropod food, the birds moult and fuelling phenology, and report on observations on the birds’ behaviour. An interesting, unexpected finding was that the pied flycatchers in Ghana were using a suppressed form of song, both in autumn and in spring.
Although there are indications – partly from this thesis research – that wintering site conditions are key in successfully preparing spring migration, the precise role is still too poorly understood. Since the wintering grounds are subject to large and very rapid environmental change, that may affect many migrants, it is particularly urgent to focus future investigations on the role of their wintering grounds. Studying wintering ecology of migrants at their highly dynamic wintering environments might not be the easiest task, it is urgent and provides many unexplored grounds that require and deserve further research.
Conservation Ecology Group, Groningen Institute for Evolutionary Life Sciences, University of Groningen, P.O. Box 11103, 9700 CC, Groningen, the Netherlands
Ouwehand 2016. Track Changes in Pied flycatchers. Annual cycle adaptation in an Afro-Palearctic migrant. PhD thesis. University of Groningen, Groningen. ISBN 978-90-367-9184-7 (electronic version). https://drive.google.com/file/d/0B54tkuGGqqbuRXFzVnVaVW5sdUU/view