Blog by Malcolm Burgess, Principal Conservation Scientist

An open access scientific paper has just published in Ibis, led by PhD student Fraser Bell at the University of Exeter, with scientists from the RSPB Centre for Conservation Science and the Swiss Ornithological Institute, and fieldworkers from PiedFly.Net. The paper reveals the full annual migration of British breeding Pied flycatchers for the first time.

The work identifies important stopover sites used on migration, some previously unknown, and refines previous work in identifying where they spend most of our winter. It also illustrates how flexible migration behaviour can be, which may help species like the pied flycatcher adapt to the changing conditions they encounter on migration in the face of a rapidly changing climate.

The Pied flycatcher is a songbird that can be found breeding in oak woodlands across western Britain, and readily breed in nestboxes. Because of this preference for nest boxes, they are one of the best studied songbirds in Europe and so we know a great deal about their breeding ecology. But like many migratory birds they are in decline. Despite our in-depth knowledge of their ecology in breeding areas, the causes of decline remain largely unknown.

Male and female pied flycatcher with food (c) Tom Wallis

One of the difficulties in trying to understand declines of migratory songbirds is that we know little about their migrations and where they go during our northern winter. We need this knowledge to direct our conservation detective work at the right places. What route do they take? How often do they stop, where do they stop and how long for? Where is their ultimate winter destination?

It is only in the last decade that we can really start to answer these questions, through miniaturised tracking technology. This study did just that, and reveals lots of new information about the migration of British breeding pied flycatchers.

Answers from the field

Thanks to funding from Devon Birds and Natural England, we tracked adult pied flycatchers from the East Dartmoor breeding population in Devon. This nestbox population grew from the 1950s and is now one of the longest monitored populations and nestbox schemes in Europe. Earlier pioneering tracking work published in 2016, tracked two male pied flycatchers from this same population to a rough area in West Africa around Liberia/Guinea. Technology and analytical techniques have since improved, now allowing us to examine this migration in more detail.

The team, including Fraser who also led the tricky analysis of the data, put on tiny geolocator tracking devices to adult pied flycatchers. The tags weighed just 0.42 grams! These data loggers record light levels against a time, enabling us to estimate daily locations from the times of sunrise and sunset. While not as precise as GPS or satellite tracking, which is not (yet) possible for such small birds, these still enable us to determine when a bird moves between breeding and wintering sites and any stop-over sites used on migration, and estimate these locations.

The small size of the tags meant we can’t download the data remotely, they required us to recapture the birds the following year to remove the tag and obtain the stored information. Breeding in nestboxes and individuals returning to the same woodland each year to breed made this possible.

Which route do they take, and where do they stop?

After breeding, pied flycatchers departed Dartmoor from mid July to mid August, first migrating along the west of France. All flycatchers first stopped in Iberia, mostly in Portugal, where they stopped for around 11 days. This must be a very important fuelling region for them. At the end of August or early September they then continued south, flying around the western edge of the Sahara, possibly including some flight over the sea. Most make the Sahara crossing in an incredible single non-stop flight which takes a minimum of 36 hours, flying continuously day and night!

Female pied flycatcher (c) Pierre-Marie Epiney (Flickr - CC BY-SA 2.0)

But not all birds made it over the Sahara in a single flight. We discovered that 3 crossings in autumn and 2 in spring were paused for up to a day. Incredibly one flycatcher aborted a spring desert crossing part way across, returning south for a rest before making a complete crossing 14 days later. It is likely interrupting crossings results from meeting poor weather such as strong winds or sand storms, this is the focus of forthcoming research from the same team. Geolocators only provide data from birds that survive so we don't know how often birds die from weather conditions faced on migration, but we learnt that they can survive such interruptions.

Immediately after the autumn Sahara crossing most, but not all, flycatchers stopped at another stopover site, mostly in Senegal or Mauritania, for about a week. We had no idea they did this! Next they flew to final destinations arriving from mid September. This was a relatively small region of West Africa centred on Guinea but encompassing Sierra Leone, Liberia and Ivory Coast. Intriguingly males and females wintered in slightly different regions, with females west of males. Females stayed in these areas for 25 days longer than males, mainly because they departed later. Females commenced the spring Sahara crossing 11 days later than males, and arrived back to Dartmoor about 9 days later.

Departing wintering areas in late March, on spring migration most flycatchers stopped after crossing the Sahara for around a week, in similar areas of North Africa and Iberia to where they stopped in autumn, before then migrating directly back to Dartmoor.

Animation showing the migration route

Next steps

This work directed us to a fairly small region of West Africa where British breeding pied flycatchers spend half of the year. Fraser and a team from the RSPB Centre for Conservation Science and the Society for the Conservation of Nature in Liberia (the BirdLife partner in Liberia), have undertaken work in two areas of Liberia. Here they occupy a variety of forest habitat types, including open and dry Savannah forest, humid rainforest and farmed habitats containing trees.

Work in Liberia has been to understand more about flycatchers ecological requirements, quite basic research but addressing fundamental knowledge gaps that is vitally important to know. We have been investigating how the different woodlands habitats they occupy influence fitness, for example infection levels by blood parasites that carry avian malaria may differ between these different habitats. Look out for the results of this work.

For the full paper in Ibis: Geolocators reveal variation and sex-specific differences in the migratory strategies of a long-distance migrant

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