Female Dotterels have multiple mates across northern Europe

The first study to track females of a socially polyandrous bird across a whole breeding season, reveals a summer of long-distance travel

To the Point

• Females travel long distances presumably to mate with multiple males: Long-distance mate-searching within a breeding season had only ever been seen in males of species where females do all the parenting. Female Eurasian Dotterels appear to behave similarly, visiting a median of three sites in a season, up to ~1,000 kilometers apart.
• Populations may be more connected than we thought: Females move between breeding areas across a broader area than field research typically covers, with implications for how populations connect.
• New questions raised: Tracking shows where the females go, but not what they do. Answering that will need studies combining tracking with field observations of behaviour.

On a high Norwegian plateau in late May, patches of bare ground begin to appear through the snow, and with them arrive some of Europe’s most approachable birds. Eurasian Dotterels breed on exposed mountain ridges and Arctic tundra from Scotland, through Scandinavia, to Siberia, and are famously tolerant of people – a trait that has long made them easy prey for hunters. They are unusual in another sense too. In this species, it is the female who is the larger and more brightly coloured of the pair, and who takes the active role in courtship. Females display and compete with other females to secure a mate, but once a female has laid her clutch of three eggs, she typically leaves the male who then incubates the eggs and raises the chicks alone.

However, where female Dotterels go next has long been a mystery. Decades of field observations have shown that most females leave the breeding site shortly after laying. It seems too early for the return migration to their North African wintering grounds, so ornithologists have long suspected the females head elsewhere in search of further mating opportunities. None of these individuals, however, had ever actually been followed to confirm this idea. A new study by an international team of researchers led by Bart Kempenaers and his group at the Max Planck Institute for Biological Intelligence, has now tracked female Dotterels using satellite transmitters, revealing striking behavioural patterns and intriguing questions about what they are up to.

“Migratory birds are traditionally thought of as making their big movements between wintering and breeding grounds, with the breeding season itself a much more settled part of the year,” says Peter Santema, a postdoctoral researcher at the Max Planck Institute for Biological Intelligence and co-first author of the study. “We have now shown that female Dotterels can travel hundreds of kilometres between breeding sites in a single summer. What’s exciting is that this looks like the same behaviour we’ve seen in males of other shorebird species where the females raise the young alone. It suggests that in species where one sex handles the parenting alone, the other sex can go looking for further mating opportunities on a scale we hadn’t imagined – opening up new questions about how female Dotterels use the landscape, what these movements mean for their reproductive success, and how they may connect populations that field researchers have long studied separately.”

Multiple layovers

In some shorebird species, like Pectoral Sandpipers and Ruffs, females do all the parental care. Earlier work by Bart Kempenaers and his team has shown that the males of such species can travel thousands of kilometres between breeding sites within a single season in search of further matings. In Dotterels, the roles are reversed: males incubate the eggs and raise the chicks. This new study is the first to investigate how far females of such a polyandrous species might travel.

The team fitted 21 female Eurasian Dotterels with two-gram solar-powered satellite transmitters and followed their movements throughout the breeding season. The birds were tagged over three seasons, on the high Hardangervidda plateau of southern Norway, and at migration stopover sites in the Italian Alps. During the roughly seven-week window when female Dotterels typically lay their clutches, individual females settled at between one and five different sites, with a median of three. Consecutive sites were typically less than 100 km apart, but one female travelled 973 kilometers between two stays, and the most wide-ranging bird covered 1,343 kilometers in total.

Seasonal pattern

The birds’ movements followed a seasonal pattern. Early on, females travelled mostly northward, possibly tracking the retreating snow line as new breeding habitat opened up. Later, the northward pattern faded and movements became shorter, as females may have searched more locally for males whose earlier nesting attempts had failed. The female Dotterels’ movements were more modest than those seen in males of polygynous species – a difference that reflects the greater investment required by females: males need only a single copulation to reproduce at a site, whereas females must form a short-term pair bond and lay a full clutch.

Intriguingly, females lingered at their final site for a median of 41 days – far longer than the brief stays at earlier sites. Two females also returned to a site they had already left, one after twenty days and 382 kilometers away. Previous studies have shown that female incubation occurs at some Dotterel nests and is most common late in the season and at the northern edge of the range. These observations suggest that some of the long final stays may be females switching, at last, to care for a clutch. However, behaviours like these are hard to interpret from satellite data alone. Fully understanding the breeding behaviour of female Dotterels will need field studies to verify remating and female incubation beyond a doubt.

“Local populations of Dotterels may in fact be connected in ways we have so far missed, with the same females moving between distant sites,” says Bart Kempenaers, head of the Department of Ornithology at the Max Planck Institute for Biological Intelligence and co-first author of the study. “The movements and breeding behaviour of these beautiful birds illustrate how the world is connected: their annual cycle stretches from the highlands and tundra of northern Europe and Russia to the semi-deserts of North Africa. Combining tracking with fieldwork at breeding sites is the next step to figure out what these movements mean for the evolution of mating systems. We also need to tag more birds in different regions to better understand how their populations are connected. What we have learnt is that even well-studied birds can still surprise us.”

[AG]