Extensive shell fishing and sewerage discharge in river estuaries could have serious consequences for the rare Icelandic black-tailed godwits that feed there. But it is the males that are more likely to suffer, according to new research from the University of East Anglia.
Research published today in the journal Ecology and Evolution reveals very different winter feeding habits between the sexes.
Both males and females mainly consume bivalve molluscs, sea snails and marine worms, probing vigorously into soft estuary mud with their long beaks. But the study shows that females, which are larger and have longer bills, are able to peck further into the silt to secure larger, deeper buried prey in areas that the shorter-billed males cannot reach. This means that human impacts on estuaries may have different impacts on males and females, depending on which prey sizes are most affected.
The godwit is a large, long-legged, long-billed migratory shorebird. It breeds almost exclusively in Iceland and winters on western European coasts, from the UK and Ireland in the north to the Iberian Peninsula in the south.
The 15-year study saw researchers weigh, measure, sex and ring-tag 1287 birds at locations including the Solent, the Wash Estuary in East Anglia, Iceland and Portugal. More than 2,000 volunteer observers took part in determining where males and females spend the winter, and the researchers measured what the birds were feeding on, how deeply they forage, and how quickly they find, catch and eat their prey.
Dr José Alves from UEA's school of Biological Sciences lead the research. He said: "We knew that females can be as much as 18 per cent larger than the males but we wanted to see what impact this difference had on their migration, feeding and segregation patterns.
"We found that the difference didn't have any impact on their distribution across the wintering area, as both sexes are able to cover the same distances. But we did find that they segregate into different foraging areas on estuary mudflats during winter.
"This is because they are selecting different types and sizes of prey to feed on. Because the females are larger, they need more food. Luckily they are also able to dig deeper and feed on larger prey - particularly the ragworm, Hediste diversicolor, and specifically on large size classes of this marine worm.
"But the smaller males are restricted by their bill size and are only able to dig out and eat smaller prey - particularly Scrobicularia plana, which is a bivalve mollusc, and smaller worms. Therefore, the amount of sexual segregation depends on the scale and variety of prey available at different sites.
"This could have knock-on effects because any environmental impacts on their food chain will affect the sexes in different ways," added Dr Alves
"In particular, the highest abundances of large ragworms tend to be found in mudflats with high levels of wastewater discharge, and females then prefer to forage on these sites. Ongoing improvements in wastewater discharging are likely to mean that large worms become less abundant, forcing the females to forage elsewhere.
"The black-tailed godwit is classified as 'near threatened' so this research will be important for future conservation work for the species."
'Sex-biases in distribution and resource use at different spatial scales in a migratory shorebird' by J. Alves and J. Gill (both UEA, UK), T. Gunnarsson (University of Iceland), P.Potts (Farlington Ringing Group, UK), W Sutherland (University of Cambridge, UK) is published by Ecology and Evolution.
This study was supported by funding from the Calouste Gulbenkian Foundation (JAA), the Arcadia Fund (WJS) and the Natural Environment Research Council (NERC).