Marsh harriers unlikely to have a major impact on breeding waders

In today’s blog, former RSPB MSc student Emily Upcott (now at the UK Centre for Ecology & Hydrology) shares findings from a scientific paper based on her MSc project, and the work of other MSc students, exploring conservation conflict between marsh harriers and breeding waders.  

 Full paper can be found here 

 Marsh Harriers tend to nest in tall marshy habitats like reedbeds, which are often found close to wet grassland where waders including Lapwing, Redshank and Black-tailed Godwit nest and raise chicks. Marsh Harriers and breeding waders are all species of conservation concern that nature reserves aim to encourage and protect, but Marsh Harriers are predators and breeding waders are threatened by predation. Their use of the same habitat in the breeding season has therefore led to a perception of conservation conflict between these two species groups. 

Marsh harrier and adult Lapwing with chick. Credit: Les Bunyan and Amy Millard (rspb-images.com).

We wanted to find out whether this perception is true: what impact are Marsh Harriers likely to be having on breeding waders? 

We investigated the overlap between Marsh Harriers and waders in terms of space (where each species nests and forages), time (periods when Marsh Harrier hunting activity may be higher, and when waders and their chicks may be more at risk of predation), and how many waders Marsh Harriers are actually eating. To do this, RSPB scientists worked with multiple MSc students to collect data in the field between 2013 and 2015, and extracted data from other studies in the UK and northwest Europe from 1942 to 2016.  

We watched Marsh Harriers flying over fields home to breeding waders and close to Marsh Harrier nesting habitat, and recorded how long individual harriers spent in a hunting mode – flying low over the ground looking for potential prey – and whether they hunted over field centres, (where the waders are) or field edges (home to small mammals and other waterbirds).  

We gathered nest hatching dates for waders and Marsh Harriers from recent and historical studies, the BTO Nest Record Scheme and North West Norfolk Ringing Group, and looked at the differences in the timings of nest hatching between different species. 

We also set up nest cameras to record what prey Marsh Harrier parents brought to feed their young, and combined this with records of prey items caught by Marsh Harriers throughout the breeding season in similar habitats from published studies in the UK and northwest Europe.  

Camera on a Marsh Harrier nest. Credit: Lucy Mason

What did we find? 

Harriers hunt most over field centres… 

Marsh Harriers do spend more time hunting over field centres rather than field edges, which does put them into closer proximity to where waders typically nest and rear their young. They hunted more as their breeding season progressed, with only a small amount of hunting during courtship of breeding pairs (April), slightly more hunting by the male during incubation of eggs (May), and most hunting activity by both male and female Marsh Harriers when they’re rearing their own chicks (June).   

…but most wader chicks hatch out much earlier than Marsh Harrier chicks… 

Marsh harrier nests hatched much later than most wader nests, so many wader chicks are already well-grown or fledged by the time Marsh Harrier hunting rates increase to feed their own young. 

…and so wader chicks are only a tiny part of Marsh Harrier diet 

Between 1942 and 2016, in studies conducted across the UK, Netherlands, France, Poland, 

Finland and Norway, wader adults and chicks contributed only a tiny proportion (1.5% on average) to Marsh Harrier prey items in the breeding season. From our own nest camera monitoring in 2013–2015, only 2% of prey items were wader chicks. Instead, the most common Marsh Harrier prey were other birds (particularly chicks of waterfowl and gamebirds, and small passerines) and small mammals (voles), as well as some amphibians, reptiles, fish, insects and carrion from larger mammals in varying proportions. There were no clear trends in marsh harrier diet over time.  

Common Marsh Harrier prey items from nest camera footage (top left: small mammal, top right: coot or moorhen chick, bottom left: gamebird chick, bottom right: small passerine).

Marsh harriers are unlikely to have a major impact on breeding waders 

Overall, we found little evidence to suggest waders are, or ever have been, a major part of Marsh Harrier breeding season diet, so conservation conflict between these species is likely to be minimal. 

However, because Marsh Harrier hunting activity increases in later summer, and their hunting is most common over field centres where waders breed, later-nesting wader pairs and species like Redshank and Black-tailed Godwit may be more at risk if their chicks have not fledged before Marsh Harrier hunting rates increase. Wader conservation strategies that encourage survival of early wader nests could therefore be beneficial, such as using fences to exclude mammalian predators, diversionary feeding to distract predators or head-starting wader chicks. Managing wader habitats to provide more of the main prey groups for Marsh Harriers, like providing patches of longer grass suitable for small mammals, could also be beneficial.  

Watching for Marsh Harriers and breeding waders. Credit: Emily Upcott and Lucy Mason

Acknowledgements 

Huge thanks to the other MSc students and fieldworkers involved in this project – Sophia Cooke, Emma Bagguley, Hannah Gooch, Michael Meadows, Rowan O’Dowd – and to the RSPB nature reserve staff, other landowners and volunteers who helped with data collection, and to the BTO and North West Norfolk Ringing Group for providing Marsh Harrier nest hatching dates. All activities were covered by a Natural England disturbance license. 

This work was joint funded by the RSPB and Natural England through the Action for Birds in England (AfBiE) partnership. 

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