By Mike Swan, GWCT Senior Advisor
“Overall, we found little evidence to suggest waders are, or ever have been, a major part of marsh harrier diet, so conservation conflict between these species is likely to be minimal.” So concludes Emily Upcott’s recent RSPB blog, summarising the results of her MSc studies.
Well, conservation conflict between marsh harriers and waders may be minimal, but the fact that waders and their chicks are a minor part of the harriers’ diet is not evidence of this, and the RSPB should know better than to suggest it. Indeed, it is almost a truism to suggest that being a minor part of the diet is evidence that marsh harriers could be major predators of breeding waders, because those waders just do not matter to them; if they disappeared, there would still be plenty of other things to forage for.
Reducing these relationships to their most simple, it is obvious that overall prey availability drives predator populations. But, the real world is not simple; very few predators are reliant on just one type of prey, and as Emily points out in her blog, marsh harriers provision their young with a very wide range of foods. This gives them enormous flexibility in foraging, meaning that if one item of food runs short, another will surely fill the gap. It also means that a scarce food resource can be put under intolerable pressure if it happens to be easy to find.
A parallel, and very well documented example of what can happen in these circumstances is the case of ravens and tortoises in California’s Mojave Desert. Here the tortoises are on the road to extinction because ravens are taking virtually all their hatchlings. The long-lived adults are largely safe against the ravens, so extinction is not imminent, but there is virtually no recruitment to the population. Ravens have clearly always taken a proportion of the baby tortoises, but there are now many more of them, sustained by farm livestock, road kill, waste food from rubbish dumps, and other resources provided by humans.
Now, I am not suggesting that marsh harriers alone are going to drive rather scarce breeding waders like lapwings, redshanks and black-tailed godwits to extinction, but I do think that we should be alive to the possibility that they may add to the pressure on these species. In fact, it is perfectly possible that the one or two percent of harrier diet that the waders constitute amounts to the entire year’s production in some places.
I was interested to hear recently that marsh harriers have more or less given up hunting over what was a struggling lapwing colony on the Isle of Wight, apparently because of persistent harrying by the newly introduced white-tailed eagles trying to steal their prey. Coincidentally, lapwing productivity has gone up to above one chick fledged per pair, so perhaps marsh harrier predation was having a significant impact?
So, I am with Emily all the way that we need to do what we can to encourage successful early breeding by the waders, so that their young fledge before the harriers start feeding theirs. I agree with her that preventing predation by other species such as foxes and crows is clearly a key priority, although my gamekeeping background suggests different approaches from the RSPB recommendation of electric fences and diversionary feeding.