A very interesting paper on Hen Harrier diet

Abstract
The hen harrier is a heavily persecuted bird of prey in Great Britain since its diet includes Red grouse, a game bird shot in driven and walked-up grouse shooting. Unlike walked-up shooting where shooters walk up moors and flush grouse with dogs, in driven grouse shooting the grouse are driven by beaters towards static shooters. Driven grouse moors (DGMs) are increasingly being managed to sustain high densities of Red grouse intensifying a long-standing conservation conflict between conservationists and grouse moor keepers. A metabarcoding approach was used with degenerate universal cytochrome oxidase I and cytochrome b primers along with hen harrier blocking primers. A novel sampling method was used to detect prey in buccal swab samples from chicks from broods across Great Britain from both managed and unmanaged moorland habitats. This resulted in detection of 62 species of prey across 51 broods with Meadow pipit, Red grouse, Wren, Skylark, and voles being most frequently detected. Frequency of occurrence data and species accumulation curves reveal high incidence of Red grouse and low prey species richness in the diet of hen harriers in DGMs but low incidence of Red grouse and high prey species richness in walked-up and unmanaged moors. Waders were only detected within walked-up and unmanaged moors and not within DGMs where they have been reported to occur at high densities. Regional species detected included endemic species such as the Orkney vole seen only in Orkney. This study represents the first metabarcoding-based dietary analysis in a raptor using buccal swabs.

https://link.springer.com/article/10.1007/s10592-019-01215-y

This paper is interesting on many levels. It is a study of Hen Harrier nestling diet from a wide range of locations in the UK (Wales, England and Scotland (including Orkney)).

I will comment on three things: how it was done, what are the findings, and what do they mean.

How was the study done?

Normally, studies of diet are done by watching what is eaten by the target species (quite tricky to do accurately in the field) or by recording what bits of food come out of the back end of the study species (also subject to biases) but this study looked at what food had been in the mouths of nestling Hen Harriers by taking buccal swabs of nestlings and examining the DNA found in those swabs. that seems very neat doesn’t it?

The paper contains a lot of information about thi8s technique which makes vague sense to me but which i am not the least bit competent to assess. So let’s take it at face value.

The paper contains data from KK broods and from both Driven Grouse Moors (DGMs) and unmanaged moors from a variety of locations, and from tweo ‘Historic grouse moors (which must, surely, include Langholm Moor in Scotland. The online paper open to the public is rather uninformativce about the locations of the data collection which is a shame (and may not be unusual for chemists buit looks a bit odd to ecologists).

What was found?

The study found evidence of 62 species of prey (mostly birds) and there aren’t many surprises here. Meadow Pipit, Red Grouse (strangely shown as ‘Red grouse’ throughout), Skylark, Wren and voles being the commonest prey species overall.

The range of species found was pretty similar to previous studies and slightly oddly the authors do not cite Watson’s monograph of the species which does contain quite a lot of diet information. There are small numbers of many species in the sample as a whole including Common Frog, Red-throated Diver, Barn Owl, Tawny Owl and a range of passerines.

There were four odd species found in samples from broods on some English DGMs; Turkey, chicken, Badger and Goat. How odd! Unless of course there is some diversionary feeding going on and a variety of species invcluding carrion are being used? Or perhaps this does just represent carrion-feeding? interesting anyway.

So far, this study confirms what we knew already (though more data are always valuable – particularly those derived from a different technique and what appears to be a geographically wider basis).

However, sufficient broods were examined to compare diets from DGMs and unmanaged moors. What comes out of this comparison is that there are relatively few species recorded in the diets of Hen Harriers on DGMs – Red Grouse and Meadow Pipits predominate. Hen Harrier chicks on unmanaged grouse moors are fed a wider variety of prey than are those on DGMs.

What do these results mean?

The larger species number in the diet of Hen Harriers on unmanaged moors is probably because Red Grouse chicks are the most numerous and easiest prey to catch on a DGM – they are so numerous and so defenceless that Hen Harriers pile into them. On unmanaged moors, Red Grouse chicks are still preferred prey items but are at natural and normal densities and therefore hunting Hen Harriers encounter plenty of other species in the course of their hunting forays.

It is theoretically feasible (but contradicted by other data) that DGMs are so poor in other species these days, after intensification of grouse moor management, that DGMs really are poor in alternative prey species but I very much doubt that that is the case. However, without a bit more information on the locations of the study sites in this study, it is slightly difficult to say. Certainly, it would be slightly unwise to rely on the Tharme et al. study (published in 2001, but fieldwork from 1995 and 1996) still to be perfectly accurate over 20 years later.

It’s a very interesting study.

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