Hen Harrier Survival: Understanding the Numbers Behind the Headlines

You may remember the media coverage of the hen harrier fate paper by the RSPB’s Ewing et al. (2023) normally. This normally focused on its striking headline:

“Illegal killing associated with gamebird management accounts for up to three-quarters of annual mortality in Hen Harriers.”

At first glance, that sounds as though three-quarters of all Hen Harriers in Britain are being illegally killed. It is easy to see how such a statement could spark outrage. Perhaps that’s exactly what the RSPB’s authors wanted.

But when you look at the data in detail, a more measured picture emerges.

What the Study Actually Found

• Ewing and his colleagues tracked 148 Hen Harriers across Britain.

• They found that survival from fledging to the end of the first year was 14% for males and 30% for females.

• Of those that did survive into the second year, annual survival rose to around 56–58%.

• When both sexes are combined, this means:

  • On Orkney (a long-studied population without grouse moors), about 22% of fledglings make it to their second birthday.

  • In the mainland study population, the figure was about 13%.

That is an absolute difference of 9% - which is real, but far less dramatic than the headline wording suggests.

A Clearer Comparison

The chart above compares Orkney (no grouse moors) with mainland Britain (Ewing et al. 2023).

• Year 1 survival: 35% vs 22%

• Year 2 survival: 75% vs 57%

• Cumulative to age 2: 22% vs 13%

In plain terms: out of 100 chicks, 22 reach age 2 on Orkney, compared with 13 on the mainland. That is a 9% drop – important, but not the catastrophic three-quarters figure implied by the title.

Why the Discrepancy?

The “three-quarters” claim refers specifically to the proportion of deaths in subadult birds (aged 1–2 years) that were attributed to illegal killing. But by that stage, most natural deaths have already occurred. So, the proportion looks large, even though the absolute numbers are relatively small.

What Counts as “illegally killed”?

A crucial point in the Ewing et al. (2023) study is how the authors defined “illegal killing.” When you look carefully at their classifications, a very different picture emerges from the headline claims.

• Out of 148 tagged Hen Harriers, only a faction (in red) were forensically confirmed as illegally killed (three shot, one poisoned, and one caught in a spring trap). These are indisputable cases.

• The authors then placed a much larger group of birds (in orange) into the same “illegal killing” category on the basis that their satellite tags suddenly stopped working, the birds were never recovered and there was no technical malfunction recorded. These are classed as “suspected” or “probable” persecution cases, but there was no carcass and no direct evidence.

• By combining the confirmed cases with these suspected disappearances, the headline figure for illegal killing is inflated to one third of the entire study population.

This amalgamation of hard evidence with circumstantial inference is what underpins the claim that “illegal killing accounts for up to three-quarters of annual mortality.” In reality, the known persecution rate was a small fraction of the total. The rest is essentially an assumption.

Tag Failures

In this study, the researchers did the right thing by removing the few tags that were proven to have failed, that’s normal practice (the ones in grey above). But then they assumed that every other bird whose tag suddenly stopped must have been illegally killed. That is a huge leap.

If just one in ten of those “suspicious” disappearances were actually down to technical failure, the difference they report between Orkney and the mainland would fall from 9% to only 5%. That makes a big difference and shows how much the headline claim rests on assumption rather than hard evidence.

Why it Matters for Policy

• Conservation requires trust in evidence. Sensational headlines erode that trust.

• This dataset confirms what we already know: survival in young Hen Harriers is naturally low and pressures differ across landscapes.

• The real challenge is to work together on constructive, balanced solutions, not to inflame the debate with oversimplified headlines.

Hen Harrier conservation is too important to be reduced to slogans. The facts show a 9% difference in survival to age two, not a wholesale collapse. That distinction matters if we are to move forward with sensible, evidence-based management that balances wildlife conservation with sustainable moorland use.

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