What to think about Hen Harrier numbers

To make sense of the figures for 2023 UK Hen Harrier numbers, released today by RSPB, they need to be seen in context.  There are five important contextual considerations.

  1. There are pretty good estimates of how many pairs of Hen Harriers could exist in different parts of the UK in the absence of illegal persecution and those suggest that, overall, the UK population is at between a third and a fifth of its potential depending on the year.
  2. We have a long run of Hen Harrier surveys, carried out in a comparable way, stretching back over three and a half decades (see Table below). There is little value in just focusing on the comparison with the last survey when we have a run of data. Particularly because…
  3. …Hen Harriers have good years and bad years in terms of whether their main prey in the breeding season are abundant enough for few or many pairs to settle and breed. Like Short-eared Owls (but less so than SEOs) we think Hen Harriers sometimes decide not to settle, or give up very early in the season, if there aren’t enough voles in their area. There are good vole years and bad vole years but these vary across the UK so there is a patchy picture. However, whether your survey year is better or worse than average for voles is likely to affect the survey results either a bit, or a lot, and without rather complex and expensive UK-wide vole data, we have to realise that there is this variation between years.
  4. Hen Harriers move around – they can move around between years and they move around by moving from the moors where they mainly nest to southern parts (sometimes out of the UK) and to lowland and coastal areas in winter. And so the three Hen Harriers I saw coming in to roost at Wicken Fen in early March might just conceivably have been single English, Scottish and Welsh birds, and their fate will be determined not just by the conditions on their nesting grounds, or their wintering grounds, but on their journeys between the two.
  5. Illegal persecution on grouse moors continues and is a potent factor in determining numbers.

Here are the results:

Hen Harrier Surveys

Potential pairs1988/89 1998 2004 2010 2016 2023
Northern Irelandc150
UK totalc2500534

What should we make of them?

  1. The UK Hen Harrier population is at about a quarter of its potential level (compare ‘2023’ with ‘Potential Pairs’) and that’s not good when we believe that this lack is largely due to illegal persecution. However, despite that, 2023 was…
  2. …overall a good year for Hen Harriers across the UK; numbers were good, not just compared with the last survey but compared with the year before and the run of surveys going back a long way. This is not true of Northern Ireland. It is true of England where the number of pairs was not just higher, much much higher, than in the previous survey year but was also higher than the previous four survey years combined.
  3. Despite having a very good year in 2023 (which we also know has been preceded by a run of improving years) England is still the worst performing country, compared with potential, of the four UK nations. Scotland is best (despite the fact from previous surveys that eastern grouse moors are basically Hen Harrier-free zones), Northern Ireland is second best (even though this year was a relatively poor year there), Wales is second worst and England, despite a much better 2023 than previous surveys is still worst. If the situation in England were similar to Northern Ireland then we would have something like 75 pairs of Hen Harrier in England now. I once supported a lower-league football team and we had good years and bad years but we were lower-league all the same – England is still lower-league for delivering Hen Harriers. It will take continued improvement (at a greater rate than other nations) for England to get anywhere near Northern Ireland in this respect.
  4. This is speculation, but informed speculation; I wonder whether the increases in Hen Harrier numbers in Scotland and Wales are partly due to reduced spring persecution in England. There must be reduced spring persecution in England because there are more Hen Harriers, and harriers passing through England to other nations may benefit from this too.

3 Replies to “What to think about Hen Harrier numbers”

  1. Really excellent news and pleased to read that harrier numbers have increased substantially over the past 7 years despite all the major challenges and hurdles this particular raptor faces.

    Perhaps we should also all remember that this total figure quoted of territorial pairs is only an estimate and its true figure would undoubtedly be higher if all suitable breeding habitat for harriers had been explored. I’m looking forward to reading the full publication which will give more information regarding statistics, etc used to arrive at this estimation.

    1. Mike – thanks for your comment, and we should both wait for the paper, but I don’t think you are right. As I recall, some areas, those covered by Raptor Study Groups as their core areas, are regarded as fully surveyed. There may be some birds missed in these but not many, and they make up a small proportion of the total potential UK breeding area.

      Other areas of suitable habitat are sampled (maybe in strata) at random and surveyed by field surveyors hired for the task (what a life!). From these sampled sites one can calculate the population in all the unsampled sites, and its confidence error. Add that to the ‘known’ pairs and you have the estimated total with confidence intervals.

      So, depending on the sampling intensity the population estimate may well be a bit wrong, but its wrongness is estimatable. And it isn’t any more likely to be an underestimate than an overestimate.

      Let’s see what the paper says. Thanks.

  2. An increase in Hen Harriers is of course good news and like Mark I suspect the figures especially for England and Wales are pretty accurate. However the number of pairs on Scottish grouse moors has apparently declined and may still be declining, almost certainly as a result of continuing persecution, hopefully the recently passed new legislation in Scotland will discourage this.
    The picture in England is also encouraging but is perhaps not as good as we might be led to believe by all the shooting lobby organisations sickeningly claiming credit for the increase in birds. They are fond of claiming how many nest on grouse moors compared to RSPB reserves, Which on the face of it is true but it is a grossly inappropriate comparison as RSPB only own about is about 12,000 acres whilst there are according to the MA 860,000 acres of members grouse moors. In fact if we take into account land where RSPB are involved in both protection and monitoring of Hen Harriers there is a harrier nest about every 1000Ha whilst on those much vaunted grouse moors a pair every 25,000 ha a 25X lower density and on grouse moors the failure rate is about 3X higher, nothing to brag about at all.
    When those grouse moors really start delivering so our English harriers are not at a mere 15% of capacity ( and the real capacity is much higher than 330pairs) but much nearer 100% perhaps persecution almost zero only then can the shooting lobby brag.

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