The investigation of brood meddling of Hen Harriers which Natural England has licensed has resulted in 14 chicks being taken into captivity and 13 of them being released into the wild again. That sounds quite good, but how would we assess how good it is?
Obviously, we can’t simply compare the number of captive-reared chicks with the fledging success of all real Hen Harrier nests over time.
Why not?
Because that would not be a like-for-like comparison as I have mentioned before.
Let’s take 2020 when two nests were brood meddled: the chicks were removed on 2 and 6 June from the two nests. I don’t think we have been told the age of the chicks when removed from their real nests but that date of early June suggests the chicks were fairly young. But we do know they were chicks not eggs. So we can’t simply take the survival rate of any old Hen Harrier nest either in 2020 or any other year as a comparator because we should only be comparing the survival rates of chicks with chicks and not with eggs and chicks.
Just in case I haven’t made the point sufficiently clearly, if the removed chicks had been taken when they were very close to fledging (they weren’t, this is for illustrative purposes) then they would be in captivity a matter of very few days. One couldn’t, obviously, compare their survival over those few days with the survival of Hen Harrier nests from egg-laying to fledging, nor with the survival of day-old chicks to fledging – neither would be comparing like with like.
Ideally, we would compare the survival of the captive-reared chicks with real nests in the same year, over the same calendar period, of the same age, also on grouse moors and in the same geographic locality. That would be quite taxing and might be impossible. However, the fact that each brood-meddled nest has to have a non-intervention nest nearby makes it perhaps possible. I don’t know, because Natural England is very secretive about the data from this ‘study’.
It will be interesting to see how Natural England seek to do this comparison.
There are several other issues around assessing brood meddling which I will come to later.
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