Even with the minimal amount of data disclosed by NE on their Hen Harrier study, it’s possible to start making some initial explorations of the data. Here are some initial thoughts which must be accompanied by the massive caveat that this is based on a few minutes playing around, and away from my home computer, so anything said here is subject to proper checking at a later date.
I’ve looked at the 59 satellite-tagged birds’ data pertaining to length of transmission and known fate of the birds – such as it is. One of these birds (an IoM bird) is labelled ‘insufficient data’ so it’s really a database of 58 birds.
Six of these 58 are known still to be alive although one of them was only hatched this year so cannot be said to be long-lived. The other five birds include two that were tagged as nestlings in 2016 and the other three are birds tagged in 2013 at Langholm. Four of the six birds known to be alive were tagged at Langholm. My guess is being tagged at Langholm and staying at the most-watched grouse moor (where they don’t shoot grouse) in the UK is a smart move. In addition, there is another bird which lived for several years after its tag stopped transmitting and bred several times. This bird is an example of how we need to remember that ‘tag not transmitting’ does not equal ‘bird dead’, and we also need to remember that ‘bird dead’ does not necessarily equal ‘bird killed’. We’d be kicking Hen Harriers off the pavements in front of us if they didn’t die of something (as is true of all species – well, the pavement bit is less true of aquatic species). So those are the potted stories of 7 of the 58 birds.
One bird lived at least as long as its third July (it was two years old) and this bird was tagged on the IoM and ceased transmitting on the IoM. That takes us to 8 of 58 birds.
Three more birds are known to have reached their first birthdays – one of them only just, the famous Bowland Betty who was shot in Nidderdale AONB. The other two birds were McPedro who ceased transmitting in France and another IoM bird whose transmissions ceased on the IoM. That takes us to 11 birds.
The remaining 47 birds all ceased transmitting before they reached the July after they were tagged. If they died, rather than this being a tag failure, then they did not (almost certainly) contribute to the next generation of Hen Harriers. We know something of the fate of 8 of those 47 birds – 6 were considered to have died of natural causes and two are known to have been persecuted.
Most of these 47 birds ceased transmissions (perhaps died) in the first 6 months of life (July-December) and rather few of them ceased transmitting (perhaps died) in the period January-June. This pattern is also true of the ceasing transmission dates (perhaps deaths) of the larger number of radio-tagged birds – just eyeing the distributions, the sat-tagged and radio-tagged birds’ last transmissions look rather similar.
We’d expect the first few months of life to be difficult for any bird but it might be that if you eat Red Grouse and spend time on grouse moors then the period from your fledging a few weeks before the Inlorious 12th and the end of the grouse-shooting season on 10 December is particularly difficult. Let’s test this experimentally by closing grouse shooting for a decade or two and seeing how Hen Harrier survival looks then…
But perhaps the most interesting analysis to be done, if and when the real dataset is released, is to calculate the daily risk of ceasing to transmit according to land use. If you spend your time on grouse moors are you more likely to die? My guess is yes – I’d love to see the data analyses (with and without the Langholm special case).
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Just out of interest (from someone who knows very little about tagging). Is it possible to demonstrate that tags on raptors are statistically more likely to fail than on other species. Therefore eliminating the technology arguement. Thus pointing to other factors I.e. Persecution