Causes of Mortality in Wiltshire Recorded Bird Fatalities: 2017 – 2020 Inclusive

The point of ringing birds is that it makes each bird individually identifiable, thus allowing ongoing, quantifiable analysis of traits, populations, etc.  This enables ringers recapturing birds to record the following on an individual basis:

  1. Age
  2. Movements
  3. Compare condition over time
  4. Monitor breeding condition (including sexing monomorphic species)
  5. Identify and collect, where possible, the details of their death

This last is something that is often informed by members of the public, reporting a ringed bird found dead to the BTO.  The finder gets a report back from the BTO telling them when and where the bird was ringed.  The ringers gets a similar report with the details of where it was found dead and the cause of death, where reported.  Unfortunately, the cause of death is not always capable of being identified, but often it is.

Phil Deacon diligently records all recaptures of birds that are recorded by the BTO that have either entered or left Wiltshire, whilst moving either on migration or post-breeding / post-fledging dispersal. Within that he also records all fatalities reported to the BTO.  This is a brief analysis of that mortality data, collected between 2017 and 2020.

The first thing I looked at was the recorded causes of death:

Clearly, the recording of death is a very patchy affair, with some 35% of causes not being recorded, or not being able to be recorded.  Unsurprisingly, the largest, identifiable, causes of death are the domestic cat, followed closely by windows, and then traffic.

Surprising to me was the number of birds that drowned in water butts / artificial ponds: 2 Barn Owls and one each of Blackbird, Nuthatch and Great Tit.

Of those shot, 3 were Teal hunted in Finland, Russia and France.  Killed illegally was a Grey Heron, ringed at Swindon Sewage Works but shot elsewhere in the Swindon area four years later.  That is hard to understand.  There was also a Cormorant ringed as a nestling near Anglesey and shot just 7 months later near Lower Woodford.  Although it is possible to get licences to kill Cormorants, to protect the profits of angling clubs, to kill a bird that was clearly juvenile seems wrong to me.

It is hard to understand why Woodcock (2) and Snipe (1) are still considered game species.  In the UK Woodcock are red-listed, and Snipe are an amber-listed species, due to population declines here. I am aware that shoots based in the Braydon Forest area restrict their shooting to the released Pheasants and Red-legged Partridge they release for the purpose but I am surprised that it is a matter of individual restraint. If there is little meat on the bones of a Snipe, who on earth shoots Redwing? Someone in France it would seem.

Of the 138 recoveries the largest cohort was the Barn Owl by a significant margin. They are striking birds that are difficult to overlook, are regularly found around human habitation and are a closely monitored species.  Looking at how their deaths break down is quite instructive:

The term building is used loosely to cover, in this case, barns and one that is a large open space, but man made and managed.

Barn Owls are known to be frequent road traffic victims, and this is underlined by the figures shown here: far and away the commonest reported cause of death.  

There are some intriguing cases here though: two of the four killed within buildings / man-made structures were found in the same barn in Pewsey on the same day. I would be keen to know what would cause two of these iconic birds to perish simultaneously but, unfortunately, that has not been recorded.

Another of the birds found dead in a man-made structure was one found in the lion enclosure at Longleat Safari Park, again the location is given but not the cause.   I suppose a slow, low flying bird might well tickle the fancy of a bored lion.

What are the chances that one of the rarest birds in Wiltshire, a Montagu’s Harrier, would end up as a road casualty? That is what happened to a juvenile ringed in Wiltshire and hit by a car just over the border in Hampshire 30 days later. 

Talking of rare birds, a Merlin ringed on the Shetland Islands in 2016 was recovered dead, having collided with electricity cables at Stanton St Bernard three years later.

The total casualty list by species is as follows:

Many thanks to Phil Deacon for his hard work in collating the data and for giving me permission to use data from outside the West Wilts Ringing Group in preparing this post.

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