Category Archives: Predator/prey relationships

Sylvia – another unusual visitor

Like a lot of other folk I gave up making new year resolutions a long time ago because the resolve would normally last until the 2nd or 3rd of January before slipping quietly unnoticed into the flotsam and jetsam of recent history, never to be seen again. But for 2013 I made two resolutions – the first was to get current with my wildlife diaries which have been appallingly neglected for far too long – and the second resolution was not to condemn the first one to the black hole into which it would normally be swallowed. And so far so good, hence I’m feeling rather pleased with myself.

This years listings can be found here at ‘Histon Wildlife Diaries 2013‘ and if you notice gaps of more than a couple of weeks opening up please feel free to leave a pointed reminder that I need to get my finger out and get up to date!

As a consequence of my girded up loins and renewed efforts I’ve been spending more time peering into the garden to see which creatures are in residence. Just before Christmas I saw the first blackcap in the garden, it was a male with his coal-black cap, like a judge about to hand down the ultimate sentence, and he stayed for all of 2-3 seconds before zooming off into the sanctuary of our neighbours orchard. And of course I was very pleased with this visitation because it’s always good to welcome a newcomer.

Then a couple of weeks ago when the winter weather was at its filthiest here in Cambridge a female appeared and spent some time refuelling on my fatball feeder:

A female blackcap (Sylvia atricapilla, Dansk: munk) on the birdcam fatball feeder

The birdcam used to be around 6m away from where it is now but there were very few takers for the lardy delights on offer.  But since I moved it to its current location it has been busy every day with numerous bird species. The reason for the change is the lack of cover in the original location which left the birds exposed to the possibility of predation by the local sparrowhawk. But now they have cover within a metre or so and and I can sit and watch them all in close up on the TV. And one of the first to arrive after I moved it was my lady blackcap, who you’ll have immediately noticed has a rufous brown cap, not the blackcap of the male. The specific name ‘atricapilla‘ means ‘blackcap‘ and the Danish name ‘munk‘ means ‘monk‘. I wonder which godly habits gave him that name, or is it simply his ecclesiastical bonnet?

She arrived early on a murky morning, fed quickly and left, and that was the last I’ve seen of her. But a couple of days later a male blackcap arrived and he’s been visiting several times a day every day ever since then:

The male blackcap feeding on an old apple

I’m puzzled as to why the female has been so conspicuous by her continued absence, I guess that now the weather is considerably more pleasant she is more comfortable feeding out in the countryside.

The male doesn’t restrict himself to ground feeding on fruit but is a more regular visitor to the fatballs.

And he tops up with water too. In the picture below he is wary of the goldfinch nibbling niger seed on the adjacent feeder. He was also aware of my presence behind a glass door around 8m away and when the goldfinch disappeared he threw numerous glances in my direction, but so long as I remained still he wasn’t too bothered.

Until recently it was thought that the blackcap was a migrant breeder here in the UK and that they spent their winters in Africa, apart from a sub population that remained here in the winter. But it is now thought that all of ours overwinter south of the Mediterranean and our winter visitors are a separate population from central Europe which migrate here to overwinter. In which case my visitors will be heading back east in the near future. After that I’ll hopefully see and hear our migrant breeders out in the hedgerows where they make a distinctive call which I think sounds like someone flint-knapping.

The British Trust for Ornithology have published a factsheet about blackcaps and their migration behaviour which is worth a read. We also have passage visitors as Scandinavian birds head south and it appears that garden bird feeders are having a major impact on the behaviour of blackcaps and other species too, such as nuthatch, which are now spreading into Scotland, assisted by garden feeding and climate change.


Barn owls bounce back

Every now and again there is a piece of good news regarding the survival of a particular species. Yesterday on the BBC news website was just such a story. It concerned the resurgence of barn owls (Tyto alba, Dansk: slørugle) in the Trossachs around Loch Lomond in Scotland.

Watching barn owls silently quartering fields on a warm summers evening is a rare treat and getting rarer, so it was good to hear that in this area the numbers of field voles (Microtus agrestis) have rocketed by up to ten fold, which has led to a concommitant increase in the breeding success of the local barn owls.

A barn owl quartering a wheat field

Paradoxically, the reason the vole population exploded was the long freezing winters we had last year and the year before, in which large numbers of barn owls perished, but the voles were able to avoid the worst of the cold and move around by tunnelling under the snow and thereby avoid detection by airborn predators.

It was also interesting to read that the owls were maximising the benefit of the vole surplus by storing slaughtered prey in owl boxes, with up to 15 dead rodents in a single box.

Hopefully vole numbers will continue to remain high and barn owl numbers can recover even further. If anyone has heard similar reports about barn owl numbers in other parts of the country please let me know.

The balance of nature

At the end of May this year the BBC screened an episode of Countryfile in which John Craven interviewed a member of the scientific staff at the Game and Wildlife Conservation Trust (GWCT). Taking things literally I’d assumed the this was a trust who’s aim was the conservation of game and wildlife. However, as the interview progressed I began to question this assumption as the discussion centred around a study to be carried out by the GWCT in which magpies were to be eradicated from a specified test area to assess the local effect on songbird populations. I don’t think that removal of a native species such as the magpie by trapping and killing them is an ethical way to study predator/prey relationships. But is it scientific?

The magpie – villain of the piece – apparently

The interviewee from the GWCT, Dr Jeremy Stoate, justified the cull on scientific grounds suggesting that increased predator numbers result in significant reductions of prey populations and the species they identified as being largely responsible was the magpie.

Magpies are bold, brassy, ubiquitous, and are generally percieved as being a tad raffish. They undoubtedly predate songbird nests, but I struggled to understand how this could justify slaughtering them. Numerous other species of bird and mammal such as sparrowhawk, kestrel, owls, great spotted woodpeckers, rats, stoats and probably the biggest killer of small birds, domestic cats, all predate songbirds and some raid nests to poach eggs and chicks.

So why was it only magpies that were to be exterminated?

The action was being justified on a scientific basis, and whilst I’m no ecologist, I am a scientist, and the whole thing seemed unscientific, so I decided to do a little research of my own on the subject. Interestingly, since I did my initial research in June, it appears that references to the study on the GWCT website have been taken down. At least, I couldn’t find any.

However, what I did find out was that the study was commissioned and financed by another ‘conservation’ organisation called Songbird Survival. Whilst I believe that GWCT do have a genuine interest in the countryside as a whole, the same cannot be said for Songbird Survival. It is a registered UK charity whose statement of activities on UK Charity Commission website is this:


However, the details on the Charity Commission website also list the trustees, and that makes interesting reading too…


All of these trustees are big landowners, or relatives of landowners, some with shooting estates, and they all support hunting and shooting. Lord Coke is the owner of Holkham Hall in Norfolk and he and several of his gamekeepers have been prosecuted for killing birds of prey by shooting and poisoning. So I’m struggling to believe that these people really have the best interests of songbirds at heart when they are funding ‘scientific’ destruction of a single predator species which has been implicated in taking gamebirds. And via the titled gentlemen listed as trustees they would be ideally placed to carry out their stated aim of ‘lobbying MP’s and the House of Lords‘.

Back to the science though. I recently attended a lecture in which Dr Mike Toms of the British Trust for Ornithology (BTO) talked about garden birds and how gardens are becoming an increasingly important sanctuary for increasing numbers of songbirds, and therefore predators, such as magpies, great spotted woodpeckers and sparrowhawks. In the course of his lecture Dr Toms talked about the decrease in songbird numbers and how magpies were being blamed but said that magpie predation was not the cause of the decline in numbers but habitat destruction and current agricultural practices were the primary reasons. I emailed Dr Toms and he was kind enough to send me several links to published work by the BTO into bird predation. This list included this publication: ‘Population changes of avian predators and grey squirrels in England: is there evidence for an impact on avian prey populations? Journal of Applied Ecology 47: 244- 252‘ describing a collaborative study between the BTO, GWCT and the University of St Andrews in which the impact of 2 groups of predators, those which predate adult birds and fledglings and those which predate nests (including the magpie) were analysed for 29 prey populations. Quoting directly from this paper:

For 22 avian prey species, there is no evidence that increases in common avian predators and grey squirrels are associated with large-scale depression of prey abundance or population declines‘.


Unexpected was a large number of positive (my italics) associations between predators and prey, particularly for native avian nest predators, which largely exonerates these predators as driving declines in passerine numbers


Analyses of large-scale and extensive national monitoring data provide little underlying evidence for large-scale impacts of widespread avian predators and grey squirrels on avian prey populations…

The paper also describes negative correlations between the remaining seven prey species and predator numbers, and accepts that some of these may be causally related. Although e.g. the relationship between sparrowhawk and tree sparrow is probably significant, that between buzzard and goldfinch almost certainly isn’t. However, in an email from Dr Toms to me he indicates that the GWCT study showed negative causal relations on  a local level but, crucially, not at a population level.

This last point is key to the argument. In a study of population dynamics, if conclusions are to be drawn about overall depletion of numbers in response to a specific influence small local studies cannot be extrapolated to enable conclusions for the population as a whole, and to do so is erroneous at best, misleading at worst.

Incidentally, I also found a publication on the website of Songbird Survival entitled ‘Detecting an impact of predation on bird populations depends on the methods used to assess the predators’, (Methods in Ecology and Evolution (2010),1,300-310, Nicoll M., Norris K.). This is an interesting paper because rather than analysing actual population data it investigates the methods used to analyse the data. And in the summary the last point recommends: ‘…the findings for studies which use opportunistic data, for a limited number of predator species, should be treated with caution and that future studies employ bespoke census techniques to monitor predator abundance for an appropriate suite of predators.’

This is interesting because it argues that the methods used to analyse opportunistic, observational data must be carefully optimised to ensure that conclusions drawn from the data adequately reflect the observations.

None of the publications suggest that slaughtering the predators is a rational scientific way to assess the negative impact on prey species. And I’m inclined to think that culling is poor science, particularly as the latest evidence suggests predator numbers don’t reflect negatively on songbird populations and in some instances there is actually a positive correlation between predator and prey, presumably because the predators remove the sick and the slow before they can add their DNA to the genepool.

I think the notion that predators destroy prey populations is bizarre because I was taught, and importantly, I believe it to be the case, that prey abundance determines predator numbers, not vice versa. If prey numbers decline then shortly afterwards so do predator numbers. Taking the reverse of this theory to it’s logical conclusion, if predator numbers determine prey numbers, all prey species would eventually become extinct and then the predators would also become extinct and the balance of nature which has prevailed for the last few billion years would be turned on its head. Which is absurd.

So population studies which are centred around the slaughter of predators seem to me to fly in the face of all current theory and are therefore morally indefensible aswell as being scientifically unsound, especially when commissioned and financed by such obvious vested interests as Songbird Survival.