I suppose the most notorious example of this in the birding world was Songbird Survival’s attempts to blame Sparrowhawks for the decline of songbirds, because their numbers were increasing whilst songbirds numbers were decreasing. However, Sparrowhawks were recovering from a catastrophic decline, due to organochlorine poisoning, primarily recognised in the 1960’s and 70’s, plus impacts of deforestation and persecution, whilst songbird numbers were declining, as they still are, due to myriad factors: primarily modern farming practices and habitat loss. Unfortunately, I find a lot of this when talking about supplementary bird feeding, particularly garden bird feeding: both in terms of spreading disease, reducing populations on the one hand, and artificially inflating populations of some species on the other.
I have a particular frustration with the discussions of whether or not we should feed birds in our back gardens. There are two “anti” arguments. The first is that feeding is spreading diseases which is leading to the decline of certain species. Secondly, is the contention that feeding birds in your garden is enabling those that take advantage to out-compete those that don’t, leading to those other species declining. The one thread that is never discussed: the educational benefit of involving humans, connecting them with wildlife, and raising their interest in what is happening to nature, balanced against those other considerations.
I have always fed the birds in my garden, wherever I have lived: urban, suburban and rural, and have always kept records of what I have seen. In 2003 I joined the BTO’s Garden Birdwatch scheme (GBW), as a way of providing data to a national database, outside of my personal interests. For six years, between 2015 to 2020 inclusive, I was a Garden Birdwatch Ambassador, doing 10 or so talks per year to groups as varied as the Women’s Institute, gardening clubs and U3A, to drum up interest in wildlife gardening and membership of the GBW scheme. During those talks I promoted the provision of feed, food plants and wildlife habitats in gardens, to encourage people to take an interest in what goes on and improve things for our wildlife. On every occasion, whilst encouraging people to feed the wildlife in their garden, I did stress the need for keeping excellent levels of hygiene at feeding stations. Most obviously bird feeders and, in the case of my garden, the hedgehog feeders. (Yes, I am lucky, alongside 60 species of bird recorded in my garden plus another half-a-dozen overflying, I also have fox, hedgehog, badger and wood mouse as regular mammalian visitors (unfortunately, I also have brown rat coming from under my neighbour’s decking, and their cats) and have recorded over 450 species of moth – including a first for the county.)
In 2009 I became a bird ringing trainee, gaining my C-permit in 2012 and my A-permit / trainer’s endorsement in 2015. Once I got my C-permit, which allowed me to ring without the supervision of my trainer, whilst he remained responsible for my activities, I started to catch and ring birds in my garden. I also got permission to ring birds in the Forestry England sites of Red Lodge (about 1km from my garden), Webb’s Wood and Somerford Common. Also, the Wiltshire Wildlife Trust gave me permission to ring in the Ravensroost complex, the Firs and Blakehill Farm. All of these sites, including my back garden, were once part of the medieval Braydon Forest hunting ground.
I have been aware of the fluctuations in numbers of some species and, particularly, the catastrophic decline in the numbers of Greenfinch visiting the garden between 2003 and 2015. This is primarily attributed to a Protozoan parasite, Trichomonosis gallinae. It is believed to have been passed through Columbidae to other species, and particularly Greenfinch. Collared Dove are the most affected member of the Columbidae, and their numbers have clearly increased significantly over the decades, almost certainly helping to spread the parasite. There is a view that it is spread at feeding stations. One would have thought, certainly with the preponderance of supplementary feeding in the UK, that someone would actually have devised methods of testing this and its subsequent transmission. That hasn’t happened. However, what you tend to see in the scientific press is conjecture: the correlation = causation conflation, as I call it. That is not to say that it isn’t true, but it is to say that it is an hypothesis in need of testing.
I have not seen a single incidence of trichomoniasis in any of my Greenfinch sightings or catches for over 7 years now. When it comes to my ringing catches, they are quite hit and miss, depending upon how many times I ring in my garden in a year, as that is where I mainly catch them. The numbers are: my garden: 127, the rest of the Braydon Forest: 6. I have seen a big increase coming to the feeders over the last few years, and this year we are regularly seeing 10 or more in or around the garden. This week I had some discussion on this with Alex Lees on Twitter. He is a senior lecturer at Manchester Metropolitan University, involved in lot of work on biodiversity and population dynamics, with lots of published papers to his name. He is one of the advocates that bird feeding is responsible for serious species declines through the spread of disease. While we were discussing something else, covered later in this post, he expressed the opinion that Trichomonosis parasites are being spread significantly through bird feeders. As evidence he provided a link to a paper that showed how the parasite could be kept alive in bird seed:
It is behind a paywall so this is their summary, which is freely available: “Trichomonas gallinae has emerged worldwide as a cause of mortality in songbirds (passerines). The congregation of numerous birds, including the reservoir hosts, pigeons and doves (columbids), at backyard feeding and watering sources has been suggested as a potential driver for the outbreaks. Evidence supporting a role for water in transmission has been established, but the role of birdseed in the transmission of trichomoniasis remained to be investigated. We assessed the survival of T. gallinae in three commercial birdseeds (mixed seed, black-oil sunflower seed, and niger seed) routinely used to attract passerine birds to local properties. Trichomonad suspensions were inoculated (low dose: 1 × 103; high dose: 1 × 105) into each of the three seed types in petri dishes, using both dry and moist (water-soaked) conditions, in triplicate. Petri dishes were incubated at 37 C and monitored for T. gallinae survival for 48 hr by wet-mount microscopy and by InPouch™ TF medium culture for 10 days. Surviving trichomonads were not detected in any of the dry birdseed treatments. In moist conditions, however, trichomonads were found to survive ≤24 hr in all three seed types and ≤48 hr in the mixed seed that contained organic debris. We demonstrate that T. gallinae has the ability to survive in moist birdseed, which suggests that public bird-feeding sites may play a significant role in the transmission of trichomoniasis.”
They state clearly that this study was done under laboratory conditions, in petri dishes, incubated at 37oC! I appreciate we live in a time of climate change / global warming, but that bears no resemblance to the reality in my garden, and I doubt whether it does in any UK garden. The only bird seed I use is sunflower hearts. I use small feeders, emptied every other day or so, the feed is never left wet and we provide clean water every day. Another person I regularly discuss things with, the estimable Alick Simmons, a well respected veterinarian, once chair of the Universities Federation for Animal Welfare (if you haven’t read his book “Treated Like Animals”, it is well worth reading, whatever your dietary proclivities) in supporting Alex, sent me a link, below, on bird feeder diseases. Alick has himself stopped feeding birds, to prevent the spread of disease from his feeding stations. I keep mine clean and disinfected, only use hanging feeders, as I find bird tables are just too difficult to keep clean, and keep a keen eye out for any signs of disease in the birds coming into my garden. If I see or suspect it, I stop feeding for a week and disinfect everything. That said, it is years since I have seen anything like that. I did have to take down and disinfect after a Chaffinch with Fringilla papillomavirus visited my garden a year or two ago, but that is not in the same category as trichomoniasis.
Interestingly, when you get to the entry on Trichomoniasis it says the following: “Unlike Salmonella, Trichomonas cannot survive for more than a few minutes outside its host.” To my mind that means that it is highly unlikely to survive to infect other birds under standard UK conditions, so I am not sure what the relevance of incubating them under laboratory conditions is, other than to reinforce a predetermined point of view. The fact is that they could have done this in a typical garden environment and got more meaningful, relevant, results.
I don’t doubt that poor feeder hygiene can lead to the spread of diseases: in particular Salmonellosis. Interestingly, that same paper says the following about this disease:
“Epidemiology: No surveys of the occurrence of outbreaks of this disease have been undertaken in the UK but it appears to be a common cause of epidemics in garden birds especially during the colder months from December through to April.”
That was the case when that was written in 2000. Subsequently there have been some studies, seemingly focused on the potential for spread from wild birds to humans through poor hygiene at feeding stations, rather than on the extent of infection of the birds involved. Apparently the key species are the poor old Greenfinch, Chaffinch and House Sparrow. (e.g. this from the BTO website, it links to the full paper: https://www.bto.org/our-science/publications/peer-reviewed-papers/epidemiological-evidence-garden-birds-are-source-human)
There have been surveys elsewhere around the world, and it seems that faecal contamination plays a large part in its spread. So, feeder hygiene is incredibly important. However, I want to point out the use of language: “appears to be”, “suggests”, “may play”. Time and again, and I have read every paper that Alex has linked me to, as much as I can, but many are hidden behind a pay wall and I don’t have the benefit of an academic institution to pay for my access, they use those sorts of phrases: not “we have data that shows that this is the case” but “we have data that shows that this might be the case”.
It seems that salmonellosis is far and away the most likely infection to be spread from dirty feeders. The thing about it, though, is that, unlike Trichomoniasis, it is not fussy about what species it might infect (as I can attest, having nearly been hospitalised from a bite from a Great Black-backed Gull). So why wouldn’t the commonest species coming to garden bird feeders be the biggest sufferers with Salmonellosis, and why doesn’t that lead to a reduction in their numbers? Why don’t we know this? Plenty of studies say that there is a level of infection within many bird populations, which can occasionally cause mass outbreaks and deaths, but is it because it is generally in the population and other stressors lead to those mass events or is it that there are occasional naturally occurring “blooms” of infection that lead to those events? Does anybody know? There must be a PhD or two in there somewhere. Given the extent of bird feeding in the UK, if this is genuinely causing severe disease problems, why hasn’t it been more intensively studied? Why don’t we have some better data? There are other diseases, like E.coli based infections, that also affect wild birds, just like they affect humans, but we know very little about how widespread they are and what impact they have on different bird populations. Almost all of the studies that I have found focus on the potential for humans to contract these diseases from wild birds.
I am not arguing with the idea that poor bird feeder hygiene is a major source of disease outbreaks: that is why I exercise the hygiene regime that I do. Also, I am pretty sure that not everybody feeding birds in their garden carries out the same levels of hygiene that I do. However, I would like to see some genuine quantification of the levels of infection in different species and some serious strategies for dealing with them.
If it is contentious when talking about disease, it is more so when talking about the impact of bird feeding on species biodiversity. I fully accept that the number of species coming to garden feeders is limited. Is that having an impact on birds in other habitats? As previously mentioned, I have registered 60 species of bird in GBW counts in my own garden, and ringed exactly 30 species. Compare that with the 67 species ringed across the Braydon Forest between 1st January 2013 and today. That is pretty decent biodiversity for a back garden!
Back in October 2021 I read and commented on the following paper:
https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2021.0480
“Faecal metabarcoding reveals pervasive long-distance impacts of garden bird feeding” by Shutt, Trivedi and Nicholls
It is a very interesting paper but there were two things about it that caused me to have a look at my own data on movements of Blue Tits, particularly around my key ringing area, the Braydon Forest. The difference between my analysis and theirs is that, because I was using ringing data, my data enabled me to identify the movements of individual birds, and my conclusions were different to theirs. Essentially the movements of the four species of Paridae that I ring are relatively small. Up until the end of 2023 my team has ringed over 4,500 Blue Tits in the Braydon Forest, and we have had just under 2,000 recoveries of those birds. Only 32 recoveries have moved further than 1km from where they were ringed. In that time, we have retrapped only 4 Blue Tits that were not ringed in the Forest: two from nearby Swindon, one from the West Midlands and our Scottish recapture, detailed in the paragraph below. Similarly with Great Tit: 2,180 ringed, 1,280 retraps, with only 23 of those retrapped birds moving more than 1km within the Braydon Forest, and only one moving in from outside, a distance of 2km and two moving out: both to the same place that we recovered that bird from: Waterhay in the Cotswold Water Park. So I had a few thoughts on why that might be, and a key difference between the two data sets, apart from our sheer volume of data, is that in the bar-coding study, in order to be able to collect their faecal samples, they set up Blue Tit nest boxes at known distances from the feeding station. However, there is no allowance in their conclusions for the potential impact of providing those nest boxes. In the Braydon Forest there have been no new titmouse nest boxes installed for nigh on 20 years, so nesting is all natural outside of the villages.
I do like irony: on the 9th January 2022 we retrapped a Blue Tit, AVF6109, that was ringed at Fort Augustus in Highland Scotland 3 years and 639km before: the second longest movement recorded for a Blue Tit in the UK. However, one movement like that does not negate the overwhelming evidence of restricted local movements being the norm.
But this brings me on to the second part of this story: in the January 2022 edition of British Birds (volume 115, pp 2-6) , there was an article written by Alex Lees, R. Schutt (of the faecal barcoding study) and Richard Broughton, who did his PhD thesis on Marsh Tits, and has been heavily involved in studying them ever since called “Rethinking Bird Feeding”. Their proposition was that feeding birds in your garden was detrimentally affecting species that didn’t make use of that feed, and was a driver in the decline of both Marsh and Willow Tit, suggesting it could be driving local extinctions. In my previous response to the Shutt et al paper, I had looked at the current state of my local Paridae populations: Blue and Great Tit were showing marginal declines, Coal Tit a somewhat stronger decline and the only species showing an increase was the Marsh Tit. When I raised these data in response to their article, Dr Shutt clearly didn’t read my piece, but felt the need to be angry about it anyway, despite the fact that my post took pains not to claim anything other than for the situation local to me and to not gainsay their findings. Richard Broughton, extrapolating from his single visits to Ravensroost Wood and Webb’s Wood, decided to say that the population of Marsh Tits in the Braydon Forest was so small that it couldn’t provide meaningful data, misrepresenting the number of Marsh Tit territories in Ravensroost Wood, and combining it with his “data” from Webb’s Wood to further weaken my data and “strengthen” his argument.
I pointed out that playing an MP3 of Marsh Tit call to count territories in two of the five woods on one occasion each, when not all of either wood was covered (I was with him at Ravensroost Wood, and know exactly how much of the wood we covered), no matter how accurate his technique is, it doesn’t really compare with the 11 years of ringing data, 24 years of birding the Ravensroost site, four years of studying all of the Braydon Forest woodlands for the BTO Bird Atlas between 2007 and 2011, plus the input from the volunteer warden and his team of observers, and their monthly breeding bird surveys of Ravensroost Wood, which showed a density of between 5 and 6 Marsh Tit territories in a 40 hectare wood, of which 30 hectares is suitable Marsh Tit habitat: rather better than the 1.4 territories per 10 hectares, Dr Broughton said were the basis for a good population. The other point to make is that there are five woodlands that I study in the Braydon Forest. For whatever reason, Webb’s Wood, primarily Beech and foreign conifer, is the least productive for Marsh Tit, and always has been. Not only that, but it had recently been subject to significant disturbance that impacted on the entire bird population: thinning of the Beech, removal of the non-native conifers, to replace them with native British species, to improve the habitat for a wide range of bird species, including the Marsh Tit, Forestry England’s priority species for the Braydon Forest. Had he gone to Red Lodge or Somerford Common and played his MP3s he might have got a completely different picture. His response was exactly as I expected and we no longer communicate. I thought the point of science was to change your theories to best fit the evidence, not ignore new or, in this case, more accurate, evidence that doesn’t support the hypothesis you are testing? The other point I have to make again, and again, as I made to Dr Shutt, is that I have never claimed that my local results are nationally applicable. Circumspection is always preferable to dogmatism. If you are interested in my data, I refer you to my blog post: Feeding Blue Tits in Your Garden: a Good or a Bad Thing?, which pre-dates their article.
Unfortunately, Willow Tit disappeared from the Braydon Forest back in 2008 / 2009. They are now only found in any numbers in the Savernake Forest area of northern Wiltshire. What the cause of its extinction was I don’t know, but I would suggest it was always marginal, with limited suitable habitat. The Birds of Wiltshire, published by the Wiltshire Ornithological Society (WOS), and using data from the various BTO atlases, plus additional surveys by members, shows low concentration of breeding territories in the summer but blank for abundance in both summer and winter. I am not sure how they hang together, but the last time I saw a Willow Tit in the Braydon Forest was on the 16th May 2008. The WOS website shows zero breeding abundance for the species in the entire county.
I have seen no empirical data that shows that feeding Blue Tits in your garden is forcing the decline, possibly extinction, of other species. As far as I can see, it is all speculation. To reiterate, my local Marsh Tit population is healthy and increasing (watch this space for future revelations), with it being a key species for Forestry England’s management of their Braydon Forest woodlands and with the Wiltshire Wildlife Trust’s sympathetic management of their Braydon Forest woodlands for this species, amongst many others. I have seen comments about “taking a precautionary approach” on a just in case basis – but that should not be the rationale for a scientific analysis.
What kicked it off again this week, and spawned this blog piece, was a post from Alex on Twitter, this time suggesting Great Spotted Woodpeckers are helping to drive the extinction of Willow Tits. I appreciate that he had reposted someone else’s tweet showing a photo of a Willow Tit nest hole under excavation, that had already attracted the attentions of a Great Spotted Woodpecker. I am not going to pretend that GSW are not a nest predator: I am sure many saw the film of them taking Treecreeper adults and young on Springwatch. Willow Tits are certainly a prey species of theirs but, with the parlous state of their population, it is hardly going to be fuelling the sustenance of the GSW population. However, a number of other species are also nest predators.
Immediately, though, it was suggested that the putative surge in GSW numbers, which Alex attributes to garden bird feeding, is helping to drive their extinction. It is a fact that garden use by Great Spotted Woodpecker has increased over time: as it has for many species. That doesn’t mean that garden bird feeding is responsible for the increase in their population overall. When I look at my ringing returns for the Braydon Forest, I get the following:
Graph 1: Great Spotted Woodpecker Processed by Year in the Braydon Forest
This shows clearly that there has been a 30% decrease in the number of individuals processed each year in the Braydon Forest. When I take out any effort bias from the results, by taking the number of individuals caught per year and dividing it by the number of sessions carried out per year, the results are very similar:
Graph 2: Great Spotted Woodpecker Processed by Session by Year in the Braydon Forest
This shows that the catch has declined by from 0.23 per session to 0.15 per session over the 11 years. Again, there is no massive increase. Perhaps it means that, after their increase from the mid-seventies, they have reached the carrying capacity of the environment? Indeed, one graph shared with me by Alex showed that nationally the GSW population has been stable, after growing during the 1970’s, due to the increased availability of insect food and nest sites as a result of Dutch Elm Disease. (Marchant et al, 2009). The BTO BirdFacts page also ascribes, amongst other things, the exploitation of garden feeding as having helped their expansion. However, looking at the trend chart for garden use by Great Spotted Woodpecker over the last 20 years, from the Garden Birdwatch data exploration site, there is variability throughout the year, with a peak in early autumn, but the overall trend peaked in 2011 and has slowly declined since. (https://www.bto.org/our-science/projects/gbw/results/long-term-patterns)
In this month’s British Birds there is a superb article: “The rapid extinction of Willow Tit in a post-industrial landscape” by Geoff Carr, Jeff Lunn and Sophie Pinder. British Birds, volume 117, pp 195-202.
The element of uncertainty on the cause of the extinction they report on is well worth the read. However, pre-dating all of the speculation about causes is this paper from 2007:
This says quite clearly that there was no difference in competitor density or predator density, including GSW, between occupied woodland and woodland where they have become extinct – but there were significant habitat differences. This is an objective study using direct measurements and coming to a clear conclusion.
In my conclusion: on the one hand some academics are bemoaning garden bird feeding for killing birds by spreading infections and then, on the other hand, blaming it for increasing populations of some species of birds to the detriment of others. Some of them hold both positions at the same time. It is hard to know what the truth is without some very targetted studies, like the one referenced above.
I can think of better reasons for the increase in the number of Blue Tits in gardens: so many people now seem to be providing nest boxes. Not only do they have more secure places to breed, there is a handy supply of food nearby. However, as previously stated, my own data shows a slight decline in Blue Tit numbers ringed, retrapped and individuals over the last 11 years and very few movements of any significance. In fact, the same BTO GBW data that I have linked to regarding Great Spotted Woodpeckers shows a reduction in the use of gardens by Blue Tits over the last 20 years. It is hard to imagine that Blue Tits that show little inclination to move any great distance within the Braydon Forest can be severely impacting other species. What the truth is, I have no idea, which is why I am not being dogmatic about it: I am asking for better science, so we can understand what is going on, and take informed action to protect more vulnerable species.
Ringing In The Braydon Forest & Beyond 