Like a large number of the dwellers of these isles, I feed the birds in my garden. As my entries to the BTO’s Garden Birdwatch Scheme show, the commonest birds in my garden are Goldfinch, Starling, Woodpigeon and, in fourth place, Blue Tit. I feed all year round in my garden: sunflower hearts, fat balls and peanuts, and provide water. I do exercise exemplary feeder hygiene, I hasten to add, and I ring the birds that use my garden as well as in the local woodlands, as the village of Purton falls within the historical boundaries of the Braydon Forest. My Braydon Forest woodland sites are fed with a seed mix and peanuts between the beginning of November until the end of February each year.
There was a recent paper looking at the potential impact of feeding the commoner birds that take advantage of our supplementary feeding on those rarer, less competitive species, that we don’t tend to see using those resources. The paper’s specific example was the potential impact of feeding Blue Tits. As I understand it, the range of food stuffs was identified by DNA analysis in their faecal samples, and those faecal samples were taken from birds at differing ranges from local feeding stations, to see how far their influence might extend. I don’t want to misrepresent the paper, so this is a link to it so you can read it for yourselves. It is open access:
https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2021.0480
One of my key projects is monitoring the Marsh Tit population within the Braydon Forest. This red-listed species is holding on well within the Forest, with its population stable. With Forestry England making this bird their priority species in their latest 10-year plan for the area, so that they will be managing the woodlands to their benefit, and the Wildlife Trust also keenly invested in the future of this species, I would hate to think that I might inadvertently contribute to a decline in its fortunes locally. They are regularly caught at the winter feeding stations, along with a wide range of other species.
Because of this paper and its conclusions, I have been prompted to have a look at the Blue Tits I have ringed in my garden and the rest of my sites in the Braydon Forest and to see how they have moved around the area. I also had a look at those Blue Tits recovered within the Braydon Forest but ringed elsewhere. Since August 2012, when I started ringing in my garden, Ravensroost Wood, the Firs and Webb’s Wood, through until the end of September 2021, 3,688 Blue Tits have been ringed in the Braydon Forest. Of those ringed, there have been 1,523 subsequent encounters, representing 1,027 individual birds. In my garden, I have ringed 238 of the total caught and have retrapped ringed birds there on exactly 60 occasions, representing 40 individual birds.
In order to test the likelihood of my feeding Blue Tits impacting on other species in my local area, I have done an analysis of those birds ringed in my garden, to identify how many were recaptured away from my garden. I have also done an analysis of the birds ringed elsewhere in the Forest that have been recaptured in my garden and, for comparison, movements within and without the Forest as a whole. The spatial distribution between my Braydon Forest sites is shown on the diagram below:
The unnumbered pink dot is the location of my garden. The numbered red dots are my Braydon Forest ringing sites and are as follows:
Of those 1,523 recaptures only 4 of them were ringed in my garden and recovered elsewhere, and only 2 of the 3,430 birds ringed elsewhere have been recaptured in my garden.
The other movements around the Braydon Forest sites are also quite interesting. I haven’t done individual records, just a summary:
As expected, the highest volume of traffic is between the sites nearest each other. Other than that, movement between sites is minimal. To put it into perspective: of 3,688 birds ringed only 71 have been shown to have moved away from the site at which they were ringed, i.e. 1.93%. Of those using the feeders in my garden, the percentage movement is 0.15%. The overall number of individual Blue Tits recaptured within the Braydon Forest (as opposed to the total number of recapture events, represented by the figure of 1,523) is 1,027, or 27.85% of those ringed.
For the sake of completeness, I have had a look at those Blue Tits ringed elsewhere and recovered in the Braydon Forest. There are very few: only five of those retrapped came in from outside of the Forest. These are their details:
I could find no records of Blue Tits that have moved from within the Braydon Forest to sites outside of the area within the studied timeframe (or outside of it for that matter).
I decided to have a look at my average catches of Blue Tit by year to see if there was any indication of population growth:
Putting that into graphical form shows the trend even more clearly:
Apart from the spike in 2013, there is no indication of significant growth or decline in the catch of Blue Tits in the Braydon Forest sites. The current slight decline showing for 2021 is because I catch significantly more Blue Tits in the last 3 months of the year than at any other time, as they form winter feeding flocks which, when caught, can number in tens of birds.
On reading the paper, one of the key pieces that I did not immediately grasp, is that they also provided nest boxes. The reason that they did that was so that they knew where to collect their faecal samples. What I didn’t see was any allowance for the potential impact of providing those additional nest boxes on their results, but they do refer to increased use of nest boxes, along with earlier laying dates, as one of their considerations in the likely impact on more vulnerable species. We don’t provide titmouse nest boxes in any of the Forestry England sites and, although there are a few extremely dilapidated boxes in Ravensroost Wood, as far as I am aware, no new titmouse boxes have been installed in any of my ringing sites since before 2009. It was discussed in the past, but the considered opinion was that these would only benefit Blue and Great Tits, which definitely would help increase their populations and, therefore, the possibility of competition for vulnerable species would be intensified.
To quote from the discussion section of the paper: “We infer from this that any impacts from supplementary feeding will be felt far wider than solely in urban environments as has hitherto been considered [1,33].
As we find that supplementary food usage is strongly associated with a dramatic increase in nest-box occupation (a proxy of breeding density) and an advance in lay date, it is perhaps unsurprising then that we find the national population trends of supplementary feeder-using woodland bird species are
increasing on average while the populations of competitor species not benefitting from supplementary feeders are decreasing.”
My key question is: is the potential negative effect just from supplementary feeding or does it need the combination of supplementary feeding and nest box provision to have a negative impact? The authors are using locally gathered data and matching that to national trend data, whereas I have focused solely on my local area. Their data is unarguable, but I would like to see some follow up to test their hypothesis.
Whilst making no great claims about the scientific rigour of my analysis (to me R is just the eighteenth letter of the alphabet), the dispersal of Blue Tits into other areas just doesn’t seem to be replicated within my ringing sites. Nor is there any obvious population trend amongst their numbers when averaged out by session by year. Clearly, I am not analysing what the paper is, I am analysing the movements of ringed birds in a relatively small area. Also, I am not looking at a few hundred specimens here: my analysis, such as it is, is based on over 3,500 individual birds ringed and over 1,500 recapture events over 9 full years.
So, how are the Braydon Forest Marsh Tits doing?
The number of sessions differs from Table 5 because it is recording those occasions on which Marsh Tits were caught, in the same way that I have only recorded the sessions in which Blue Tits were caught. Three of the Braydon Forest sites, the two at Blakehill and my garden in Purton, are entirely the wrong habitat for Marsh Tit and will never produce a catch. This year’s decline in actual numbers to date is entirely down to my being unable to access three of the most productive sites for the species for a significant part of the year. Ravensroost and Webb’s were out of bounds until September and July respectively, and Red Lodge has been out of bounds for the last two months, due to vandalism / fly-tipping and the access road being blocked as a result. However, there is no obvious trend either up or down. To put that into perspective, in two sessions in Ravensroost Wood in October I have ringed a further 3 Marsh Tits.
References:
Shutt JD, Trivedi UH, Nicholls JA.
2021 Faecal metabarcoding reveals pervasive long-distance impacts of garden bird feeding.
Proc. R. Soc. B 288: 20210480.
https://doi.org/10.1098/rspb.2021.0480
Ringing In The Braydon Forest & Beyond 