I started my ringing career in January 2009. Having been a keen birder up to then, and Ravensroost Wood being one of my key local patches since I moved into the area in 1997, I was keen that we should get to ring in the site. I set about persuading the Wiltshire Wildlife Trust to allow us access. I provided the rationale, and swapped loads of data with the Trust, from my 11 years of birding there, and they gave me the information gathered from regular breeding season surveys that had been carried out, and the agreement was reached. My trainer provided a project plan and a risk assessment, and we gained access after Natural England approval (it is a SSSI) and first ringed there in July 2009.
To be fair, it wasn’t my trainer’s favourite site, and it was agreed that, when I got my C-permit and could take it on, it would become my site to manage and work. I got my C-permit in June 2012 and took over the site, with a new project plan, in September 2012. Soon after starting work there I bumped into a staff member from the Forestry Commission and I enquired about getting access to their Braydon Forest sites. They were really helpful and welcoming and I got access to Somerford Common, Webb’s Wood and Red Lodge Plantation soon after. Since then the vast bulk of my ringing has taken place in this area.
Unsurprisingly, the commonest species in my catch is the Blue Tit, Cyanistes caeruleus. People who don’t understand ringing often ask “Why bother with such a common species?”. Some ringers choose not to ring them (at 27p per ring cost one can understand the reluctance for those who do not get external funding for their rings, although I largely fall into that category, my view is that if I have “inconvenienced” them by catching them, the least I can do is ensure that we get some data back from them). The argument itself is fallacious: once upon the time the BTO would not allow House Sparrows, Passer domesticus, to be ringed, because they were so common. That worked out well, didn’t it? Besides, it seems the whole rationale for the Edward Grey Institute at Oxford University is studying Blue Tits, and if it is good enough for them, who am I to argue?
The key thing about Blue Tits: you are going to get a good sample size upon which to make judgements on trends. So, with the vagaries of the weather disrupting my ringing activities more often than I would like at the moment, I have been having a look at my Blue Tit records since I took over the ringing in the Braydon Forest, with complete years from 2013 onwards. The numbers caught have been as follows:
This year’s numbers have possibly been skewed a bit by lockdown, but only really in March / April, and I have managed to get sessions in every wood in just about every month of 2020.
One of the perennial questions ringers have differing opinions on is about sexing Blue Tits on wing length. They are a sexually monomorphic species, so the only reliable way of telling the sexes apart is in the breeding season, when the females develop a pronounced brood patch and the males cloaca becomes engorged and is known in the trade as a cloacal protuberance. I have analysed my data for males and females sexed by their breeding condition and mapped the numbers against wing-length to see whether it is possible to reliably sex them on that biometric outside of the breeding season:
As you can see, a Blue Tit with a wing-length of 62 mm is almost certainly female, and an individual with a wing of 66mm and longer is almost certainly male. In fact, amongst my catches I have had 1 male with a wing of 62 mm and one female with a wing of 66 mm. Some ringers look at a combination of wing-length and brightness of plumage but I prefer to err on the side of caution.
With Blue Tit broods being typically 8 to 10 eggs, with perhaps 6 to 8 nestlings surviving to fledging age, one would expect the numbers of juveniles ringed each year to be significantly higher than the number of adults and, as figure 3 shows, that is usually the case. From looking at figures 1 and 3, it is clear to see that 2016 was a terrible year for Blue Tits in the Braydon Forest. Not only were the numbers down significantly across the board, but the breeding success was remarkably poor.
It was a particularly wet spring in 2016, and the summer wasn’t much better. Other species also suffered in the Forest, particularly Long-tailed Tits, Aegithalos caudatus. Whilst Blue Tit numbers have recovered somewhat, Lotti’s are taking a longer time to build up again. However, 2020 looks to be the second worst for juveniles and the best year for new adults.
Whilst the bald numbers in figure 3 are themselves of interest, what really stands out is when you show adults vs juveniles as a proportion of the total, as shown below in figure 4:
As mentioned above, this shows that 2020 has been a depressed year for juvenile production as a proportion of the whole, but not as bad as 2016 was. What is surprising is that this was not how it felt whilst ringing juvenile birds in the summer, so I had a look at how this split might have occurred by analysing the proportion of birds ringed in the breeding season (May to August inclusive), as shown in figure 5, and those ringed in the early winter (October to December inclusive), as shown in figure 6.
This shows that, proportionately, 2020 in the breeding season was very much in line with all years, except the remarkably different years of 2016 and 2017.
Figure 6 does show the ratio to be depressed compared to all years except 2016, but not significantly so. So why the overall variation in numbers? I thought that it might have something to do with adult survival over the winter of 2019 / 2020 being better than in previous years. So I have graphed up the numbers of birds ringed over each complete winter, using the months November to December in the preceding year and then January to March in the following year.
Figure 7 shows the numbers of juveniles and adults ringed in that period:
As you can see, these numbers are second only to the winter or 2013 / 14. The proportion of juveniles to adults is also somewhat interesting:
Essentially, it is consistent with all previous years except 2012 / 13, which was the first year of study, and the winter following the awful breeding season in 2016. I suppose that what I am saying is: I have no idea what has caused the change in the balance this year. Blue Tits might be common, but they are interesting.
Ringing In The Braydon Forest & Beyond 