Since the beginning of 2013, I, and latterly, me and my team, have been ringing in the Braydon Forest woodlands, with the permission of the Wiltshire Wildlife Trust and Forestry England. The bulk of our catches over the years in the woodlands have been Blue Tit, Cyanistes caeruleus; followed by Great Tit, Parus major; and smaller numbers of Coal Tit, Periparus ater and fewer for Marsh Tit, Poecile palustris. Of them, we have ringed the following numbers of each species:
Whilst the numbers are as I would expect for the frequency of each in our catches, I decided to do a bit more digging into the figures. I wanted to have a look at how they divided up into adults and juveniles, and also look at the numbers processed per session. To be clear, and possibly a bit contentious to any other ringers, I have defined juveniles as those fledged within the year, plus those identifiable as fledged last year, prior to the start of the breeding season. These are age codes 3J, 3 and 5 in the BTO recording scheme. I also wanted to look at the trend in numbers ringed over the period. To do that, I counted the number of ringing sessions carried out in the Braydon Forest woodlands throughout the period. I used those figures to calculate the percentage of each species caught per session. What this is not looking at is survival / mortality rates. Many of the juveniles will not have survived their first winter. It is purely about trends in the numbers ringed.
For each species I graphed the numbers ringed per annum and the average number ringed per session. These were done for the total, adults and juveniles ringed. For each graph I also chose a trend line for the average ringed per session, to identify whether there were any significant changes over the period.
These were the total number of sessions per year:
The overall trend in ringing Blue Tits is stable. However, the juvenile trend is slightly on the increase and the adult trend is slightly on the decrease. Whilst I haven’t (yet) carried out any statistical testing, neither trend looks significant.
Although there was a short spike in 2021, the overall trend for Great Tit ringing is downward. It would seem to be mainly a reduction in the ringing of adult birds, with a reduction from 1.4 to 0.4 over the period. That certainly looks like a significant reduction. Compare that with the juveniles, where the trend is much shallower: from 2.8 to 2.2. Overall, the reduction is from 4.3 to 3.7. Interestingly, I have looked at the catch for 2022 to date and they are quite interesting: 110 have been ringed so far this year, 98 juveniles and 12 adults. It looks like it will be very similar to 2021.
Again, the overall trend is a reducing one. However, in this case the key reduction is in the number of juvenile birds being ringed, from 1.30 to o.9. The adult trend is slightly positive, but at less than 0.02% per session, definitely unlikely to be significant. Overall, this gives a downward trend of 1.5 to 1.2.
So to Marsh Tits, the least common titmouse in our catch. They are the only one of the four species that has shown a positive increase in frequency – albeit starting at a very low base. It has increased from 0.36 to 0.47. However, what is really weird about this is that the result for adults being ringed has shown a significant decrease, from 0.14 to 0.03, with juveniles showing an increase from 0.17 to 0.47. Obviously that is counter-intuitive, and could just be an anomaly based on the comparatively low catch of this species.
Conclusions: Obviously, the Blue Tit data is the most reliable simply because of the volume of data. That their figures seem to be stable across both juveniles and adults does rather fly in the face of the idea that their numbers are increasing significantly. This also ties in with the data from the BTO’s Breeding Bird Survey which shows zero change in results between 1995 and 2018.
Looking at the results for Great Tit, although the BBS doesn’t split between ages as I have, they show two distinct trends: overall from 1995 to 2018 their population has increased by 36% in England but the recent trend, from 2008 to 2018, there has been a reduction of 5%. This rather reflects what is being illustrated by our data.
The downward trend in Coal Tits is not reflected in the national figures, with the BTO Bird Trends showing an increase of 9% to 10% in England, as opposed to our decrease of approximately 16% over the same period in the Braydon Forest. This is possibly down to changes in habitat: with Forestry England replacing much of the non-indigenous conifers in the Forest with native broadleaved trees as part of their management plan for the Forest.
Marsh Tits are well known as a species in trouble, with huge population decreases since 1995 of over 37%. Many theories have been put forward as to why. For a species that is largely sedentary, habitat loss and fragmentation is bound to have had a major impact. We have only ever had one Marsh Tit recovered more than 1km away from where it was ringed: one ringed in Webb’s Wood and recovered in Red Lodge about 5 years ago. The upturn in numbers does correspond with a recently noted upsurge in both England and the UK since 2016.