Great Spotted Woodpeckers in the Braydon Forest: 2013 to 2021

It is wet, it is windy, no ringing, no fun, so I thought I would spend some time today having a look at some data. As regular readers will know, I have recently done some analysis of Blue Tits in the Braydon Forest, as a result of a paper by Shutt et al¹, which was followed by an article in January’s British Birds², expressing concerns about the impact of garden bird feeding on vulnerable species. Although I expressed no criticism of the source paper and, in fact, was at pains to make that point several times in my post, it certainly ruffled enough feathers for me to be attacked on Twitter. I am not going to go into details of the individuals concerned, but I hope that they put more rigour into their science than they did into their criticism of my post. One critic was particularly egregious, criticising me for something that I never claimed, and then getting his data and analysis so horribly wrong that, when I pointed it out, he did what any self-respecting academic would do and admitted he was in error (don’t be silly – he blocked me). My post pre-dated the article in British Birds, and I only got the chance to read it during the criticisms, not that it was relevant to the points I was making anyway. It helped that I have over 9 full years of data for me to analyse.

At the risk of sparking off more Twitter Wars, I thought I would have a look at the population of Great Spotted Woodpeckers in the Braydon Forest, as represented by my ringing data, as this species was one of the species identified as having had a significant increase in numbers and, being a successful nest predator, capable of impacting on populations of vulnerable species. Anyone who has watched recent Springwatch episodes in the last few years cannot have failed to have seen examples of their predatory behaviour. Particularly gruesome was the predation of Treecreeper pulli / near fledglings from their nest.

I have had regular visits of Great Spotted Woodpecker on my garden peanut feeder over the years. This year I am pretty confident that we have seen a juvenile that has matured into a fine looking male – but I cannot be sure it is a single bird. I did catch and ring a single male adult back in April 2020. It hasn’t subsequently been recaptured.

For this analysis I have looked only at the captures within the woodlands of the Braydon Forest, ignoring the wide open spaces of Blakehill Farm. Also, I have counted the total number of sessions in those woodlands in each year, not just the number of sessions in which Great Spotted Woodpeckers were caught, and analysed as per the total number of sessions.

Table 1: Percentage of visits in which GSW were caught per annum

Fig 1: Percentage of visits in which GSW were caught per annum

As you can see, over the nine years there has been a near 5% increase in the number of sessions in which we have caught this species. The next thing I looked at was the actual size of the catch:

Table 2: Average numbers caught, ringed and retrapped per session by year

Fig 2: Average numbers caught, ringed and retrapped per session by year, with the trend for number of birds ringed

As you can see from these figures, there was a significant spike in the number of birds ringed in 2017. Despite that, the overall trend shows a slight decline in the number of birds over the nine year period.

Next I looked at the proportion of the catch that are adults and juveniles over the period:

Table 3: Actual and average numbers of adults and juveniles ringed per session by year

Fig 3: Average numbers of adults and juveniles ringed per session by year

As this graph shows, whilst numbers of juveniles ringed each year are lower than those of adults, the trend for juveniles is a steadily increasing line. Conversely, the trend for adults ringed is steadily declining:

Fig 4: as per Fig 3 but showing the declining trend for adults ringed

Whilst I have analysed the numbers ringed and retrapped, a single bird can be recaptured multiple times over multiple years, so I have counted the actual number of individuals that have been processed per annum:

Table 4: Average number of individuals processed per session by year

Fig 5: Average number of individual GSW’s processed per session by year

As this shows, the actual incidence of interacting with individual birds shows a slow decline. In all of the figures shown, the 2017 spike has clearly had a positive impact on the population trend shown, so I have removed that spike from the data to see what impact it might have:

The impact is relatively minor: a reduction of 3% in Fig. 5 to a reduction of 4% in Fig 6, albeit from a lower starting point (27% as opposed to 30%).

Given that we carry out our sessions with consistent net positions and consistent, although seasonally variable, session lengths, this does not look as though it shows a population that is growing rapidly within the Braydon Forest. As per my previous posts, I make no claims about the validity of this analysis, outside of the specified geographical parameters and the limitations of bird ringing for analysing population dynamics.

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