Today's blog is by Rob Shotton, MRes student at Worcester University, about his final year thesis.
Following on from my previous blogs about the study that I carried out investigating how birds are using solar farms, I have now completed my thesis and am in a position to give overall results.
In total nine solar farms (Fig.1) were surveyed over a period of two years with each site visited 3 times per year during the breeding season (March-August). The solar farms chosen were located across central England and needed to be a 90-minute drive from my home in Worcester. Further information about the choice of sites can be found in my original blog.
Figure 1. A map of the solar farms studied
Site usage compared to control sites
Solar farms are being used by birds at a similar level compared to other land use types (Fig.2). There was also a significantly higher variation of species found on solar farms compared to arable fields which suggest that solar farms provide a habitat for a range of farmland birds. In essence the solar farm is a piece of permanent grassland with a grass height that is generally taller than pasture but more intensely managed than a wildflower meadow.
Figure 2: Box plot of bird abundance found throughout the study across treatments: The line in the middle of the box represents the median, and the lower and upper ends of the box are the 25% and 75% quartiles respectively. Open circles indicate outliers where large bird numbers were recorded.
The most common species that were recorded over the two years doesn’t differ too much from the interim results with the same species dominating. There were fewer birds recorded on all sites during the second year perhaps due to 2018 being a very dry and warm spring and summer.
Solar farm usage
There was a highly significant difference found between bird abundance and the location used within the solar farms. The centre of the solar farms (Where the arrays are) was used far more than margins. This could be the result of there being no significant difference found in the grass height between the margins and the centre. It may be that the wide margins are less appealing for foraging than the narrower areas between the arrays where birds can make frequent short trips whilst returning to an elevated position to remain vigilant particularly when the grass is tall.
Yellowhammer were in the top ten birds which were found in my study
The arrays within the solar farm are a valuable addition to the landscape with birds of all types from buzzard to wren recorded using them for resting, singing or foraging. Birds would often enter the solar farm from the established boundaries and fly directly to the arrays then hop down to the ground between and underneath the arrays to feed. Birds were using the arrays in a similar way to hedgerows when feeding themselves by making foraging trips between the arrays before returning to the arrays to eat whilst remaining alert to nearby threats. Birds that were raising young behaved differently making trips from the hedgerow over the margins to the arrays before returning to the nest with invertebrates for chicks.
Solar farm age
Although the relationship between solar farm age (Fig 3.) and bird usage was found to be not significant further study into this would be valuable. The hot and dry spell reduced the number of birds recorded across all sites during the second years surveys.
Figure 3: Bird use (abundance) across the nine solar farms in relation to solar farm age.
Differing aspects of land management such as later mowing may have played a part as the oldest sites studied were mown later and coincidently surrounded by arable farmland.
Sward height and composition
The number of birds recorded decreased with the increase in grass height (Fig.4). It was also noted that once the solar farm had been mown in its entirety the height was reduced to around 3cm and the number of birds recorded dropped dramatically. The optimum height for bird usage on solar farms appears to be between 7cm-20cm which possibly matches the desired height for solar farm management.
Figure 4: Bird abundance found on solar farms by the mean sward height (cm) recorded on each visit.
The sward make-up on the solar farms studied were found to be similar regardless of the location within the site margin or centre. The opportunity to grow wildflower meadows leaving them to seed in the margins and unused areas of the site was not being taken up by the solar farms studied and it was noticed that the areas of grass outside of the security fence that surrounds the solar farm were left untouched throughout the entire season.
Perhaps these external areas could be sown with a wild bird seed mix or native wildflowers and left to seed if doing so in the margins of some sites is not viable. The presence of weeds was a problem at all of the sites studied particularly underneath the arrays.
Weeds, left unmanaged, could become a problem for the panels
Conclusion and recommendations
There is huge potential for solar farms to replace the grassland lost due to the intensification of farming in the later part of the twentieth century. Solar farms have demonstrated their value in the farmed landscape with little evidence to suggest that solar farms are having a negative impact on farmland birds. While it is positive that birds are using solar farms at a similar level to arable, pasture and meadows. Changes to management such as mowing later in the year and leaving margins to set seed where possible would benefit both stakeholders and nature.
Solar farms provide an opportunity for the long-term existence of land in which wildlife can thrive, which could go a long way to help slow the rate of decline of farmland birds.
However, it must be remembered that the primary function of the solar farm is to produce low carbon electricity, rather than being nature reserves. Consequently, management to increase a sites biodiversity value could increase costs by encouraging large flocks of birds to nest in and forage within the site. Solar farms need careful management to ensure that the fragile state of our farmland birds is not made worse and with the suitable management systems in place for each site and, with time solar farms can be a place in which both its value to biodiversity is increased and management costs are reduced.
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