I contacted the BTO about a report they produced for Natural England and have had this rapid response from Andy Clements:
Thank you for contacting me requesting a copy of the report to Natural England, Kettel, E & Siriwardena, GM 2018. Comparisons of breeding bird population and abundance trends within and outside two specified areas located in SW England. You are correct that this report is not available on the BTO website.
We are unable to fulfil your request at this particular time. The work was carried out under contract to Natural England, and the terms and conditions of the contract determine that we cannot release the findings of this scientific work without the agreement of Natural England. Their current view is that they do not wish us to release the report.
BTO and Natural England aim to have the work published in the peer-reviewed literature, and a revised manuscript is almost ready for resubmission to a relevant journal. Publication now, as a report, would compromise journal publication. The study is limited by sample sizes and spatial extent of the cull, so it does not provide strong evidence for or against any ecological effects but represents the strongest evaluation BTO could undertake.
We have taken longer than would have been ideal to get to this point. Due to the limitations of the study as a piece of science evaluating badger culling effects on birds, it has proven difficult to find a journal that will publish the work. However, we regard the process of independent peer-review that is intrinsic to publication in a scientific journal as critical quality control. Further, the final product, after that review has taken place and the paper has been revised accordingly, will be improved as a result. Indeed, dealing with comments from reviewers has also contributed to the delay in finalising this piece of work.
We recognise that the public interest in this issue is high, and there is societal need for the available evidence to be in the public domain. A peer-reviewed publication will represent an improvement on the original report with its limitations clarified, more complete interpretation of the results and additional checking analyses. It will provide a product for public scrutiny in which audiences should therefore have more confidence.
I recognise that your support for BTO generally, your membership and your valuable contributions to our surveys, including BBS, make an additional powerful argument for having access to BTO science. I’m sure you would want that science to be the best that BTO can make it, given the effort that you and many others contribute in gathering the data. We believe that the way set out here will ensure that the public interest, and the interests of our highly valued volunteer surveyors, are served in the most scientifically rigorous way.