Earlier quoted context omitted.
You provide quotes but no attribution for those quotes.
Fair point. This is the original article in nature. https://www.nature.com/articles/ncomms2380?WT.mc_id=FBK_NCOM... "Methods: We searched JSTOR, Google Scholar, and the Web of Science database [...] within the Web of Knowledge search engine [...] to identify studies that document cat predation on birds and mammals" "We initially focused this search on US studies, but due to a limited sample of these studies, we expan…
If you base your search on bibliography you are pruning all the negative results. This is like removing all negative numbers before calculating an average value.
We need to remember also that "Local cats killed minus 1000 birds", would be a perfectly acceptable result [1] that is excluded from the search and the article.
[1] (= Cats save 1000 birds by killing egg-eating rats)