This video from the USA says about itself:
9 July 2013
Learn more here.
From the NestWatch eNewsletter in the USA, September 2017:
Predator Guards: The Verdict Is In
To NestWatchers, it might seem obvious that nest boxes should be protected with predator guards for the best possible chance at success. However, no one has formally tested whether predator guards make a tangible difference in nesting outcomes for songbirds at a large spatial scale. Furthermore, no one has tested which of the popular styles is best (although everyone has an opinion on this subject), or if having more than one is even better. And are there any species which don’t reap the benefits?
We are thrilled to announce the publication of a new ground-breaking study by NestWatch wherein we analyzed more than 24,000 records submitted by participants to answer these questions. After nearly 50 years of speculation on the topic, read our results in the latest NestWatch Blog post.