Good point; I initially interpreted the article's analysis as an attempt to calculate the prior probability of a bird appearing in a specific photo. But then I

Good point; I initially interpreted the article's analysis as an attempt to calculate the prior probability of a bird appearing in a specific photo. But then I switched to thinking about the probability of the bird appearing within a sample of photos, which of course depends on the sample size. Of course, you need to know the prior probabilities in order to calculate the posterior probability distribution, and in this scenario, there doesn't appear to be a reliable way to obtain the prior probabilities, especially considering that there's only a single instance of the event we're looking at in the sample data.