In a study conducted in Paris, France, equal amounts of pigeon feed were spread on the ground in two adjacent locations. A human feeder was present in both sites, with one acting as a hostile feeder running at the birds to scare them away and the other acting as a neutral feeder and just observing. The human feeders were randomly exchanged between the two sites throughout and the birds quickly learned to avoid the hostile feeder’s site and to eat at the site of the neutral feeder. At the end of the training session, both feeders behaved neutrally but the birds continued to remember which one was hostile. In the most interesting part of the experiment, when the feeders exchanged coats (orange worn by the hostile feeder and yellow by the neutral feeder throughout training), the pigeons were not fooled and continued to recognize and avoid the hostile person. The quantity measured is difference in number of pigeons at the neutral site minus the hostile site. With n = 32 measurements, the mean difference in number of pigeons is 3.9 with a standard deviation of 6.8. Test to see if this provides evidence that the mean difference is greater than zero, meaning the pigeons can recognize faces (and hold a grudge!).