They discuss this in the paper. Scientists aren’t just randomly doing shit, they do actually put in thought.

Video Playback Techniques Pigeons are social animals. They often communicate with each other by exhibiting distinct behaviors, such as courtship displays in front of potential mates. Accord- ingly, the occurrence of a courtship display indicates that a bird ‘recognizes’ the visual object as a potential mate [Shimizu, 1998; Partan et al., 2005; Patton et al., 2010]. In studies in our laboratory, as well as that of Barrie J. Frost and Nikolaus F. Troje [Frost et al., 1998], subject pigeons were exposed to live stimulus pigeons, videotaped pi- geons, and photographs of pigeons to examine which vi- sual features can trigger spontaneous courtship displays.