I was going to make a joke about the aliens returning and presenting us with a bunch of birds.
But that got me thinking, if an intelligence goes around collecting species, how would they keep their specimen populations from evolving? How could they possibly keep so many specimens alive over the eons?
Perhaps their strategy would be to take a vast amount of genetic samples and preserve those species as data. Rather than abduct dozens of T-Rexes for some zoo exhibit, they take the genomes from tens of thousands of T-Rexes and make a population snapshot. After they depart from Earth they could clone or create as many as they want, as needed.