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On the tour of Jurassic Park, the guests are told that the hypsilophodonts will respond to a mating call.
“The main herd of animals can be found in the grassy plain below you,” said the voice. “We can rouse them with a simple mating call.” A loudspeaker by the fence gave a long nasal call, like the honking of geese.
The thing is, if they didn't breed any males, how could they know what a mating call sounds like?
Because Jurassic Park (Isla Nublar) is a theme park in more ways than one. The true work of breeding and farming the dinosaurs is actually taking place on Isla Sorna in a hidden base known as "Site B".
In the sequel, "The Lost World" Ian Malcolm identifies that there are simply too many dinosaurs to have been bred in the miniature laboratories seen at Jurassic Park.
Taken together, the whole complex had a utilitarian quality that reminded Thorne of an industrial site, or a fabrication plant. He frowned, trying to put it together. "Do you know what this is?" Thorne said to Malcolm. "Yes," Malcolm said, nodding slowly. "It's what I suspected for some time now." "Yes?" "It's a manufacturing plant," Malcolm said. "It's a kind of factory." "But it's huge," Thorne said.
"You see," Malcolm said, "visitors to Hammond's park at Isla Nublar were shown a very impressive genetics lab, with computers and gene sequencers, and all sorts of facilities for hatching and growing young dinosaurs. Visitors were told that the dinosaurs were created right there at the park. And the laboratory tour was entirely convincing.
...
"But actually, Hammond's tour skipped several steps in the process In one room, he showed you dinosaur DNA being extracted. In the next room, he showed you eggs about to hatch. It was very dramatic, but how had he gotten from DNA to a viable embryo? You never saw that critical step. It was just presented as having happened, between rooms.
Given that they were suffering high levels of unexplained deaths (and becoming increasingly desperate to improve mortality rate) the Site B staff released non-carnivorous dinosaurs from the factory bays.
The book explictly mentions that they were working with both male and female embryos so it's reasonably likely that the Site B scientists were allowing them to breed in mated pairs or flocks which explains how they'd know what the mating calls sound like.
Out of universe, the film's producers created the various mating calls and dinosaur noises using sounds from real-life amphibians, mammals and birds. It's at least possible that the Jurassic Park scientists used the same technique, mixing contemporary sounds and using trial and error until they found something that regularly attracted the female dinosaurs.
