Antelope Flying From The Pitchfork Ranch, Montana to The San Diego Zoo 1937

Does anyone know anything more about this shipment of antelope to the San Diego Zoo in 1937.  Were they solely for exhibit, or was there a reason a large number of calves had to be flown off of the Pitchfork Ranch in Montana?  I don't have access to my copy of 'Mr. Zoo: The Life and Legacy of Charles Schroeder' and the only thing of significance for 1937 from the San Diego online history site is: 

 A second flight cage, originally called "the Great Eagle cage", is built to house birds of prey on the north side of Primate Mesa. It is the world's largest bird cage of its time. (This aviary is later expanded and converted to a walk-through tropical rain forest aviary.)  Dedicated 03/07/1937

Reptile Mesa is completed with outdoor pits, a Galapagos tortoise enclosure, land tortoise pens and pools for alligators and crocs.

First emu hatching.

That is Charles Schroeder above with the pilot Bill Monday.  I don't know if they have landed or are preparing to leave.  The plane was a Ryan Brougham.  With the exception of that little information, I have not been able to find anything else.