The zoo workers, who spoke on condition of anonymity, brought their concerns to CBS News.
They said problems began a year ago when the zoo made the decision to add a half dozen new species to the Cheetah Conservation Station, but failed to add extra space.
Two newly acquired hornbill birds had to be kept indoors for seven months until their exhibit yard was completed, and when a volunteer complained and the birds were moved outdoors, they had to share a space with a wallaby, CBS News reported.
The wallaby became so stressed by the presence of the birds, it bloodied its nose and spent its time hiding, the zoo sources said.
In another instance, a red river hog became malnourished and died of an infection, and another hog fought with two antelopes with which it was housed.
"I call it 'fetch and pray.' You get the animals and then you pray that the project will work out," said Mark Bekoff, animal biologist and ethicist. "There's not one single excuse that I would accept. Basically this is a form of animal abuse and animal neglect. ... They get an 'F' in this project."
Pamela Baker-Masson, zoo spokeswoman, said animals at the National Zoo always get the best care.
"We take great care in introducing our animals to these various habitats and transitioning them. Ideally, these things wouldn't happen, of course," said Baker-Masson. "I think my colleagues were very well aware and they know these species and they know how to take care of these species. ... Every single animal has a very specific course of action and a plan."