The National Zoo has begun to fill staff vacancies that contributed to the deaths of three animals and a zebra attack on a zookeeper last winter, zoo officials told a congressional oversight committee.

Zoo spokeswoman Pamela Baker-Masson said the zoo has hired seven animal caretakers and has plans to hire an additional 10 people for various other jobs. The zoo’s staff had been “spread too thin,” Baker-Masson said.

“Some of the problems that arose reflected some staffing imbalances that, while temporary, did put stress on the system,” National Zoo Director Dennis Kelly and Smithsonian Conservation Biology Institute Director Steven Monfort said Wednesday in a statement to the Committee on House Administration.

The positions had been left open after budget negotiations fell apart last year, and many federally supported agencies were forced to implement a series of cuts known as sequestration. Smithsonian officials opted to implement a hiring freeze that lasted from July through October. The institution also offered a buyout in the summer 2013, and 19 National Zoo staff left by October.

Soon after, the zoo found itself dealing with a number of animal deaths. In December, a young Przewalski’s horse and an antelope reportedly broke their necks after each charged into fences. The zoo also reported the death of a hog due to malnutrition and scars of unknown origin.

“The rate of deaths has been shown that it’s pretty normal,” Kelly said.

A Grevy’s zebra also attacked a zookeeper in December. The zoo said staff had not followed protocol, allowing the zebra into the area where the attack happened. A vulture almost escaped as well, and its flying feathers were more carefully clipped. The zoo called the incident “a learning experience.”

Kelly told committee members that to pay for the competing missions of animal caretaking, scientific research and public outreach, he has worked to secure funding from outside sources, including philanthropic organizations, research grants and income from selling merchandise and food to visitors. Admission to the taxpayer-funded zoo is free.

“These non-federal sources of income supplement our federal support, but they cannot replace it,” zoo officials said in a statement to the committee.

According to the zoo, alternative sources of revenue have increased in the past four years.

“We recognize the reality of the federal budget,” Kelly said.

When cuts had to be made, Kelly said he turned first to departments that aren’t essential to animal care, often meaning fewer zookeeper talks and tours and more cuts to administrative departments.

“It is a tough balance, we’re balancing that all the time,” Kelly said in an interview. “But our first priority is animal welfare and human safety. But at the same time we understand our mission is to provide a great guest experience, to do research and to do outreach.”