A Delaware animal shelter is working to find new homes for thousands of chicks that were left abandoned in a U.S. Postal Service truck for three days.

Delaware’s Department of Agriculture said it received a call earlier this month from USPS saying the Postal Service had an “undeliverable box of baby birds.” About 12,000 chicks had been shipped from the Pennsylvania-based Freedom Ranger Hatchery to farms across the country.

State agricultural officials say that when they found the chicks at a USPS distribution center in Delaware, around 4,000 were dead.