i agree that it's probably just a cold, but like pac said, it could be due to a disease, possibly fiv (feline aids).
since they spent time out in the wild, they definitely should be taken to a vet to be tested for fiv, and felk/felv (feline leukemia). it's called a snap test.
fiv isn't necessarily deadly, but a cat that has it should be kept away from cats that have not contracted it. some cats can have feline aids and show no symptoms, but they are carriers. it can be given to other cats through a playful bite, and the other cat may end up dying from it.
cats that have fiv and show no ill symptoms can live with other cats that also have it but are not ill from it, however.
the downside is they do have compromised immune systems, which makes them more susceptible to getting colds.
felk/felv (leukemia) is unfortunately an automatic death sentence for cats. if they have it, they should be put down immediately, according to our vet.
in any case, good luck with your new cats, gigg.
A true lover of nature does not despair now that his mistress has turned a colder cheek.