It startled postal customers who were appalled that anyone could do that.
A witness told KOMO News he saw someone pickup the animal in a cardboard pet carrier on Thursday. The container is meant to transport a small animal very short distances; animal advocates say it is not meant to house any animal for days.
"I was outraged," said Michelle Weber, "just outraged."
Weber was so upset by what her friend told her that she immediately contacted the KOMO 4 Problem Solvers. Weber's friend did not wish to be identified, but agreed to share what he saw.
"They said, 'It's been sitting here for two days. We've been trying to get a hold of you,'" the witness said. "I distinctly remember them saying that."
The witness said another postal customer complained that the animal had been left for two days without food or water, and called it abuse. He added he was sure the animal, which was in a plain unmarked cardboard box, is a dog.
"My impression this entire time was yes, that it was a dog," he said.
The post office said its policy doesn't allow mailing dogs or cats. The clerks said they are convinced that the animal inside the box was a rooster, which is legal to put in the mail with special handling. The clerks believe the animal had been mailed the day before.
KOMO News asked for verification, but the post office could not locate the shipping information.
The witness said he has kept both roosters and dogs, and he does not think what was in the box yesterday was a rooster.
"It didn't sound like a rooster," he said. "In my opinion, it would have made a lot more noise if it was a rooster. It would have been pretty obvious if it was."
The Seattle Humane Society said no matter what kind of animal it is, keeping it in a box for two days or more is not humane.
"I've never heard of someone just putting an animal in a box -- dog, cat or otherwise, and just licking a stamp and hoping they're going to safely get to where they belong," said Seattle Humane Society CEO David Loewe.
"If you're not providing food and water, any kind of problem becomes magnified and all of a sudden what's coming out of the box is in much, much worse shape than how it went in," said Dr. Brad Crauer, a Seattle Humane Society veterinarian.
Even if it was a bird that had been mailed, both the Seattle Humane Society and the witness hope there is an investigation.