Chickens are dying off...

So I traded for 3 chickens, which seemed like 1 more than I had to pay for just to be sure, but now they're just disappearing from the pasture. I thought maybe they got old and were slaughtered, but it doesn't show up on the yearly output of that pasture. My one thought was that I pulled the herdsman off for a short while to go work a new sheep farm and forgot to add another to compensate. But I wasn't expecting the population to drop from 3 to 1 within a year, even if it didn't have a worker (they died off while the field was being worked.)

Anyone else experience this? Will my 1 chicken be able to make more or do I need to trade for another couple of them? Thanks