I haven't had one of these dreams in a while (I used to have them fairly often), but last night I dreamed that I had some pet birds. The dream followed the usual scenario: I had gotten them and then sort of forgotten I had them*; of course that meant I had forgotten to check their food and water; out of an original number of 5-6, a couple were dead in the bottom of the cage. Another variation I've had on this dream is that the birds are not dying, but escaping from cages with too much space between the bars. I've also had rather harrowing dreams featuring other pets that I've forgotten to feed/water, usually dogs (I don't recall one where the dog actually died, but there have been some close calls).
*Interestingly, I have a vague memory of also recently having a dream in which I had forgotten about being enrolled in a math class for an entire semester. Except at the end of the dream I realized that I had actually dropped the class and there was no reason to be so frantic.