[mongol]

I think you should not worry too much about that. There are usually special methods for those cases when it is important to get this right - such as, you need to observe for a set amount of time, and you need to report also the species that you know about but that you don't observe. For the popular "backyard counts", the scientists working with the data knows that it has been collected using a method where these details are not as accurate. Your data can still be valuable, but not to be used for the same purposes.