Not all of a person's money goes into one charity. A person can spend their money on many different things, and can choose how much to spend on each different thing. Think of willingness to pay to actually be a measure of how much you care. Basically, the bird situation is crazy because humans barely if at all feel a difference in terms of how much they give a damn between something that has one positive effect, and something that has 100x that positive effect!

To Luke: This person was reading about the biases you breifly outlined, and he ended up confused by one of the examples. While the linking helps a good deal, I think your overview of those biases may have been a little too brief, and they might not really hit home with readers of your site, and personally I think it might be difficult particularly for those who may not be familiar with the topics and content of the site. I don't think it would be a bad idea to expand on each of them just a little bit more.