I'm not the one who programmed it, so I'm not certain, but my guess is that each bird is part of an enum, and it gets averaged just like all the other stats. So while the other stats have a sensible numerical value when averaged, since they're supposed to be interpreted numerically, the ones you referred to make no sense because what's being averaged is just the numerical representation of the birds, which means nothing at all since there isn't a "bird scale" to help you interpret what you're seeing.
(If you're not a programmer and thus not familiar with what an enum is, here's an example (not in any particular programming language):
Enum bird {
Sparrow = "Sparrow"
Duck = "Duck",
Warbler = "Warbler"
}
So anytime you want to use the word "Sparrow", you can write bird.Sparrow instead. Why would you do that? Imagine your program outputs the word "Sparrow" in a lot of places. In each of those places, you could write the word "Sparrow" in the code-- but then if you realize you need to change it to say "sparrow" instead, you have to find everywhere that says Sparrow and change it to sparrow. However, if you use the enum, you can just change the enum's value so it's Sparrow = "sparrow", and then everywhere in the code will have the correct version without you finding every instance.
Each enum also assigns a numerical value to each entry-- sparrow would be 0, duck 1, and warbler 2, in this case-- and I expect that's what we're seeing with those nonsense statistics.
Sorry, long aside that may not actually be relevant, but now you know more about enums)