Vampire Birds

I remember what I wanted to tell you!
We've all seen them- those birds that sit on the backs of cows. I've heard them called cowbirds and cattle egrets. Supposedly, they sit on the cows and eat the bugs that bother the cows. Mutualism. A win-win for both parties. You learned about it in biology. Right?
I read in my evolution book that a guy decided to watch these irds for a while. Here is what he noticed:

1. birds only sit on cows with sores
2. cows with birds had bigger sores than cows without sores.
3. The sores tended to bleed more and took longer to heal.
4. The birds were actually drinking the blood of those cows!
5. The only time he observed the cowbird eating a tick was when the tick was full of blood.

Damn- vampire birds. That's all I need, something else to give me nightmares about.