What happens when you free mice on an open field?

So one of my friends has some mice.  One day her mice produced a lot of mice.  Like maybe 12 or so.  So we set out on a journey to free the mice because she didn't want to feed them anymore.  We get to this open field with a little artificial lake in it.  One of my friends who came along decided to throw them in the lake to see what will happen. They didn't really do anything. They just all swam back to us...so we kept throwing them back in the lake.  Even if we throw the mice almost to the other side of the lake...they decide to take the long way all the way back to us.  I think mice like being thrown into lakes or something.  So after thirty minutes of that we just left them in the field.  Then as we look back...all the pigeons came and ate them.  Whoopsies?  So much for freeing them.  Oh well.  If you ever freed anything and it backfired on you.  Please share.