Q:

Validity stemming from contradictory premisses

I'm a student new to the study of logic, and having had my first tutorial on it yesterday, while I generally understand the characterisation of logical validity, there were a couple of examples my tutor used which I can't quite get my head around. 
The argument: 

Birds can fly
  Birds cannot fly
  Therefore there are no birds

Is apparently valid, while:

Fish can swim
  Fish cannot swim
  Therefore there are no animals

Is invalid. 
My textbook (Halbach's Logic Manual) states that an argument is "logically valid if and only if there is no interpretation under which the premisses are all true and the conclusion is false," but I have no idea how the conclusions in both of these situations is related to the premisses, and then how one is logically valid and the other isn't. 
My tutor said that it was something to do with the first conclusion specifying about birds, but I still don't really get where he's coming from.  Any help explaining the two arguments would be much appreciated. 

A:

There are animals other than fish.   
You want "no interpretation", "no X does Y" is easiest to address by contradiction.   So let us imagine an interpretation as a counterexample. 
We only need one counterexample, to contradict "no interpretation". 
It has to be one with animals.   So let's choose the simplest easy interpretation with animals: one animal -- me.   So the conclusion is false. 
But fish are animals, too, and I am not a fish.   So we have ruled out fish entirely. 
Then both premises can be true.   The nonexistent, necessarily swimming fish still don't swim.