Can cows starve to death?

No. Cows and other animals cannot starve to death. It's possible that some of them glitched into a wall and suffocated, but besides that, I can think of no other possible explanations.

All mobs can't starve to death. Food for passive mobs just allow them to enter "mating mode". When they eat grass this does not make them go into "mate-mode". Some food (for horses) allow you to tame them more easily. What may have possible happened is that a cow may have died from suffocated like UnionHawk said. However passive mobs can also die form the following:

[Cows] also tend to jump off high cliffs and make no attempt to stay out of water. They also can climb ladders in certain conditions.

If you have any ladders to get in or out of your pen the cows may have "jumped the fence". You may have also left the gate open (which I have done several times. It stinks).