Why does my horse try to eat birds?

of these wonderful creatures we must consider how long is too long to coop a horse up in what is a relatively tiny space for such a large beast.
Is bird eating a problem though?
Put simply yes. But lets delve a bit deeper. Firstly as alluded to above horses with pica the tendency to eat nonfood items tend to do so in order to relieve something askew in their diet andor life. For welfare reasons it is vital that we take steps to improve their situation. Secondly eating such things is bad for their health. Damage to teeth gastrointestinal damage injury from birdssmall mammals who fight back for reasons of not wanting to be eaten this is simply not part of an appropriate diet for the equid.
Horses have a long gastrointestinal tract which is designed to absorb fibre and plantbased nutrients. Carnivores meat eaters by contrast have short gastrointestinal tracts suited to meat digestion. Horses also dont have the teeth for it! Their teeth are shaped to graze and process grass and other forage they dont have the incisors or hind teeth required to chew flesh and bones into manageable pieces. No teeth no horse is a useful phrase to consider because good dentition is vital for horses to sustain life. Wear and tear and outright damage to the mouth and teeth through abrasion from hard bones and the like can be detrimental to their health. There are a whole host of reasons that horses must eat near constantly from managing stomach acids to preventing toxins reaching the bloodstream. But this is a large subject for another day. 
So the bottom line
Horses and birds is an interesting topic and the crux of it is that if a horse is eating them or small mammals or indeed anything odd and unnatural we
must look for what is missing in their life in terms of nutrition health and welfare and take immediate steps to fill that hole. Assistance from your vet can be invaluable in these cases as they will help you assess body condition and the quality and appropriateness of their diet