I don't think thats always the case, there are many different subtypes of avian flu. There was a recent outbreak of a H7N9 variety of avian flu on the borders of Vietnam and southern China. The main problem with that type is that it only mildly affected poultry and showed up when people started...
Probably looked like this one a Coopers hawk. Very common in SoCal suburbia and i think you'll have to look out in future or have an enclosed run. Sorry for your loss