I was in the garden with my hens, the first time I began to suspect that "birdbrain" might not be the insult we intend. I was digging holes, the hens were looking for grubs in the clumps of dirt. One of the hens started making this grawling type noise, then they all took cover under a nearby shrub. I looked up to see a hawk flying over the garden. After observing this same behavior several more times, I began to recognise the hen vocialization that obviously means, "Danger, take cover".
While I have know for a while my hens have language, I never new they could do math until I read this article in TreeHugger. The artical goes on to lists several other hen behavors that suggest chickens might be smartet than we think. They can even count.