The companion call is rarely indicated in my birding field guides. The call is commonly uttered by both the female and the male of a mated pair, usually in an alternating pattern. It seems a way of staying in close, auditory contact – a kind of checking in with one another – a brief chirping back and forth that lets each bird know the other’s whereabouts while both are foraging. Slight variations in the call may serve to indicate that the caller has found a good food source – “come on over” – or any number of other nuances. Whenever we hear the companion calls of one species flowing back and forth in a regular rhythm, it’s a clear indication that the birds are in a relaxed or “baseline” state, a condition of ease that expends little unnecessary energy. If the rhythm is interrupted – if one mate stops replying – then the other will call again in an irregular pattern, sometimes raising the volume. This interruption in the normal pattern is instantly noticed by other wildlife in the area (not only by birds but by squirrels, deer, fox and others), alerting them to the possible presence of a predator or intruder. The alertness rapidly deepens if there’s no response for a long stretch. But if the first bird finally responds, then the rhythmic pattern of chirps will reassert itself, and the neighboring animals will relax. – David Abram, "Becoming Animal," 2010