If you watch birds at your feeder for a while you see that some birds are dominant over others, not only between species (blue jays rule!) but among the same species (some cardinals are bossier than others).
As with humans, dominance is expressed in both behavior and outward appearance. The dominant birds tend to be physically larger than their subordinates and sometimes are marked differently. This is especially true of male house sparrows who wear their status on their chests.
Scientists call the male house sparrow's bib a "badge of status" because it's a clear outward sign of dominance. The bibs become are fainter winter plumage but at any given time of year the bigger and darker the bib, the more dominant the bird. In a contest between the two birds pictured above, the one on the left wouldn't even attempt to challenge the one on the right. Mr. Big Bib wins, just by showing his chest.
Unevenly matched birds are unlikely to start a fight but males with similar badge size fight often, perhaps because it's not obvious who's in charge. Eventually the contests work themselves out and everyone knows his place.
Blue jays can avoid contests altogether by figuring out dominance from afar. Here's a hypothetical story showing how they do it:
Two jays, Charlie and Bob, are in the same flock. Charlie knows he's subordinate to Bob. Then one day Arnold shows up. From a distance Charlie can see that Bob is subordinate to Arnold so Charlie knows, even before he meets Arnold, that Arnold is dominant over him. This saves a lot of trouble in the long run!
It sounds almost human. 😉