In contrast to feather pecking described in Part 1, aggressive pecking (and aggression in general) is not motivated by exploration or re-directed foraging. Instead, it is motivated by dominance, competition, and the development of a social structure or hierarchy.
Aggressive pecks are forceful downward pecks often directed at head or neck (sometimes feet) of another hen. The goal of these pecks is for the opponent to give up and move out of the way. Other agonistic (or aggressive) behaviours include threats, chases and fights as well as avoidance behaviour. When I am scoring a flock, feather loss on the head of the bird typically points to aggressive behaviour (see photos below). I also look at the combs of the birds and score the number of wounds (scabs) that I see as further evidence of aggressive pecking.
Initial encounters between birds are often aggressive because they need to figure out where everyone stands. Subsequent encounters are less aggressive because chickens are able to remember their relative rank. Because the physical cost of physical aggression (namely injury) is so high, it is really only used as a last resort. Threatening behaviour (raising hackles, standing tall) is a more common first step.
The goal of this initial aggressive encounter is the formation of a stable hierarchy where the dominance relationships are clearly defined. If this happens, the group will show very little in the way of aggressive behaviour going forward. Think of hierarchy formation as an investment in a future of peaceful co-existence. Unless a stranger is introduced, in which case the group has to aggressively determine where they fit in to the new hierarchy.
Dominance hierarchies or peck orders can be simple and relatively linear structures (below left) or more complex networks (below right), often depending on the number of individuals involved and their status relative to one another. The larger the flock, the more complex the hierarchy.
For very large flocks (greater than 100 birds), chickens seem to change their strategy. Instead of building dominance hierarchies based on recognizing and remembering specific individuals, they use key signals of status like body weight and comb size to determine where they fit in. In evolutionary terms, these are called honest signals because they are outward indicators of fitness (or good genes).
For example, if one bird is a heavyweight champion and the other is a scrawny subordinate, there is no contest and the latter bird will surrender to the former immediately. However, if two birds are similar in their honest signals, their interaction may turn aggressive in order to decide who will gain priority access to the feeder, for example. This is another time when birds may get aggressive: competition.
I found the diagram below while doing a quick search for images of peck orders in chickens. I take issue with it for a couple reasons. I think it confuses the issue a little bit by first showing an oversimplified dominance hierarchy, but also by confusing aggression with feather pecking. The target of these birds seems to be the tail area, which is a common feather pecking hot spot, as opposed to the head or neck area. Not to mention hierarchy formation does not typically lead to the death of the most subordinate individual, whereas feather pecking can lead to cannibalism.
There may be one exception: pariah birds. These birds receive excessive negative pecking attention, both aggressive and feather pecking, from their flockmates. They are victimized to the point of needing to hide and quickly dart out of their hiding place to eat or drink. This abnormal behaviour then attracts further unwanted attention. These pariahs have a very poor quality of life and should be euthanized if they are identified. Pariahs may be found in any size flock. It is unclear what initially marks a bird as a pariah but it seems like birds that look or behave differently tend to become targets.
Pecking is one of those behaviours that is both essential to the survival of chickens, it is how they consume food and explore their environment, but it can also be destructive. Some aggression and gentle feather pecking are expected. The challenge is to notice the subtle changes in a flock that may indicate a problem and to take action. Part 3 explores some prevention and treatment strategies for both types of injurious pecking.