Pecking; not all pecks are equal.

A chicken’s beak is extremely sensitive. It is packed with tiny receptors that scientists believe provide the chickens brain with similar tactile information that humans receive from their hands.
So far scientists have identified three main types of receptors in a chickens beak, one type detects vibrations another temperature and a third, texture.
When combined with the chickens binocular eyesight this enables chickens to make qualitative and quantitative assessments of pecked objects.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1166494/

Observing the free range chickens here it became obvious very quickly that the force behind various pecks was different depending on circumstances. One might easily believe for example that some of the so called pecking order pecks given from the vocal reaction of the recipient, were fairly hard and caused some pain.
More careful observation showed that some of these pecking order pecks hardly disturbed the feathers of the recipient; various factors including the ranking of the recipient and their personality determined the level of vocal and physical response.

The range of pecks and the force of each is extensive. Learning to peck with the correct force and accuracy takes practice. Cockerels and pullets have to learn how to control the force of their pecks and acquire accuracy in coordination of beak movement and positioning. Watching chicks eating demonstrates that learning the most basic skills of picking up small items of food takes perseverance. I’ve observed three day old chicks only achieving a 20% success rate trying to pick up a cooked grain of rice for example. As new items are discovered and tested different beak movements with different forces may produce better results than others.
I can find no reason not to believe that providing new challenges are found, this learning process continues throughout the chickens life.

Chickens do not just use their beak for eating, their beak also acts like a humans hand in their social interactions.
I have yet to find any scientific study on the various pecks used by a chicken, so I’ve classified them from my observations in not very scientific language.

There are many different types of peck. What should be born in mind that a peck to a bouncy pile of feathers isn’t likely to produce much damage, if even felt by another chicken. A peck to a patch of bare skin is likely to be felt, no matter what the intention of the chicken. I have with some perseverance and the minimum of pain managed to convey to a couple of the roosters here that my skin is rather more sensitive than a covering of shock absorbing feathers and more gentleness was required.

With the help of a balance and some simple mathematics I’ve been able to very roughly assess the force of a chickens peck. The experiment involved placing a treat on one end of the balance and with the aid of a Dial Test Indication I was able to measure the deflection of the balance with various food types. To encourage more aggressive pecking I used double sided tape to stick the food to the end of the balance.
Grains of cous cous were picked up with an average force of 4 Newtons.
Half a peanut with flat side down on the double sided tape 15 Newtons.
The chicken tested was an adult rooster weighing 2.3 Kilos.

The pick up feeding peck.
It’s done with a partially open beak and the object is grabbed as the beak closes. Watching chicks pecking at food demonstrates just how difficult the coordination required is.
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The investigation peck.
Chickens will peck at anything they consider worth investigating and this is usually done with the beak slightly open.
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The warning peck.
This peck is about status and is most commonly seen among hens. The first warning peck while looking aggressive and done with a closed beak often hardly disturbed the other hens feathers. If this is ignored the pecks increase in force.

The ‘Hello’ peck.
I have only noticed this with cockerels. Many of the cockerels I’ve had a lot of contact with as they matured give this peck. It is usually done in the morning when I open the coops up. The cockerel will come out of the coop, shuffle alongside my boot and give it a quick soft peck and then attempt to herd me. I have put my hand over my boot a few times to gauge the force of this peck and it’s surprisingly gentle.

The grab.
This is done with an open beak. The grip is strong enough to pull out feathers. Roosters and cockerels will use this grab to the back of the neck, or back of head, to try to ‘persuade’ a hen to crouch for them. Uncooperative hens will usually struggle to get away and this is often responsible for a hens feather loss in these areas. With cooperative mating pairs the rooster places his feet on the hens shoulders first and grabs the feathers on the back of her neck to maintain his balance.

The dig
The digging peck is done with a closed beak and the peck end of the peck is often a scraping movement.

The wipe.
Not strictly a peck but this movement is used for both cleaning the beak and organising feathers.

The stroke.
This is done only by roosters ime. The rooster strokes his closed beak along the feathers of the hen as if he is cleaning one side of his beak. It seems to be a sign of great affection and I have only seen this with long standing established pairs. The first time I saw this was when a hen had just shown her partner their chicks. The rooster imprinted the chicks and then carried out this stroking action along the hens back a couple of times. The hen made no attempt to avoid this attention.

The groom.
Usually seen when a hen is grooming her rooster but hens will do this to each other. From casual observation one might think that the hen is aggressively pecking at a roosters comb, wattles ears and eyes. The fact is these are areas the rooster cannot remove mites, dead skin, fight scabs, and feathers from on his own. Most hens will groom their rooster. The rooster will bow his head, keep perfectly still.
I believe the saying hen pecked probably came form observing this behaviour but in fact, this pecking is has nothing to do with being domineering.
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Tuck and shove.
A peck from a mother hen to one of her chicks, when encouraging a chick to take up a position under her body are easy examples to see.
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The attention peck.
There is a subtle difference between this and the warning peck and the context in which this peck is given is important. I have a few hens here that use this peck, most often to attract my attention if they want food, but also to get me to move objects for them, or in the case of the picture below, to be picked up. This became obvious with a couple of hens sitting on a clutch of eggs outside. In both cases a branch has fallen across their nest. Usually I could slide my hand under them to feel their abdomen without any response. In the above cases once I had moved the branch the pecking stopped.
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Aggressive pecking.
This is done with an open beak with a range of strengths.
There are too many reasons for aggressive pecking to cover in this article.

While the above classifications may seem whimsical the importance of understanding which peck is being used and in what circumstances can help the chicken keeper to differentiate between aggressive behaviour and mating behaviour.

The Pecking Order.
The pecking order is testimony to the incredible adaptability of the chicken. The chickens ancestors the jungle fowl live in small family groups usually comprising a senior male, a senior female and for a period of time their offspring. Given that the probability of hatchings over time is 50% of each sex this one to one ratio would seem as nature intended. The only order required in such an arrangement is the natural parent and offspring order common to many species.

The common keeping arrangement of a single rooster with harem of hens has meant that in order to avoid conflict with regard to mating and to ensure that the most successful genes are passed forward a hierarchy among the hens is established. This has enabled hens to live within a large group without constant conflict over resources, attacks on other hens being mainly restricted to preventing a junior hen from mating with a senior rooster.

For the chicken keeper this hierarchy has some implications for those who wish their hens to sit and hatch. Here, it has not been uncommon for the senior hen of a group to drive a junior hen off the nest. What has tended to happen in the small groups here is the pullets and junior hens are more likely to make nests away from the groups coop in order to avoid being driven off by the senior hen and in the case of very aggressive senior hens for the safety of her chicks once hatched.

Roosters are not in the pecking order.

This may seem a trivial article. However, reading some of the posts on BYC it became obvious that many have not learnt how to discriminate between the various types of peck and have a tendency to label all pecks apart from the obvious feeding peck as aggression.

https://www.sciencedirect.com/science/article/abs/pii/S0031938496005355