Introduction The chicken (Gallus gallus domesticus) is a domesticated subspecies of the Red Junglefowl. Chickens were originally domesticated for cockfighting, and barely used for meat or egg production. Only around the nineteenth century the poultry industry commenced, starting out with producers keeping only a handful of chickens of dual purpose (eggs and meat), which had access to the outdoors and brooded their own young. Today there are over 100 million chickens in the Netherlands, of which almost 35 million are laying hens. The laying hens are spread out over 700 farms, which means the mean population of laying hens per farm is around 50.000 animals. With this commercialisation of the industry come a lot of welfare problems, of which Feather pecking is a major one. There has been a lot of research on the causes and solutions to feather pecking, but the right solution has yet to be found. As the ban on beak trimming, which up until now has been the main solution against feather pecking, in 2018 nears, the want for an alternative solution is greater than ever. The goal of this review is to elucidate what causes feather pecking behaviour and advising what would be a valid solution for this huge problem. Because feather pecking is seen as a negative influence on the state of welfare in the laying hens, the concept of animal welfare will firstly be discussed. Next, the many causes of feather pecking will be highlighted. To maintain a clear structure, the causes have been categorised in respectively Nutritional influences, Environmental influences, Endocrinal or Hormonal influences and psychic influences. Finally I will discuss the solutions for feather pecking behaviour and what would be an option when beak-trimming is forbidden in 2018. Material and Method This literary study has been mainly conducted on digital platforms. Through the Vet Portal on the Utrecht University website, there was access to complete publications of scientific research. The search engines used were Scopus, CABabstracts and Google Scholar. I searched using the terms: Feather pecking, beak trimming, laying hens, nutrition, genetic manipulation, housing systems, chicken welfare, light intensity and rearing. The keywords were combined to narrow down the articles. To ensure the quality of the used articles, only articles published in scientific journals or conducted by researchers at Universities were used for this paper. Furthermore I tried to use the most recently published or most often cited articles. References in cited articles were also used. 1. Animal Welfare 1.1 Definition Animal Welfare Animal welfare is a complex concept. The definition of welfare varies between cultures, regions, time and individuals. To be able to measure and compare welfare in animals there has to be a clear definition of welfare known to all parties. Throughout this review the concept of animal welfare will be formulated according to the five freedoms defined by The Brambell Committee (Ohl & van der Staay, 2012) : An individual is in a positive welfare state when it has the freedom to adequately react to: – Hunger, thirst or incorrect food; – Thermal and physical discomfort; – Injuries or diseases; – Fear and chronic stress, and thus, – The freedom to display normal behavioural patterns that allow the animal to adapt to the demands of the prevailing environmental circumstance and enable it to reach a state that it perceives as positive To measure fear and chronic stress the most important pillar is the analogy postulate. This postulate takes into account the anatomical and physiological similarities of vertebrate nervous systems and the comparability and homology in the behavioural and physiological responses to discomfort of humans and other vertebrates in similar situations. 1.2 EU-project Welare Quality Based on the five freedoms, the University of Wageningen has developed a system to measure animal welfare. This system’s framework emanates from four classes: Behaviour, health, nutrition, and housing systems. In the following paragraphs each class will be shortly explained and also its relevance to laying hens. 1.2.1 Behaviour For one there is the behavioural component, which is pretty much the most important. Natural behaviour is the product of evolution. The survival of animal species throughout evolution has been based on their ability to adapt to their environment. By domesticating animals we have suddenly drastically changed their environment, thereby limiting their ability to show natural behaviour. This is not necessarily a bad thing. For example, prey animals such as chickens no longer need to fear for predators. Some behaviour is rewarding to animals and if they can’t preform this behaviour this affects their welfare negatively. This rewarding behaviour is referred to as ‘behavioural needs’, for example dust bathing and foraging for food. For good welfare it is necessary that this rewarding behaviour can be performed. Otherwise you stand at risk of the animals developing stereotype behaviour, which is seen as negative for welfare. 220.127.116.11 Coping styles A coping style may be defined as: ‘a coherent set of behavioural and physiological stress responses which is consistent over time and which is characteristic to a certain group of individuals.’ (Koolhaas et al., 1999) The ways animals cope with stress have been categorised into two categories: pro-active or reactive. An animal with the pro-active coping style is characterised as having a response ruled by territorial control and aggression. Animals with the reactive-coping style are described to respond with a conservation-withdrawal response. Although these coping styles are often used to distinguish between two types of responses to a stressor, it is not without discussion. One of the reasons for this discussion is the question to which degree distinct coping styles can be differentiated. Still the research up till now (Korte, Beuving, Ruesink, & Blokhuis, 1997) suggests that High Feather Pecking lines show the pro-active coping style, and that Low Feather Pecking lines show the reactive coping style. A study in mice showed that pro-active animals have the tendency to develop routines or habits where as reactive animals do not. If you apply this assumption in chickens, it confirms the theory that high feather pecking lines show the pro-active coping style. 1.2.2 Health Secondly there is the health component. Health can give us a lot of information about how an animal functions in its living environment. Animals in good welfare are free from injuries, diseases and discomfort by treatments. Every treatment that causes pain is seen as a negative influence on an animals welfare. To measure health in laying hens, the Louis Bolk institute has composed a list with health characteristics. These characteristics are only useful if they comply with three conditions. Firstly there must be a different outcome in sick and healthy animals. Second the characteristic has to be objectively measurable. Last the characteristic has to correspond with what is described in literature and with experience of researchers or otherwise consulted experts. A few examples are the size and colour of the comb, bodyweight and skin and feather condition. 1.2.3 Nutrition Eating and drinking behaviour are essential behaviours for animals to survive. If there is a shortage on food the motivation to eat and drink will increase. If hunger and thirst feelings continue to exist for a certain amount of time this can lead to negative emotions, such as anger and aggressiveness. This influences the welfare state negatively. Note that this does not mean an animal should never feel hunger or thirst. These are natural feelings and it is only when an animal cannot react to the feeling of hunger or thirst by eating or drinking that welfare is influenced negatively. 1.2.4 Housing Housing has a direct influence on physical and physiological discomfort. Comfort consists of three aspects, that all influence animal welfare in a positive way. First is the ability to rest and lie down comfortably. For laying hens this means the disposal of perches. Second animals should have the ability to move around freely. They shouldn’t be confronted with too many obstacles that could possibly injure them. Floors should offer sufficient grip to prevent animals from slipping. Lastly there should be a good thermoregulation. For example research by the University of Wageningen indicates that exposure to UV-light decreases stress levels in chickens, and that they show more aggressive behaviour in light with a long wavelength (red-light) than with a short wavelength. 2. Feather pecking Feather pecking (FP), pecking directed to and damaging the feathers of other birds, is a behavioural disorder occurring in laying hens and other poultry (Sedla??kov??, Bil’?k, & Ko”l, 2004). It is a huge welfare and economic problem in the poultry industry, especially since the banning of conventional cages in 2012. Estimating the economic consequences is difficult, because of the intricate relationship between FP and other forms of injurious pecking. What can be stated is that FP causes conspecifics to have poor plumage cover and therefore less thermal isolation resulting in loss of more body heat in cooler environments. Less plumage cover is associated with a lower food conversion ratio and study by Blokhuis et al (H. J. Blokhuis et al., 2007) showed that bald chickens need up to 40% more feed to maintain body temperature and are also less effective in transposing energy derived from feed into egg mass. A huge misinterpretation is that feather pecking is a form of aggressive behaviour. It is true that there is a form of pecking that is used to dominate over another chicken. However, this is a form of natural behaviour, and is not the behaviour that is causing all the problems. A way to distinguish this ‘natural feather pecking’ (NFP) from the problematic feather pecking (PFP) is the location that is being pecked at. NFP is usually aimed at the head or the back of the neck of conspecifics, whereas PFP is aimed at the back or the tail (T. van Niekerk, Lezing welzijnsimplicaties kippen, University of Utrecht, 23-02-2015) There are two types of PFP: Gentle feather pecking (GFP), and severe feather pecking (SFP). GFP is the act of gently pecking or nibbling at the feathers and is mostly ignored by the recipient. GFP can be sub-divided into pecks to various feather targets that are thought to be driven by exploration and repeated pecks to a single location on a feather (Dixon, 2008). The recipient is usually not injured by this behaviour. It may be a form of stereotype behaviour in the pecking animal. SFP consists of forceful pecks and pulls of feathers that are frequently eaten and results in feather loss on the back, vent and tail area (Rodenburg et al., 2013). The recipient is often hereby injured and will either move away from or confront the pecker. If the pecking continues, there may develop bald patches on the recipient, which can lead to tissue pecking. This is a form of pecking at the skin, leading to injuries. It can eventually lead to the death of the recipient due to excessive blood loss and tissue damage. Another form of feather pecking is vent pecking. This form is aimed at the region around the cloaca. It starts primarily as curious pecking but can become so impetuous that it can turn into cannibalism. This form will not be further discussed in this review because it is a separate form and therefore not relevant for this thesis. 2.1 Ontogeny Ontogeny is the development or course of development especially of an individual organism. In this review ontogeny will be used to describe the process of developing feather-pecking behaviour. The development of feather pecking starts from day one. In the first weeks of their life, chicks learn what is edible and what is not. Naturally, the mother hen demonstrates this by her eating behaviour. In the poultry-industry chicks are raised separate from their mother hence they don’t have an example to learn from. As a result chicks peck at everything, and by doing so also learn what is and what is not edible. Note that this is natural behaviour, and that the absence of adult animals might be the cause of excessive pecking, but not the occurrence of feather pecking behaviour. If you take into account the fact that the surroundings of the pullets consist mainly of other chicks, it is almost inevitable that they also peck at each other. The form of feather pecking chicks perform is GFP. There have been a lot of studies testing the hypothesis that GFP can develop into SFP, but in all these studies there has been no relation found between the two forms of feather pecking. This suggests that SFP stands separate from GFP. 2.2 Causes There are many factors that influence feather pecking behaviour. It is still unclear how these factors influence each other, and a lot of research is still conducted on this topic. The challenge is also that some studies seem to contradict each other in the outcome (for example the influence of light intensity). In the following paragraphs the known influential factors will be shortly discussed. These factors are divided into four categories: (1) diet/nutrition, (2) environment, (3) hormonal influence and (4) psychic factors (differences between individual birds). An overview of all known causes is shown in Figure 2 at the end of this document. 2.2.1 Diet/nutrition One of the current hypotheses on the influence of nutrition on feather pecking is that chickens are in want of more structure inside their digestive track. Feathers are indigestible and offer this structure. As a result chickens that lack structure in their diet eat feathers instead. In a study by Kamendal & Bessei (Kalmendal & Bessei, 2012) the feed preference in HFP and LFP laying hens was studied. The different lines were housed separately and given the choice between a wheat-soy control diet, and a diet supplemented with 8% spelt hulls (FIB). The FIB diet differed only from the control diet in the 8% spelt hulls, which were on the expense of wheat. Spelt hulls were used because it comprises large quantities of insoluble nonstarch polysaccharides, which have a similar effect on the digestive system to feathers. The result was that HFP hens consumed significantly more of the FIB diet than LFP hens. This suggests that HFP hens are more wanting for structure in their digestive track compared to LFP hens. Other studies, for example Kriegseis et al (Kriegseis et al., 2012) hypothesize that HFP hens prefer more palatable diets than LFP hens. In this experiment it was seen that diet supplemented with 10% feathers significantly lowered the amount of severe feather pecking in HFP hens compared to HFP hens on the control diet that did not contain feathers. Another, more important, hypothesis is that the composition of the diet is of great influence on feather pecking behaviour. Of all the substances in available diets for chicken, amino acids seem to have the biggest impact. McAdie & Keeling (McAdie & Keeling, 2000) found that ruffled or trimmed feathers aroused significantly more severe feather pecking than unaffected plumage. The highest amount of feather pecking was seen on the back ant tail area where feathers had been manipulated by either being cut very short or removed entirely. This emphasizes the importance of good feather development. Feathers consist of protein for 89-97%. Amino acids derived from feed intake therefore play a crucial role in feather development. The biggest components of feather keratin are the sulphur-containing amino acids: methionine and cysteine. A small shortage of these amino acids is enough to cause abnormal feathering (Elwinger, Tufvesson, Lagerkvist, & Tauson, 2008). This resulted in more severe feather pecking directed to aberrant plumage. Another finding, by van Hierden, Koolhaas & Korte (Van Hierden, Koolhaas, & Korte, 2004) seems to confirm that high levels, 21 g/kg diet, of Tryptophan ‘ an essential amino acid – decrease the amount of feather pecking. Tryptophan is a large neutral amino acid, and is a precursor for serotonin synthesis. Previous to their study in 2004, they found that the serotonergic (5-HT) system played a causal role in the occurrence of feather pecking. They found that an acute decrease of 5-HT turnover in the forebrain seemed to cause higher amounts of feather pecking behaviour. This led to the hypothesis that high levels of 5-HT turnover in the forebrain should reduce the development and performance of feather pecking. To test this hypothesis, 200 white leghorn chicks were used. From age 1 day to age 34 days they were all fed the same commercial diet containing 1,6 g Tryptophan (TRP)/kg feed. From 34 days onwards half the group was fed a diet containing 2% TRP, which comes down to +/- 21 g TRP/kg diet. The other half stayed on the commercial diet. At 49 days of age the chicks were observed during a period of 30 minutes. The results are in line with expectation and show that an increased concentration of Tryptophan in the diet leads to a decrease of feather pecking behaviour. 2.2.2 Environment Another very common theory is that feather pecking is a misdirected behaviour. It could be (1) derived from ground pecking. If the surroundings are void of appropriate substances, pullets and hens may substitute its absence by pecking at feathers. It could also be that chickens ‘mis-imprint’ on feathers instead of dust for dust bathing. This theory has been tested in several studies. For one an experiment carried out by H.J. Blokhuis in 1989 (H. Blokhuis, 1989). Here he shows that litter influences the frequency of feather pecking. In this experiment 32 chicks were divided in four groups at birth. Two of these groups were housed on litter (L), and the other two were housed on non-litter (NL). During fifteen weeks each chick was observed. Based on the results Blokhuis concluded that NL chicks displayed more feather pecking than L chicks. Rearing of laying hens also plays an important role in the development of feather pecking (Gilani, Knowles, & Nicol, 2013). Thirty-four flocks from 29 rearing farms were observed during the beginning, middle and end of the rearing period, and at the start of lay. The results were processed in three models, which contained 21 significant variables. A third of these variables related to house climate. An especially interesting one was that SFP occurred more in flock that had been exposed to higher sound levels at the end of rear or at the beginning of the laying period. It was first thought that the higher sound levels weren’t the cause, but the result of feather pecking. Birds can make loud cries, which are seen as a sign of frustration. However, this doesn’t apply to pullets during the rearing period, for they cannot yet produce these cries. The noise during the rearing period originated from mechanic machines, which has been suggested to be aversive to birds in earlier studies(MacKenzie, Foster, & Temple, 1993). This relation could form problems in creating good housing systems. Because there is little to no ventilation in stalls, mechanical fans are needed to maintain a good climate. At the same time these fans, may influence the development of feather pecking. Secondly, the models showed that flock that had not started feather pecking during the rearing period had better plumage condition than flock that had displayed feather pecking during rearing. Light intensity also seems to influence the amount of feather pecking. Many studies have been done, but there has yet to come a uniform explanation. What is clear are a couple of matters; Chickens rely on their sight for recognition of conspecifics, chickens should not be held in continuous light respectively darkness (this causes morphological changes in the eyes), and chickens need a minimal light intensity of 5 lux to be able to see. In the European law Council Directive 2007/43/EC wording regarding light is stated that: ”6. All buildings shall have lighting with an intensity of at least 20 lux during the lighting periods, measured at bird eye level and illuminating at least 80 % of the useable area. A temporary reduction in the lighting level may be allowed when necessary following veterinary advice.’ In an experiment by Kjaer & Vestergaard (Kjaer & Vestergaard, 1999) 450 ISA Brown were housed in groups, each of which exposed to a light intensity of 3 or 30 lux. The pullets were observed at 10 weeks, 28 and 45 weeks. At 16 weeks the pullets were regrouped and reduced to a total of 320. The results of this experiment were that gentle feather pecking at ten weeks was a ten-fold higher in 3 lux and the amount of severe pecks was about 3 times higher in 30 lux. At 28 and 45 weeks there was significantly more SFP in 30 lux than in 3 lux. These results are in line with the theory that SFP and GFP are not caused by the same factors. A possible explanation for the high levels of GFP at 3 lux could be that this light intensity impaired the eyesight of the pullets. Earlier research by Helle Halkjaer Kristensen (Helle Halkjaer Kristensen, 2007) showed that only at a light intensity of 5 lux hens can jump from one perch to another. This suggests that chickens are not able to see clearly at a light intensity lower than 5 lux, otherwise you would expect to see the jumping from one perch to another at lower light intensities. As a consequence the amount of exploratory pecking increased in order to compensate for the lacking sight. The suggestion that the frequency of feather pecking is higher in high intensity lighting than in low intensity lighting is a reason for lots of farmers to keep their hens in the dark. This is not the best solution for welfare, and will be discussed later in this review (3.3). 2.2.3 Hormonal/Endocrinal influences It is thought by many researchers that feather pecking is a form of stereotype behaviour and that it is stress related. In an experiment (Kjaer, Hjarvard, Jensen, Hansen-M??ller, & Naesbye Larsen, 2004) this theory was tested. Laying hens were first observed for 50 minutes pre-treatment. Then they were injected with haloperidol (a dopamine D2 receptor antagonist) and subsequently observed during 50 minutes. Feather pecking decreased significantly after treatment (from a total of 1.70 bouts pre-treatment to 0.29 bouts post-treatment). Haloperidol had no influence on aggressive behaviour. This outcome leads to assume that feather pecking is influenced by the dopaminergic system and possibly stress related. This experiment is not enough to say so with certainty, so more experiments will need to be done to confirm this theory. An important hormone for the development of feather pecking is the steroid hormone corticosteroid. It is known that in a number of species, including birds, corticosteroids from the circulation enter the brain, where they bind to mineralocorticoid (MR) and glucocorticoid (GR) receptors in the hippocampus and amygdala (mammals) c.q. archistriatal complex (birds). If the MG/GR function becomes unbalanced it is thought to alter how animals respond to the environment, promote receptiveness to stress, change behavioural adjustment, and influence learning and memory processes. It is hypothesized that the development and performance of feather pecking between LFP and HFP chicks is associated with differences in behavioural and physiological response to environmental stimuli and differences in learning processes during early development. Furthermore, we hypothesize that a different MR/GR balance in the brain of LFP and HFP chicks may be underlying these differences. This difference is seen in the results of an experiment (Van Hierden et al., 2002), in which plasma corticosteroid-levels were measured. Plasma corticosterone levels were higher in HFP and dopamine and serotonin turnover were lower in HFP chicks as well. Once again this is not enough to confirm the relationship between feather pecking and corticosteroids, but it certainly means we cannot rule it out. 2.2.4 Psychic factors One of the current theories is that feather pecking may be a hereditary behaviour. This would be very useful for farmers. This way they can select the animals with a low tendency for feather pecking to bread further generations. Research suggested that one Quantitative Trait Locus (QTL) was found, showing that many genes, each with a small effect, are probably involved in the causation of feather pecking (Jensen et al., 2005) (Buitenhuis et al., 2005). The genes for feather pecking seemed to be linked to multiple other traits, such as growth. Study showed that hens with a wild-type plumage are predisposed to become victims of feather pecking (N??tt, Kerje, Andersson, & Jensen, 2007). This has to do with the PMEL17 colour gene. An inter-crossed red jungle fowl (wild type) and White Leghorn (domesticated) were used to investigate this. Chickens with I/I showed white plumage, and Chickens with i/i showed pigmented feathers. This study concluded that birds with wild-type plumage were more often victimized then white birds. Note that there is yet a lot to be found on the genetics influencing feather pecking. Difference in stress sensitivity between different lines selected on the occurrence of feather pecking have been studied by Kjaer & J??rgensen (Kjaer & J??rgensen, 2011). In total they observed 48 hens. 16 hens had been selected on High feather pecking (HFP), 16 on low feather pecking (LFP) and 16 were the control group. Of each hen they composed an electrocardiogram (ECG) before and during physical restraint and during a social test. The results showed that laying hens on average showed higher stress reactions during physical restraint than the social test and that genetic selection for lower feather pecking decreased the autonomic neuro-system reaction to physical restraint, and selection for higher levels of feather pecking increased the response to higher levels of social contact. This result supports the hypothesis connecting feather pecking to higher stress sensitivity. 3. Solutions As seen above, there are numerous influences on feather pecking behaviour. Consequence is that it is hard to find a solution for the problem that completely eradicates feather pecking in laying hens. 3.2 Beak trimming Currently the conventional method to prevent feather pecking is beak-trimming or beak treatment. The terms Beak trimming and beak treatment are often mixed up. The difference is that beak trimming is done when chickens are older than six weeks and beak treatment has to be done before the chicken is ten days old. Beak treatment and beak trimming used to be done manually, by a professional beak-trimmer. This way each individual pullet was assessed and the beak was trimmed down to a length specifically for that pullet. Now the process has been automatized. A machine is programmed to cut the beak down to a certain length, based on the average pullet in a flock. It is therefore of great importance to have a homogeneous flock to prevent unevenness in beak lengths. There are two ways to apply beak treatment: (1) using a hot-blade (HB) or (2) an infrared beam (INF). 3.2.1 Hot-blade vs. infrared beam Carruther et al studied the differences in the effect of beak trimming using respectively a hot blade or an infrared beam in commercial flocks (Carruthers et al., 2012). In this study four flocks were used, each observed between 21 and 24 weeks of age and between 53 and 60 weeks of age. Overall results showed that pullets that had received INF treatment showed a lower frequency of abnormal beaks than pullets that received the HB treatment. Also there was less variability in beak length measurements in INF-treated pullets than in HB-treated pullets. Also T. Niekerk (T. van Niekerk, Lezing welzijnsimplicaties kippen, University of Utrecht, 23-02-2015) explains that INF treatment is thought to be less harmful than HB treatment. HB treatment uses a blade to cut off 1/3 ‘ ?? of the beak. This blade can become blunt with time and as a result cause more damage than a sharp blade would. Also, by cutting the beak off with an actual blade the beak gets slightly dented, as every substance does when something cuts through it. INF treatment uses an infrared beam that burns off 1/3 ‘ ?? of the beak. After the treatment all you see is a thin line on the beak. After a couple of weeks the part of the beak distal from that line crumbles off. This way there is no open wound and there is no indentation of the beak, which minimizes damage done to the beak. Since 2011 it is only allowed to perform beak-treatment using the INF treatment in the Netherlands. 3.2.2 Method in the Netherlands Worldwide beaks are either treated before ten days of age or after the age of six weeks. In the Netherlands Novatech performs beak-treatment using an exclusive machine. In this machine pullets are placed inside a carrousel and subsequently treated with an infrared beam. This carrousel has a set size and can only be used on one-day-old pullets. This is something hatcheries have to take into consideration for their logistics. Young hens lay smaller eggs and as a result give smaller pullets compared to older hens. To ensure a homogeneous flock of pullets the eggs that are hatched have to be sorted according to age of the mother hens. 3.1.3 Effect of treatment at different ages As previously stated beaks are treated at either 6 weeks of age. Freire, Glatz and Hinch did an experiment in which they researched among other things the effect of beak-treatment performed at these different ages (Freire, Glatz, & Hinch, 2008). Research was done on 80 Hyaline Brown pullets divided in 4 groups, each getting a different beak-treatment or none at all. Pullets receiving beak-treatment at ten weeks of age (BTD) pecked less at new objects and with less force than pullets receiving beak-treatment at one day of age (BOD). These results are in line with the hypothesis that beak-treatment at one day old is less harmful than beak-treatment at ten weeks of age. 3.2.4 Neuromas The beak is a complex structure and is very sensitive. This gives birds the means to be able to differentiate between structures during foraging, and gives them the opportunity to choose between what they do and do not consume. When part of the beak is cut off, so are many of the free nerve endings. This could result in the development of neuromas (Breward & Gentle, 1985). A neuroma is a proliferative mass of nerve processes that develop at the severed end of a nerve. This can cause pain similar to phantom pain (pain felt after a limb has been amputated). This indicates that birds may feel pain long after beak-treatment has been performed, which effects negatively on animal welfare. Research by Gentle (Gentle, Hughes, Fox, & Waddington, 1997) found that neuromas don’t seem to arise if beak treatment is done at a young age. In pullets beak-trimmed at an age younger than ten days no neuromas were found, whereas pullets beak-trimmed at an age older than five weeks, there was a significant amount of neuromas found. In comparison to pullets that received beak treatment at an olderage, pullets receiving beak treatment at a younger age healed faster and showed no scar tissue. 3.2.5 Acute effect of beak treatment When part of the beak is cut off it is inevitable that the pullet feels pain. Pain in animals is defined as ‘an aversive sensory and emotional experience by the animal in response to damage or a threat to the integrity of its tissues’ (Molony & Kent, 1997). Chronic pain is presumably caused by neuromas (as discussed in 3.1.4). Acute pain is seen in the phase between dealing with the origin of the injury and the lead up to recovery. This phase could last varying from a few minutes to a few days. However, it should not last longer than the healing process. Research has confirmed that beak trimming causes acute pain in pullets. For one Glatz (Glatz, 1987) found that heart rate of pullets increased significantly after the beak was treated, indicating there was short term pain. Another finding was a reduction of appetite, less locomotor activity, longer periods of sleep, and a decrease of social behaviour. 3.3 Light intensity As previously (2.2.2) discussed, light intensity seems to have an influence on feather pecking behaviour. Farmers often keep their hens in the dark or use light with specific wavelengths, such as red or blue light, to prevent feather-pecking behaviour. It is thought that red or blue light impairs the eyesight of the birds. This way they cannot sufficiently discriminate between other hens, resulting in less feather-pecking behaviour. Red light also seems to minimize the pullet’s ability to detect blood and naked skin (Mohammed, Grashorn, & Bessei, 2010). Red light should not be used as a universal pre-emptive measure, as photoreceptor sensitivity in the retina can be cankered by the absence of other wavelengths reaching the eye. Keeping pullets in continuous darkness causes disproportionate development of the eyes (Nickla, Wildsoet, & Troilo, 2001), which is a negative influence on animal welfare. 3.4 Genetic selection This topic too, has been shortly discussed earlier in this review (2.2.4). This is an upcoming alternative for the current solutions, which has yet to be further implemented. Layer hens with a brown and white plumage phenotype are more likely to become the victim of feather pecking than layer hens with an only white plumage phenotype (Keeling et al., 2004). This is probably because the combination of white and brown feathers offers a clearer contrast than solely white feathers. This seems to be more attractive to peck at. Up until today commercial breeding companies have mainly focused on individual selection for egg laying performance. However, selection against feather pecking should also be seriously considered. This could have beneficial effects, for example lower stress responses (Kjaer & J??rgensen, 2011). 4. Conclusions After studying many researches it is clear that feather pecking is indeed a huge and very complex problem in the laying hen industry. Solving one problem often leads to another, to great frustration of the farmers. Many farmers have now agreed to come together to find a solution against feather pecking, so that when beak treatment is no longer allowed in 2018, a good alternative will be available. The many influential factors on feather pecking are divided roughly into internal and external factors (figure 12). What I would advice the farmers (mag ik hier ik zeggen?) after critically analysing the available data, is the following: Firstly, as seen in the review, feather pecking already starts in one day old pullets. It is therefore essential that rearing farms also join in and ensure a good environment, preferably as similar as possible to the laying farms, to rear the pullets. Secondly, performing genetic selection on the laying hens can facilitate farmers’ work. Selection on low feather pecking, will probably decrease feather pecking levels significantly. Thirdly there are the external factors. These are divided into the two categories environment and nutrition. Environment measures should be taken by ensuring the presence of litter, perches and enrichments. Also group density should be minimized. Light intensity should not be to low and light of multiple wavelengths is necessary for development of the chickens’ eyes. If feather pecking levels increase too much, lowering the light intensity can be used as an emergency brake, but should not be used as a preventive measure. Nutrition is of great importance as well. Sufficient levels of amino acids (especially methionine, cysteine and tryptophan) should be added into the diet, to ensure plumage condition is optimal. Lastly the health of the chickens is essential as well. Flock challenged by disease will be more prone to feather pecking incidents. It is therefore important to inform the accompanying vet, so that he or she can advise farms how best to prevent outbreaks.