According to the prevailing theory feather pecking is considered a misdirected food pecking behavior. It is assumed that food pecking is independent of satiation. Under intensive feeding conditions satiation may occur before the drive of feather pecking is consumed. In the absence of suitable foraging opportunities the food pecking activity is directed towards the feathers. Consequently the strategies to control feather pecking have been focused on avoiding early satiation (e.g. pelleted feed), increase of time spent feeding (fiber diluted diets) and the presentation of foraging opportunities (e.g. deep litter, free range). A favourable influence of high fiber diets, litter on other enrichment devices on feather pecking has been reported in numerous publications. However, the reduction of feather pecking was only gradual and in some studies there was no effect at all. In addition the risk of feather pecking and cannibalism in deep litter systems and free range is obviously very high. Recent studies using lines selected for high and low feather pecking have shown, that high feather pecking birds have a particular preference of eating feathers and fiber and prefer feathers over fiber when given the choice. It has further been shown that high feather pecking birds differ in their intestinal microbiota from low feather pecking birds. It is therefore assumed that feather eating is the primary motivation for feather pecking. The specific microbiota of high feather pecking birds may be the cause of their appetite for feathers. Although the incidence of such primary high feather pecking birds may be low in commercial flocks, their pecking activity may be transferred to flock mates through imitation and learning. This explains the deleterious effect of group size on feather pecking. High light intensity stimulates feather pecking through increasing the general activity. A similar effect is attributed to the generally known effect of nutritional deficiencies. The conventional environmental measures against feather pecking and cannibalism can delay or attenuate feather pecking, but they do not prevent it. There is a need to unravel the genetic basis of this behavior. Quantitative genetic studies have proved sufficient genetic variation for severe feather pecking.
Transition from free range towards indoor deep litter and cages systems have been reported as influencing factors. Similarly the changes in feed formulation, e.g. replacement or fish meal and meat meal by soybean meal or other protein sources of plant origin have been considered a major cause of feather pecking and cannibalism (Walser, 1997). Finally selection for high egg production and feed conversion rate is being discussed as potential influencing factor. The return from conventional cages towards deep litter and free range systems has increased the problems related with feather pecking and cannibalism. The problem will become more important when beak trimming is prohibited.
The underlying motivations of feather pecking
The most widespread theoretical explanation of feather pecking and cannibalism is based on the assumption that feather pecking is misdirected food pecking and food searching behaviour. Indeed the pattern of feather pecking is similar to feed pecking. It is further assumed that the motivation of food pecking and foraging is independent of satiation, and that under intensive feeding conditions satiation occurs before the drive for pecking is consumed. In the absence of suitable materials the birds would address their activity towards the feathers of group mates. This theory is supported by a large number of different experiments. Reducing the time spent feeding through pelleted or crumbled feed increased feather pecking and cannibalism.
Time spent feeding is even more severely reduced under restricted feeding schedules and the risk of feather pecking and cannibalism is increased under these conditions. High fiber diets showed a positive effect on feather pecking, feather score and cannibalism.
This has been explained by the extension of time spent feeding. The effect of dilution on damaging pecking was, however marginal in many of the above cited experiments. It not only depended on the level and material of dilution, but also on the breed used. In some cases there was no response to fiber even though 10 percent were supplied. Some authors consider food pecking not as an isolated element of behavior but as component of a more general motivation of foraging, which comprises food searching, scratching and ground or litter pecking. There is abundant information that feather pecking increases in non-litter management systems.
Nutrition and feather pecking
It is well documented that nutrient deficiencies increase the risk of feather pecking and cannibalism in poultry. This effect has been reported for total protein, amino acids and various minerals. The effect of deficiencies can be explained by an increase of exploratory behavior in general. Since feather pecking also occurs, when the birds are fed diets fully meeting their requirement the question remains whether hidden deficiencies exist. Commercial diets are usually based on the mean performance of flocks and the birds with the highest production may not receive adequate amounts of nutrients. Jensen et al. (2005) reported that feather-pecking hens grew faster, came earlier into lay and were more active. However, experiments have shown that the risk of feather pecking only increases when the known standards for energy, protein, specific amino acids and minerals are not fulfilled. Higher levels of the essential nutrients did not show positive effects on feather pecking. Therefore it is not likely that feather pecking is caused by hidden nutrient deficiencies. But it cannot be excluded that feather-pecking birds are not able to adjust their nutrient intake according to their requirement, or that feather pecking birds differ from the non-feather pecking birds in their nutrient selection behavior. In a choice feeding experiment using high and low feather pecking birds, high feather pecking birds consumed more protein than required when soybean meal was fed separately from the rest of the diet. High feather pecking birds also showed a high preference for fiber when given a high-fiber and a low-fiber diet in a choice feeding experiment. The differing response of high feather pecking birds in choice feeding experiments indicate differences in the metabolism and confirms the assumption of Jensen et al., (2005) that feather peckers differ from non-feather peckers in their resource allocation. As described above, the intake of indigestible fiber has obviously a positive effect on feather pecking and cannibalism. In relation to the foraging theory this effect has mainly been discussed under the aspect of increased time spent feeding and pecking. There are, however, various other effects of fiber, which need to be considered. Dietary fiber stimulates the activity of the gizzard and the gut motility. Feed retention time is increased in the foregut, but decreased in the hindgut. It also stimulates the excretion of bile acid and enhances the gastrointestinal reflux. Since chickens eat considerable amounts of litter, the above mentioned positive effect of litter on feather pecking may also be attributed to the intake of fiber. This aspect is confirmed by the finding that sand, as litter had no influence on the development of feather pecking. Dietary fiber plays also an important role through its influence in the composition and activity of the intestinal microbiota. The potential role of the microbiota in feather pecking will be dealt with below.
It has been observed that feathers cast during moulting are frequently eaten and the absence of feathers on the floor of pullet houses is taken as an indicator of later feather pecking. Ramadan and v. Borell (2008) could reduce feather pecking when the feathers cast during rearing were frequently removed from the litter. This led to the speculation that eating cast feathers in early live may facilitate feather pecking in later periods. Benda (2008) raised commercial layer hybrids with diets containing 10 % of shredded raw feathers. These birds showed a lower feather pecking activity during the rearing period than birds receiving a conventional diet containing the same levels of nutrients. When the birds were switched to the conventional diet they performed significantly more severe feather pecking than the control birds. When the birds receiving the high feather diet during rearing were given the choice between the conventional and the high feather diet, they ate significantly more of the high feather diet than the birds reared on the conventional diet. This confirms the assumption that feather eating in young birds generates a preference for feathers in adults, raising the question on the role of feathers as dietary component. Since the digestibility of protein in raw feathers is low and the birds are usually fed diets, fully covering the nutrient requirement of the birds, other causal factors are sought.
Eating whole feathers or coarsely chopped feathers included in the feed reduced the passage time of the digesta in the digestive tract. In this regard feathers show a similar effect as insoluble crude fiber. Birds from the high feather pecking line clearly distinguished between feathers and fiber, and consistently preferred feathers to fiber when given the choice. This shows that dietary fiber does not substitute feathers in these birds.
Assuming that feather eating is a main motivation for feather pecking attempts have been made to control it through repellents. A wide range of bitter or irritant substances applied on feather, have been identified to reduce the acceptance of feathers. Methods of application however have to be developed for the application under practical conditions.
The role of microbiota
The special preference of high feather pecking hens for feathers raises the question of the underlying mechanisms. One of the potential influencing factors is the development of a different intestinal microbiota. The first study of the microbiota role on feather pecking was carried out by Meyer et al. (2012). The inclusion of 5 percent raw feathers in the diet of commercial layer chicks increased the keratinolytic bacteria in the digestive tract of chicks. DNA extracts of the caecal microflora showed a reduced diversity and also significant changes in the microbial metabolites, such as short chain fatty acids, ammonia and lactate. Comparisons of the microbiota of birds of a high and low feather pecking line confirmed differences in the metabolites. Besides the above-mentioned components biogenic amines showed different levels in the high and low feather pecking lines. It is known that both, the microbiome itself as well as its metabolites influence the mental state and play an important role in the regulation of appetite in humans.
The modification of the microbial activity through the ingestion of feathers indicates the potential connection between feather pecking behavior and gut function. It is known that the intestinal microbiota can be determined by genetic factors. The microbiome of chicken lines selected for high and low body mass differed significantly by line and gender.
On the basis of the above results it can be hypothesised that high feather pecking birds have a specific microbiome, consisting of keratinolytic bacteria, such as Enterococcus faecium and Lactobacillus strains, and that this microbiome generates the specific preference for feathers in high feather pecking birds.
Genetic factors of feather pecking
First information on the genetic background of feather pecking and cannibalism stems from differences between lines and breeds. There are numerous studies which report variation among different pure lines or commercial hybrids for feather pecking behavior, pecking related feather damages or mortality through cannibalism. Heritability estimates reported by different research groups showed low heritability coefficients from .06 to .12 in pullets and higher values in laying hens. There is obviously sufficient genetic variation to select against feather pecking. There exist several experimental lines, which differ in their propensity for feather pecking and cannibalism. These lines are now being used for genomic studies on the level of QTL, gene expression (micro arrays) and SNPs. It is expected that these studies not only detect genetic markers related to feather pecking, but also unravel the underlying behavioral and physiological mechanisms. Buitenhuis et al. (2003) found significant QTLs in F2-crosses of commercial lines for gentle feather pecking, but not for severe feather pecking. Similarly Jensen et al. (2005) detected one suggested QTL with low explanatory value for severe feather pecking in F2-crosses of Jungle fowl and White Leghorn. Keeling et al. (2004) found a significant QTL for feather damage, which was related with white feather colour. Further QTLs related to feather pecking have been reported by Wysocki (2006). SNP studies indicated that feather pecking is related to the dopamine and serotonin pathways. It is expected that information on feather pecking on the molecular genomic level will provide tools to identify the birds performing feather pecking and cannibalism and this can be used for selection in commercial breeding programs.
(From Proceedings of the “Australian Poultry Science Symposium”, New South Wales, Australia)