INTRODUCTION
Most farm animal species have long been subjected to intense and strictly targeted selection for improvement of their performance traits. Programs implemented in chicken breeding are mainly focused on increasing weight gain, laying performance, or internal and external egg quality traits. This has led to substantial changes in the anatomy and physiology of this species, which resulted in considerable differences between birds reared currently and their wild ancestors, also in terms of their behaviour. Natural hens’ behaviour represents a repertoire of their ancestors’ behaviours, provided that their rearing conditions allow demonstration of such behaviours [
1]. Behavioural problems arise when chickens are motivated to show certain behaviours but they are unable to express them due to limitations such as the size of the cage or absence of enrichment elements. Consequently, other variants of behaviour arise and can often lead to behavioural disorders, e.g. feather plucking. Behavioural patterns depend not only on birds’ habitat and experiences but also on the genetic background, environmental conditions prevailing during embryonic development, and epigenetic effects [
2,
3]. Therefore, it seems that each breed of laying hens reared on farms can exhibit diverse behavioural needs for maintenance of homeostasis of the organism. However, the differences in behavioural needs between chicken genotypes are not considered during adjustment of rearing standards to ensure bird welfare. Therefore, standardisation of the conditions for rearing laying hens on farms does not necessarily improve the welfare of all breeds.
The aim of the study was to determine whether there is variability in behaviour as well as differences in emotional reactivity and preferences of laying hens depending on the breed.
DISCUSSION
A major hen welfare-related problem in modern industrial poultry farming is the lack of adjustment of rearing conditions to birds’ behavioural needs [
11]. The natural behaviours of domestic fowl, e.g. free movement, pecking, scratching the ground, wing flapping, self-grooming, or quiet rest and sleep, may be limited by the impossibility to express them. The tests carried out in this study were aimed at verification whether the many-generation hen selection, which indirectly determines birds’ behaviour [
12], exerted an effect on the behavioural variability and whether birds from different breeds characterised by dissimilar performance value exhibited different behaviours.
The results demonstrated such differences between the analysed breeds. Noteworthy is the relatively long time devoted by the Zk birds to explore the enriched feed; this parameter had several-fold higher values than in the case of the Pb and LG breeds. The birds did not only ingest the feed but also expressed the need for scratching and searching, thus satisfying one of the basic needs, i.e. curiosity [
13]. However, it seems that the scratching and searching need is strongly developed mainly in the Zk birds, which represent primitive breeds that have not been selected towards high performance value. This element of the environment was not preferred by the hens from the other breeds. The LG hens, which are mainly reared as high-yield layers in intensive production, exhibited considerably greater interest in the commercial feed, which has a form of a homogeneous granulate and ingestion thereof does not require searching for edible parts. These results indicate that selection can significantly change hens’ behaviour and food preferences. High-yield breeds are targeted at feed intake that will fulfil their physiological rather than behavioural needs. For economic reasons, bird breeds with the highest feed conversion rates and absence of the scratching and searching behaviour are preferred in breeding.
Another enriching element, i.e. the mirror, showed considerable differences in curiosity among the breeds. The longest time for examination of the object was devoted by the LG hens. The birds pecked at their reflection in the mirror. The interest in the mirror may indicate great curiosity in this breed. It should be noted that this temperament feature cannot be fully satisfied in the farm rearing conditions. A stimulus-poor and monotonous environment does not offer opportunities to satisfy curiosity. Hence, it is possible that boredom and an attempt to satisfy curiosity is one of the causes of feather plucking, which is quite a common phenomenon in the LG breed (own unpublished observations).
There were also differences in the birds’ interest in the shelter. This object was clearly avoided by the Pb breed. Simultaneously, none of the enriching elements was found to define the preferences of this breed, as in the case of Zk and LG. This may be a result of the origin and the components of this breed, i.e. heavy meat breeds and the primitive Zk breed. Hence, the behaviour of these hens is characterised by elements differing them from light layer breeds such as the LG and from the Zk breed. It can therefore be concluded that hybrids of layer breeds reared in a farm breeding system will differ in their preferences and behaviour from parent breeds. This is an important finding, as it indicates that the assessment of chicken behaviour cannot be limited to testing pure-bred birds.
There were differences in the behaviour of the analysed hen breeds in terms of emotional reactivity. Animals respond adequately to the degree of emotional arousal, which is strongly associated with the breed in addition to individual traits. The indicators of the emotional status comprise the locomotion speed, vocalisation, defecation, and self-grooming. The present investigations demonstrated differences in the level of these indicators depending on the breed. Within a similar locomotion time (no statistical differences), the LG hens covered a several-fold higher number of squares on the floor, which evidenced a fast locomotion rate. Additionally, there was a significantly greater probability of defecation in this breed. These indicators might suggest increased anxiety and higher fearfulness in LGs [
14]; however, another group of behaviours associated with comfort activities (e.g. cleaning feathers, flapping wings, ruffling feather, scratching the body) was noted in this breed more often than in the others (
Table 5). As reported by Zimmerman et al [
14] increased locomotion is correlated with anxiety about an upcoming aversive event, whereas anticipation of a positive event is associated with comfort behaviours (e.g. cleaning feathers, flapping wings, ruffling feather, scratching the body). It should be underlined that application of a standard open-field test for assessment of fearfulness [
15,
16] without modifications consisting in enrichment of the environment with additional elements would suggest a high level of fear in the LG hens. In this case, however, the presence of the interest in the elements of the environment with the absence of significant differences between the investigated breeds should be emphasised. Simultaneously, if the animal shows interest in objects, increased locomotion should not be regarded as expression of stress, as a stressed and terrified animal does not explore the environment. The relationships between the emotional reaction to the environment and the decision to avoid or approach the environment are key elements of animal welfare [
17]. The behaviour of the LG hens indicates that the breed is characterised by very strong emotional arousal and high reactivity, but not necessarily fearfulness. The present results agree with investigations demonstrating that birds with white plumage exhibit higher emotional reactivity than birds with coloured feathers [
18–
20]. There were no differences in the level of reactivity between the coloured Zk and Pb breeds. This result should not be surprising, given the origin of the Pb breed. Importantly, only the LG breed has been intensively selected for many generations towards higher laying performance and it is currently characterised by very high laying rates. The Zk and Pb hens belong to conservative herds and constitute a genetic reserve; therefore, no selection targeted at birds’ performance traits is being carried out. In conclusion, the higher reactivity of LG hens may also be a result of indirect selection and a behavioural response to the high physiological requirements of the organism.
The present investigations also included a tonic immobility test, whose results can be an indicator of the level of fearfulness, as suggested by various researchers [
8,
21]. Tonic immobility is an unconditioned reaction that can be easily induced manually and is reflected by bird’s immobility and lower reactivity to external stimulation [
9]. It is believed that the latency of the first head movement and the TI duration are positively correlated with fearfulness [
22]. In the present study, the shortest duration of immobility was recorded in the Pb breed; however, the results of the open-field test do not show lower fearfulness in this breed in comparison to the others. There were no differences between the breeds in the time required by the birds to start exploration of the environment or in the locomotion time, which can undoubtedly be indicators of the level of fear [
10]. TI is thought to be a behaviour protecting birds from predators [
23,
24]. Given this theory, it can be assumed that primitive behaviours related to species survival can be eliminated by selection. The LGs needed twice as long latency of tonic immobility, which did not influence the duration of immobility. Possibly, selection eliminates the physiological reaction of immobility but not its duration. The duration of immobility was the longest in the primitive Zk breed; yet, statistically significant differences were noted exclusively between Zk and Pb, and not between the specialised LG breed. As indicated by the other research results, tonic immobility is difficult to achieve by excitable birds with increased emotional reactivity. Thus, the latency of immobility may be a specific marker of emotional excitability of an individual.