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Broiler chickens: A tolerant social system?

Authors:
  • Neiker- Basque Institute for Agricultural Research and Development

Abstract

It was hypothesized that the social organization of com- mercial broiler chickens (Gallus gallus domesticus) kept in large groups (50 or more chickens) is based on the development of peck orders within sub-groups. Predictions of this hypothesis are (1) decreased use of space as group size is increased within a constant area, with the majority of birds restricting movement to avoid aggressive encounters with unfamiliar individuals, and (2) increased inter-individual variability in body weight of chickens with increasing group size due to monopolization of resources by despotic individuals. Groups of 50, 100, 150 and 200 chickens were kept in identical pens, with or without access to an outdoor patio. Use of space by focal individuals was analysed by the harmonic mean method, which is more sensitive than previously used methods to assess use of space by domestic fowl. The results showed that space use at the 30, 50 and 70% isopleth levels remained stable across group size. Space use increased at all group sizes when the birds were given access to the outdoor patio. Body weight decreased with increasing group size. However, the coefficient of variation in body weight was similar across group size, and the frequency of threats declined with increasing group size. The results suggest that access to resources was not impaired by agonistic behaviour in larger groups. The results do not support the sub-group hypothesis for broiler chickens under commercial conditions. The chickens showed plasticity of social behaviour according to environmental conditions, with increased tolerance at larger group sizes.
... In layer hens, Keeling et al. [18] reported that groups of fewer than 15 birds used aggression to establish dominance and maintain stable relationships, groups of 30 were too large for a stable hierarchy to develop, but too small for the tolerant social system that was observed in groups of 60 and 120 birds, where the birds were relatively non-aggressive [18]. Increased tolerance in larger groups (up to 200 birds) has also been observed in broilers [19]. Current broiler production systems in the EU are focused on maximising returns, and it is assumed that this is best achieved by housing thousands of birds in a single space. ...
... Our data clearly demonstrated (1) similar growth rates at 12, 14, 16, 18 and 20 birds/m 2 (all three trials) and 22 birds/m 2 (trial three only), where any differences in growth rates were at best marginal and (2) a faster growth rate in the biosecurity cubes even when the stocking density was similar to the main flock. Higher growth rates have been previously observed in smaller groups of broilers, but the magnitude of the differences depended on the study design, being more pronounced in smaller (50 to 200 birds) groups and/or at lower stocking densities (0.05 to 0.11 birds/m 2 ) [19,20]. ...
... One possible explanation for the faster growth rate in the cubes, even when the stocking density was similar to the main flock, is improved access to feeders and drinkers [16,17], but the feeder and drinker to bird ratio was similar in the cubes stocked at 20 birds/m 2 and those in the general flock. Other studies that observed higher growth rates in smaller broiler groups have also ruled out better access to resources [19,20]. Roosting behaviour, facilitated by simulated walls provided by the cube, could also account for improved performance, but this was only observed toward the last week of the study when the test birds were already bigger than the control flock [21]. ...
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This study investigates the effect of stocking density and population dynamics on broiler growth rates and productivity, while further validating the ability of the biosecurity cubes (BC) to protect birds from Campylobacter. In our methodology, six BC were constructed in a commercial broiler house containing approximately 28,500 birds. During three trials, the BC were stocked at densities of 12, 14, 16, 18, 20 and 22 birds/m2, with the main flock (20 birds/m2) considered the control. Periodically, 10 birds per density were weighed and examined. The Campylobacter status of the birds was monitored via faecal samples using the ISO 10272: 2017. The stocking density for maximum calculated yield was 20 (trials 1 and 2) or 22 birds/m2 (trial 3), followed by 18, 16, 14 and 12. At the stocking rate of 20 birds/m2, the birds in the pen grew faster than those at the same density in the main flock achieving 2 Kg 3–6 days faster. Birds in the BC were observed to be generally healthier, and in some cases, remained Campylobacter negative, even after the main flock was infected. Our results conclude that dividing the flock into sub-flocks of approximately 20 birds/m2 using BC could increase productivity up to 20%, while preventing Campylobacter.
... For example, removing the most dominant or most aggressive animal in a group may alter the dominance hierarchy (e.g., cichlid fish, Chase et al., 2002) and individual experiences of the animals remaining in the group (e.g., cows, Woodbury, 1941). Similarly, increased group size can cause individual recognition of other group members to become more difficult and reduce the frequency of interactions between individuals (Pagel and Dawkins, 1997), leading to situations where dominance relationships are either absent or only weakly established (e.g., Estevez et al., 1997Estevez et al., , 2003. Even though the effect of group size on individual recognition and dominance in cattle has been discussed (e.g., Fraser and Broom, 1980;Grant and Albright, 2001), it has not been experimentally investigated. ...
... However, larger groups may yield less steep dominance hierarchies (e.g., Balasubramaniam et al., 2011), conditions expected to decrease the reliability of dominance calculation methods, or require even larger data sets (Sánchez-Tójar et al., 2018). Large group sizes may also result in a greater proportion of unknown or poorly established dominance relationships (Pagel and Dawkins, 1997;Estevez et al., 1997Estevez et al., , 2003, potentially influencing the performance of calculation methods (Saccà et al., 2022); further research is required in this regard. ...
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Cattle are gregarious animals able to form social relationships. Dominance is one of the most widely studied social behaviors of dairy cattle, especially cows confined indoors. However, much of the past dairy cattle research has used an unstandardized approach, differing in definitions and conceptual understanding of dominance, as well as their methods of data collection and dominance calculation. The first of the 3 aims of this review is to evaluate how dominance relates to the social behavior of housed dairy cows. Cows engage in agonistic interactions to establish and reinforce dominance relationships. An individual's dominance is influenced by intrinsic characteristics, such as personality, and extrinsic factors, including group composition. When competing for resources, agonistic interactions can also be influenced by individual motivational differences, such as hunger, which may diminish the role of dominance in regulating competition. Our second aim is to critically review methods used to assess dominance in cows. This includes discussions on the effect of time and location of data collection on measured values as well as the viability and limitations of some dominance calculation methods. We propose that different methodologies lend themselves to different types of research questions. For example, the use of data stream-based methods that consider the sequence of interactions are useful for estimating how dominance fluctuates with changing conditions and can be used in a dynamically changing group. In contrast, matrix-based methods that aggregate social interactions may be best for identifying the social position of individuals and understanding how social characteristics influence the attributes of a stable hierarchy. Our third aim is to discuss the future of dominance research. We use a flowchart to illustrate guidelines for a more standardized approach to measuring dominance in cattle. We also identify areas in need of further conceptual clarification, suggest practical applications of dominance when managing dairy cattle, and discuss some limitations of dominance research.
... Sørensen et al., 2000;Sanotra et al., 2001;Dozier et al., 2005;Estevez, 2007 (Bolton et al., 1972) ‫بلعکس،‬ . Proudfoot et al., (1979) ‫که‬ ‫دادند‬ ‫نشان‬ ‫(محدوده‬ ‫باال‬ ‫تراکم‬ Deaton et al., 1967;Estevez et al., 1997;Sørensen et al., 2000 ) ‫یافته‬ ‫این‬ . ‫توسط‬ ‫آمده‬ ‫بدست‬ ‫نتایج‬ ‫با‬ ‫ها‬ Cravener et al., (1992) Proudfoot et al., 1979;Shanawany, 1988;Cravener et al., 1992 ) ‫نهایت‬ ‫در‬ ‫و‬ ‫سود‬ ‫این،‬ ‫بر‬ ‫عالوه‬ ‫شد.‬ ‫خواهد‬ ‫بیشتر‬ ‫گله‬ ‫آوری‬ Puron et al., (1995) (Feddes et al., 2002) ‫طور‬ ‫به‬ . ...
... ‫تراکم‬ ‫در‬ ‫ران‬ ‫و‬ ‫پا‬ ‫سوختگی‬ ‫شیوع‬ ‫همچنین‬ ‫جوجه‬ ‫در‬ ‫باال‬ ‫های‬ ‫ها‬ ‫ی‬ ‫می‬ ‫افزایش‬ ‫گوشتی‬ ‫گونه‬ ‫به‬ ‫یابد،‬ ‫شاخص‬ ‫با‬ ‫پارامتر‬ ‫دو‬ ‫هر‬ ‫که‬ ‫ای‬ ‫هستند‬ ‫همبستگی‬ ‫دارای‬ ‫ضعیف‬ ‫گام‬ ( Sørensen et al., 2000; Sanotra et al., 2001; Arnould and Faure, 2003; Dozier et al., 2005 ) ‫گام‬ ‫شاخص‬ . ‫ضعیف‬ ‫های‬ ‫با‬ ‫ارتباط‬ ‫در‬ ‫است‬ ‫ممکن‬ ‫تر‬ ‫تراکم‬ ‫در‬ ‫که‬ ‫باشند‬ ‫پرنده‬ ‫تحرک‬ ‫کاهش‬ ‫عدم‬ ‫دلیل‬ ‫به‬ ‫باال‬ ‫های‬ ‫است‬ ‫ممکن‬ ‫یا‬ ‫و‬ ‫افتاده‬ ‫اتفاق‬ ‫کافی‬ ‫فضای‬ ‫وجود‬ ‫کاهش‬ ‫علت‬ ‫به‬ ‫تراکم‬ ‫در‬ ‫بستر‬ ‫کیفیت‬ ‫سریع‬ ‫عامل‬ ‫یک‬ ‫عنوان‬ ‫به‬ ‫که‬ ‫باشد‬ ‫باال‬ ‫های‬ ‫می‬ ‫شناخته‬ ‫پا‬ ‫سالمت‬ ‫در‬ ‫مؤثر‬ ‫شود‬ (Estevez et al., 1997;Sørensen et al., 2000;Škrbić et al., 2009 ) ‫از‬ ‫ترکیبی‬ ً ‫معموال‬ .‫بستر‬‫کیفیت‬ ‫سریع‬ ‫کاهش‬ ‫و‬ ‫کافی‬ ‫فضای‬ ‫وجود‬ ‫(عدم‬ ‫عامل‬ ‫دو‬ ‫هر‬ ‫تراکم‬ ‫علت‬ ‫به‬ ‫است.‬ ...
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To maximize the genetic growth potential of broiler chickens, optimal environmental conditions should be provided. In this regard, any failure to provide optimal conditions may negatively affect birds' performance. Stocking density has critical implications in the broiler industry since higher returns can be obtained as the number of birds per unit of space increases. Previously, stocking densities were determined by a simple cost-benefit analysis in commercial broiler farms; however, it has been proved that excessive densities did not only result in lower economic profits, but also reduced bird performance, health, and welfare. Currently recommended densities by producers are rather variable, and therefore, it is necessary to establish management guidelines based on practical and scientific outcomes. Recent studies, in agreement, indicated that the broiler chickens’ performance, health, and welfare are compromised when the stocking density alleviates to less than 0.0625 to 0.07 m2/bird (equivalent to about 34 to 38 kg/m2 ~14-15 birds/m2 for 2.5 kg final body weight). In this context, negative consequences include a peduction in both feed intake and final body weight, and in severe cases, foot-pad dermatitis, scratches, bruising, poor feathering, tibial dyschondroplasia, physiological stress, and mortality have been well documented. In conclusion, it can be stated that a high and limited stocking density without sufficient control over environmental factors may lead to serious damage to the growth performance, health, and welfare status of the broiler chickens. In this review study, attempts have been made to establish the effect of various stocking densities in broiler farms on their growth performance, feed and water intake, carcass quality, leg weakness, litter microbial load, hysiological stress, and immune status.
... Furthermore, the housing conditions for the broiler chicks in the present study differed from those in the conventional broiler farming in terms of stocking density group size and enrichment, all of which have been shown to affect physical (Thomas et al., 2004;Estevez et al., 1997), physiological (Beloor et al., 2010), and behavioural (Fairhurst et al., 2011) responses. Consequently, it is possible that the set-up of the present experiment with comparatively small groups of animals, low stock densities, and much enrichment alleviated or even reversed the stress experienced during transportation. ...
Article
Incubation and hatching commonly takes places at hatcheries, separate from the grow-out facilities where broiler chicks are raised. This means that chicks are sorted and transported immediately after hatch, during which time they typically do not have access to feed and water, and are subjected to transport stress. Recently, innovative housing systems are being developed in which fertilized eggs are transported on embryonic day 18 (E18) from the hatchery to the grow-out facility, where they hatch on day 21. In chicken, the hypothalamic–pituitary–adrenal (HPA)-axis becomes functional around embryonic day 14-16. It is therefore conceivable that transport of eggs at E18 may lead to a stress response in the chick embryo. Exposure to prenatal stress may affect the coping capacity of the individual and negatively impact its further development. We investigated whether prolonged transport on E18 has effects on the development of a slow growing broiler chicken strain (Hubbard JA257). E18 eggs were transported for either 41 minutes (short transport, ST) or 219 minutes (long transport, LT). Transportation significantly increased embryonic heart rate after ST. This increase continued during an intermediate measure at 120 minutes. The increased embryonic HR then remained high at measurement immediately following LT. We did not find effects of prolonged transport on behavioural parameters measured in the juvenile chicken in the tonic immobility and open field test. Concentrations of feather corticosterone as well as faecal corticosterone metabolites did not differ on postnatal day 36. We showed that transport leads to an autonomic stress response in chicken embryos at E18, but that this elevation had no further effects on other indicators of prenatal stress. Nevertheless, our results emphasize that transport of incubated eggs should be as refined as possible to minimize the exposure to stress.
... Broiler behavior and movement are, in addition, affected by factors such as density and location within the house under assessment (Leone and Estevez, 2008;Mallapur et al., 2009;Buijs et al., 2010). Indeed, the distribution of broilers inside a house is not homogeneous, as individuals will tend to stay close to walls to seek protection (Newberry and Hall, 1990;Estevez et al., 1997), especially those showing health/welfare problems (BenSassi et al., 2019a). All these aspects may influence the results of welfare assessments, and therefore, they must be carefully considered when interpreting assessment outcomes. ...
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Full-text available
A combined welfare assessment protocol, including indicators from the Welfare Quality ® and AWIN ® EU funded projects, was tested on commercial fast and medium growth commercial broiler flocks to determine differences in their assessments as measured with the used of animal welfare indicators. Ten commercial fast (Ross 308, Cobb 500, or a mix of both) and 10 medium growth (Hubbard JA×Ross 308), mixed sex commercial flocks were assessed at 32 and 48 days of age, respectively. Two observers simultaneously collected data on each flock. Observations included transect walks on central and wall areas to assess the AWIN ® welfare indicators, bedding quality, environmental parameters and positive behaviors, all of them collected with the i-WatchBroiler app. According to the WQ protocol, welfare assessment indicators including the human avoidance tests, gait score, body weight and hock burns were also measured on each flock. Novel object tests were also carried out. The results of the study show that fast growth flocks had a higher incidence of welfare issues shown by the higher percentage of immobile, lame, sick, featherless, and tail wounded birds. Positive behaviors such as play fighting, wing flapping and running were more frequently observed in medium growth flocks on central locations, while fast growth flocks had a more limited expression of such behaviors. Fast growth flocks also had worse gait scores. Medium growth flocks expressed a different response to behavioral tests depending on the house location, likely attributable to their better mobility and welfare state, and also to the smaller stocking densities at which they were housed, while on the other hand the behavior of fast growth broilers during tests was similar regardless house location, being likely affected by mobility problems and the higher stocking density specific to their management. These results provide quantitative evidences on the differences in animal welfare assessment outcomes in fast and medium growth broilers. Nevertheless, results also suggest that some of the test responses were associated with the physical state and movement ability of the birds and house location that limit their response capacity. Such limitations should be considered when interpreting animal welfare assessment outcomes. These results add to previously published scientific evidences showing the potential of the method and app technology for practical on-farm broiler welfare assessment, including positive indicators, with farmers, technical personnel, certification bodies or scientist as potential end-users.
... Poultry farms in Europe and North America are increasingly adopting large, open systems with flocks of tens of thousands of individuals, where animals can freely access various distinct areas, whereas the ancestors of domesticated chickens typically formed groups of two to 20 individuals [58]. While chickens develop a pecking order in small groups, a theoretical study and several empirical studies with broiler chicken and domestic fowls report a change from a high aggressive dominance relationship towards low aggressive alternative strategies with increasing group sizes [59][60][61][62]. A study by D'Eath and colleagues has suggested that groups containing more than 70-80 animals will adopt an alternative strategy due to limitations in recognizing animals and the level of aggression needed to maintain a dominance position within such large groups [63]. ...
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We explored the relationship between social associations and individual activity patterns in domestic hens. Out of 1420 laying hens, 421 hens were equipped with RFID tags attached to RFID-specific leg bands (leg bands from Company Roxan, Selkirk, Scotland) to continuously track their change in location across four different areas (one indoor and three outdoor areas). Using a combination of social network analysis for quantifying social relationships and dynamic time warping for characterizing the movement patterns of hens, we found that hens were consistent in their individual variation in temporal activity and maintained stable social relationships in terms of preferred association partners. In addition to being consistent, social associations correlated with movement patterns and this correlation strengthened over the period of observation, suggesting that the animals aligned their activity patterns with those of their social affiliates. These results demonstrate the importance of social relationships when considering the expression of individual behaviour. Notably, differences in temporal patterns emerge despite rather homogeneous rearing conditions, same environment, and low genetic diversity. Thus, while variation in behavioural phenotypes can be observed across isolated individuals, this study shows that the social environment within a group can shape and enhance variation in general movement patterns of individual animals.
... Mean (± SEM) latency to approach (s) all cues (positive, P; near positive, NP; middle, MID; near neutral, NN; and neutral, N) of birds from both high-and low-density pens in the judgement bias test for 4 rounds (n = 9). 1 42.08 kg/m 2 at day 50. 2 www.nature.com/scientificreports/ increased motivation to receive a food reward during judgement bias training and testing. ...
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Affective state can bias an animal’s judgement. Animals in positive affective states can interpret ambiguous cues more positively (“optimistically”) than animals in negative affective states. Thus, judgement bias tests can determine an animal’s affective state through their responses to ambiguous cues. We tested the effects of environmental complexity and stocking density on affective states of broiler chickens through a multimodal judgement bias test. Broilers were trained to approach reinforced locations signaled by one color and not to approach unreinforced locations signaled by a different color. Trained birds were tested for latencies to approach three ambiguous cues of intermediate color and location. Broilers discriminated between cues, with shorter latencies to approach ambiguous cues closest to the reinforced cue than cues closest to the unreinforced cue, validating the use of the test in this context. Broilers housed in high-complexity pens approached ambiguous cues faster than birds in low-complexity pens–an optimistic judgement bias, suggesting the former were in a more positive affective state. Broilers from high-density pens tended to approach all cues faster than birds from low-density pens, possibly because resource competition in their home pen increased food motivation. Overall, our study suggests that environmental complexity improves broilers’ affective states, implying animal welfare benefits of environmental enrichment.
... Our results were comparable with the majority of studies that mentioned increasing stocking density had a detrimental impact on growth performance. Estevez et al. (1997) and Sørensen et al. (2000) reported a decrease in body weight when space per bird was lower than 0.066 m 2 , moreover, the body weight gain was declined with increasing the stocking density from 25 to 40 kg/m 2 ) that supported the current results. There was no significant difference regarding the growth performance between low and medium stocking density. ...
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The aim of the current study to investigate the potential impact of different stocking densities on growth performance, carcass traits, indicators of biochemical and oxidative stress and meat quality of Arbor Acres and Ross-308 broiler breeds to recommend the better stocking density with low production cost simultaneously with high quality. A total of 312 one-day old of each Arbor Acres broiler and Ross-308 were randomly classified into 3 experimental groups with different stocking density, each of 6 replicates. The first group (SD1) was 14 birds/m² (28kg/m²), while the second group (SD2) was 18 birds/m² (36kg/m²) and the third group (SD3) was 20 birds/m² (40kg/m²). The growth performance, carcass traits, meat quality hematological and biochemical parameters were measured. SD3 group possessed the lowest body weight. Alanine transaminase in Arbor Acres was 15 and 14% higher in SD3 when compared with SD1 and SD2, respectively. While, was 21 and 20% of Ross-308, respectively. SD3 revealed the highest values of cholesterol, TG, MDA and LDL of both breeds when compared with SD1 and SD2, with the lowest levels of HDL, GPX and IGG. Birds of SD3 was the nastiest carcass weight 873 (p=0.000) and 1411.60g (p=0.000); dressing percentage 63.07% (p=0.000) and 75.83% (p=0.000); breast weight 513.10g (p=0.000) and 885.50g (p=0.000); thigh weight 359.90g (p=0.000) and 526.08g (p=0.000) when compared with SD1 and SD2 of Arbor Acres and Ross-308, respectively. The dressing % of SD1 and SD2 was approximately 19% better than that of SD3 of Arbor Acres, while it was 4% of Ross-308. The cooking loss and drip loss of breast and thigh muscles were higher in SD3 of both breeds. Moreover, SD3 possessed the highest bacterial count. In conclusion birds reared in medium stocking density revealed better performance and welfare than high density but similar to low density. Therefore, from the economic point, medium density was the best.
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Barren housing and high stocking densities may contribute to negative affective states in broiler chickens, reducing their welfare. We investigated the effects of environmental complexity and stocking density on broilers’ attention bias (measure of anxiety) and tonic immobility (measure of fear). In Experiment 1, individual birds were tested for attention bias (n = 60) and in Experiment 2, groups of three birds were tested (n = 144). Tonic immobility testing was performed on days 12 and 26 (n = 36) in Experiment 1, and on day 19 (n = 72) in Experiment 2. In Experiment 1, no differences were observed in the attention bias test. In Experiment 2, birds from high-complexity pens began feeding faster and more birds resumed feeding than from low-complexity pens following playback of an alarm call, suggesting that birds housed in the complex environment were less anx-ious. Furthermore, birds housed in high-density or high-complexity pens had shorter tonic immobility durations on day 12 compared to day 26 in Experiment 1. In Experiment 2, birds from high-density pens had shorter tonic immobility durations than birds housed in low-density pens, which is contrary to expectations. Our results suggest that birds at 3 weeks of age were less fearful under high stocking density conditions than low density conditions. In addition, results indicated that the complex environment improved welfare of broilers through reduced anxiety.
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One hundred and thirty five 18weeks old Bovans Nera Black strain pullets were used in a 10week study to determine their heat balance and blood profile under varying stocking density in locally fabricated metal-type cage system. The cages were stocked 2, 3 and 4birds/cell. Daily ambient temperature and relative humidity of the cage and rectal temperature of the birds were taken and heat balance calculated. Record of Packed cell volume (PCV), Haemoglobin concentration (Hb), Red blood cell (RBC), White blood count (WBC) and differential of the birds were taken at beginning and end of the study for the haematological indices while blood glucose, total protein, Albumin and blood urea were taken for the bio-chemical measurements. Ambient temperature, relative humidity, and heat balance showed no significant (P>0.05) difference with cage stocking density. Cage stocking density had significant (P<0.05) effect on rectal temperature of layers. Bird stocked 3/cell recorded the least (41.14oC) rectal temperature while those stocked 4/cell recorded the highest (41.27oC). All the haematological parameters of the birds were not significantly (P>0.05) influenced by stocking density of the cage type. Bio-chemical measurements were not significantly (P>0.05) different among layers under varying stocking density of the cage except total protein (P<0.05). Layers stocked 4/cell recorded highest (5.22g/dl) total protein while those stocked 3/cell had the least value (4.37g/dl). However, the values were within the normal range recommended for healthy chicken. The study concluded that locally fabricated metal-type battery cage could be used to rear layers and stocking density of 3birds/cell is ideal without compromising the welfare of the birds.
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Movements of broiler breeders were studied in commercial flocks of nearly 4000 birds kept on deep litter by tagging samples of individuals. In Flock 1, locations of females in the house were recorded on 52 days over 34 weeks. Individual ranges analyzed were all more than half the area of the house, with a median of 505 m2 (73% of the area). Most or all ranges must have overlapped extensively. In Flock 2, locations of males and females were recorded on 10 days over 7 weeks. Male and female ranges were not significantly different. Implications of these results for welfare and productivity are discussed.
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A postulated curvilinear relationship between area per bird and frequency of agonistic interactions is based on data collated from several preliminary experiments.
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Commercial meat-type birds are commonly reared in large groups at high population densities, and may also be severely feed-restricted in breeding flocks. Although these factors are all recognized to affect aggressive behavior in domestic animals, there have been few studies of aggression in meat-type birds. The present experiments were designed firstly to investigate whether selection for growth has modulated the development of aggressive behavior in male broiler chicks, and secondly to determine the influence of feed restriction on aggression.In the first experiment, broiler males were compared with males from a brown egg-laying strain and a white egg-laying strain. No significant differences were found between brown and white egg-type males in the frequency of pecking and threatening, with both groups showing a rise in these behaviors as expected until 8 weeks of age. Pecking and threatening in broilers, however, remained extremely low throughout this period. Sparring did not differ among the three strains. In the second and third experiments, broilers fed either ad libitum (AD) or placed on skip-a-day feed restriction (SK) were compared. SK sparred less but were more aggressive than AD until 15 weeks of age. Aggressive activity in SK was particularly elevated on feed-off mornings, and decreased to its lowest levels after SK were placed on full feed for only 4 days. Selection for growth thus appears to have resulted in a decrease in aggressiveness, regardless of actual body weight, at least when food is provided ad libitum. It is concluded that problems associated with aggressive behavior are likely to be of little significance in commercial broiler flocks, which are marketed at 7–8 weeks of age, but are of importance in breeder flocks maintained on feed-restricted diets.
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THE peck order, observed in domestic poultry flocks, has four main components—aggressiveness, submission, acceptance of submission and recognition. Intra-specific aggressiveness arises as a result of sexual selection for mates, territory and food1, and submission signals and their acceptance are necessary to divert aggressiveness into harmless channels2. Recognition is necessary in a gregarious species to allow these other three forces to be integrated into a peck order. Aggressiveness is only lowered within a group; strangers are always attacked.
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The movements of individual birds within flocks were recorded by marking a sample with coloured tags and photographing the flock at fixed intervals using eight time-lapse cine, cameras, each camera covering a field amounting to one-eighth of the total area. The birds were housed on deep litter in an area 13·4×5·2 m, and flock sizes of four to 600 were used they ranged in age from 9 weeks to adult. The films were analysed, the positions of the marked birds were plotted and the probability of the observed distributions occurring by chance were calculated. Previous workers have found that individual birds move over, only part of the total area; in our study the movements of individuals lay on a continuum ranging from apparent randomness at one extreme to clear non-randomness at the other. Moreover, in all experiments most birds were sighted in most camera fields, indicating that their movements were incompatible with the accepted definition of a delineated home range. These results were explained by postulating strain differences in aggressiveness and assigning an important role to illumination level. At very low light levels individuals may be able to move freely in a flock because they cannot be recognized as strangers and, in any case, individual recognition may be exceptional in large flocks. Even so, birds did differ in the extent to which they moved around and some did exhibit preferences for particular parts of the pen; such tendencies were consistent over a 4-week period in adults but not in juveniles.
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Some of the criteria which have been used for assessing the welfare of farm animals are briefly reviewed. A relatively new method employing techniques of animal learning is discussed. A prerequisite for this is knowledge of the preferences of the animals themselves for different environments. The measure of preference used in the present study was how quickly hens moved from a starting box into a given environment. Experimental data are presented which show that battery-kept hens move more quickly into a battery cage than into an open run in the garden. For both these environments, the presence of other birds increased their preference for the hens. In a second experiment, the previous experience of the birds was shown to be crucial: although battery-kept birds again moved more quickly into a cage, hens used to living outside moved more quickly into a hen run. When hens were familiar with both environments, they showed no significant preference for one environment over the other.