都知道豬群有社會地位一說，其實(shí)雞也同樣呢！
Space allowance and social requirements

There is inevitable interaction between social requirements, group size and space allowance (Keeling, 1995). Modelling shows that at a given space allowance crowding is worse in small enclosures and groups (Appleby, 2004). Often the floor space allowance per hen in colony systems is similar to that for hens in cages, but they can usually make use of vertical space and the fact that other birds do not often spread evenly over the floor of a large shed.

Laying hens seldom perform activities such as wing flapping, stretching, body shaking and tail wagging (Albentosa and Cooper, 2004). However, when space is so restricted that they cannot perform them, as in conventional cages, they exhibit rebound behaviour and perform them for much longer when subsequently given more space (Nicol, 1987). Moreover, Albentosa and Cooper (2004) found a significant reduction in the number of wing or leg stretches and tail wags in birds housed in groups of 8 in cages at 762 cm2 each, compared with pairs of birds at 3084 cm2 each. Other relatively infrequent activities such as dustbathing may be performed more in smaller group sizes (Abrahamsson and Tausson, 1997). So far the value to hens of infrequently performed comfort activities such as wing flapping has not been measured (for example by using consumer demand theory or operant conditioning methodology).

There is evidence that hens prefer to have personal space and where stocking densities are high will maximise this by spacing themselves out evenly both in cage systems (Albentosa and Cooper, 2003) and in colony systems (Lindberg and Nicol, 1996). At lower stocking densities hens may space more randomly or clump according to environmental resources such as feed (Albentosa and Cooper, 2003). Based on research findings and to account for the crowding effect, Appleby (2004) has suggested that minimum space allowances in furnished cages should vary with group size from at least 800 cm2 per bird in groups of 8 or more, up to at least 900 cm2 for groups of 3 or fewer, plus a litter area.

It is not easy to extrapolate research results for individual hens’ preferences for more space both horizontally (Nicol, 1986) and vertically (Dawkins, 1985) to their preferences in a different social context with other birds and in commercial environments. Work by Faure (1986) with groups of hens trained to key peck suggested that 100 mm per bird was adequate feed trough space, and that for most of the time a cage size of 400 cm2 per bird was sufficient. However, the hens would work to obtain a cage size of up to 6000 cm2 per 4 birds, suggesting that they valued greater space for up to 25% of the day.

Relatively little is known about the social priorities of hens (i.e. how they value belonging to different group sizes or different group compositions). Laying hens seem to be able to discriminate between different individuals within their own social group (Bradshaw, 1991) and to associate with familiar rather than strange individuals or groups of hens (Hughes, 1977; Bradshaw, 1992). Unfamiliar hens may be aversive to others (Grigor et al., 1995; Freire et al., 1997b). Although the maximum number of flock-mates that can be recognised by each hen is not known, it is thought to be slightly less than one hundred individuals (Nicol et al., 1999) so we might expect to Weeks 3/30/2006 Page 9 of 15 LayWel, WP4 Task 4.1 find that hens prefer to belong to groups of this size or smaller. Lindberg and Nicol (1996) reported that hens showed a strong preference for a group of 5 hens over a group of 120 hens in the same-sized space, but tended to prefer the larger group in a large space over the smaller group in a small space. They concluded that whilst smaller group sizes may be preferable to hens this would need to be combined with sufficient space. Thus interpreting preference tests for group size is difficult because test outcomes appear to be influenced by the context in which testing is carried out as well as the prior experience of the test bird (Hughes, 1977).
Aggressive behaviour is infrequent in large flocks compared to that reported in small to medium-sized flocks, possibly due to hens not recognising flock mates as familiar or unfamiliar (Hughes et al., 1997). Alternatively, when kept in large groups, hens may switch from their normal social system of individual recognition and remembered social hierarchies to a ‘rule of thumb’ system (Pagel and Dawkins, 1997; Nicol et al., 1999). Here any aggression results from direct assessment and comparison of body and comb size (D’Eath and Keeling, 1998). Therefore, at least from a biological functioning perspective, certain individual hens might experience reduced social stress in larger flocks, though whether such factors would influence hens’ choice of flock size is yet to be determined.