much less consistent in their choice of roosting site. A similar analysis of the daytime locations showed that the sections of the night roost were used significantly more than
Ž other sections during the daytime by end group birds single-sexed, P - 0.05, mixed,
. Ž
. P - 0.05 ; but again for mid-pen groups, this was not the case Fig. 3 .
3.2. Aggression in small sample groups The results show that during the observations the first day, there were significantly
Ž less aggressive pecks in ‘‘close’’ groups than in ‘‘apart’’ groups from mixed flocks Fig.
. 5, P - 0.05 . But in groups from single-sexed flocks, there was no significant difference
Ž .
Ž .
Fig 4, P s 0.43 . When the data was pooled ss q m , there was a tendency for higher Ž
values in groups of birds that had been roosting far apart from each other F s 4.42, .
df s 1 and 14, P s 0.05 . There were no significant differences between groups during the second day.
There were no significant differences between the treatments in the numbers of threats, avoidances, gentle and severe feather pecks in all small sample groups from
mixed flocks as compared to groups from single-sexed flocks. In the sample investigated in this report, there were more performers of aggressive
Ž pecks in day 1 as compared to day 2 mean number in ‘‘close’’ groups: 2 compared to
. 1.5, and in ‘‘apart’’ groups 2.5 to 1.5 . The mean number of performers in ‘‘apart’’
groups from mixed flocks were 2 compared to 1, whereas in ‘‘apart’’ groups from single-sexed flocks, the means were 3 compared to 2. Retaliations of the dominance
order were evenly spread between the groups and over the days. No hypothesis testing was performed because of the small number of registrations.
4. Discussion
4.1. Localisation In the present study, birds roosting at the ends of pens were in most cases found
significantly more often in these areas during daytime and at night, which would indicate the existence of home ranges. However, groups roosting in the middle of the
pens were much less persistent in their choice of roosting site as well as where they spent most of their day. Hens, like other birds, probably use visual cues to localise
themselves in their habitat. In an artificial environment, like an intensive housing system, each section looks quite similar, except for the end sections with the limiting
walls. In this light, the results of the present study seem rather logical; birds roosting near the ends of the system are more constant in their choice of roosting site and
movements during the day, simply because localisation is easier for them than for birds in the middle part of the system. It might also be that these more easily recognized areas
are the most desirable sites in the system, which in fact seems to be the case, as end areas most often are quite full at night whereas there is still a lot of space in the middle
of the pen. It is therefore highly probable that subgroup formation is facilitated by structures, such as pen walls, that help the birds to localise themselves, as also suggested
Ž .
by McBride and Foenander 1962 . Even for hens in a natural habitat, localisation may Ž
. be difficult if they cannot rely on well-known environmental clues. Collias et al. 1966
found no signs of a homing ability in red jungle fowl transferred 300–400 m from their home territory.
4.2. Reduced aggression During the first day together, one would expect a higher incidence and more
performers of aggression in groups of unacquainted birds than in groups of birds that know each other. This was also the case in this study, though no hypothesis testing was
performed on the number of performers. The difference in aggressiveness was signifi- cant in groups from mixed flocks but not in groups from all-female flocks, and the
results therefore indicate that the males had an effect. The reducing effect of males on
Ž female aggression found in a previous study of laying hens in the large flocks Oden et
´
. al., 1999 , however, seems to depend more on the fact that males dominate females
socially by their immediate presence than an enhanced subgroup formation, as there was no overall lower level of aggressiveness in the small sample groups from mixed flocks.
In fact, the higher aggressiveness among the unacquainted hens from mixed flocks when taken out and put together in small groups with restricted space might well be a rebound
effect of the males’ influence.
During the second day, there were no significant differences between the groups, Ž
. which is in accordance with the findings of Zayan 1987b that hierarchies in small
groups of laying hens could be formed rather quickly. Zayan also discussed the idea that large groups are built up by subgroups and that information about aggressive encounters
could be used transitively by the birds, which would explain the relative stability also in large groups.
Ž .
According to Lindberg and Nicol 1996b agonistic behaviour in semi-familiar and unfamiliar groups tends to be expressed by aggressive pecks, rather than by threats
which are more commonly used in familiar groups. This means that birds that are acquainted to one another do not have to fight with each other for dominance. The
present results are in accordance with this idea as there were no significant differences regarding threats but by aggressive pecks.
Ž .
Oden et al. 1999 found no significant difference in feather pecking between mixed
´
and single-sexed flocks, and also no less feather damage in mixed groups. This is in accordance with the results of the present study.
4.3. The method Ž
. In the present study the different resources were unlike most situations in nature in
close proximity to the hens in all parts of the pen, and consequently there was no need Ž
. for them to move far about. McBride and Foenander 1962 found that the average area
of movement of hens in a flock of 80 birds in captivity was about one third of the total
2
Ž .
available area, or 13 m . Furthermore, Collias and Collias 1967 found that jungle fowl in a semi-natural zoo habitat were more stable and persistent in their choice of night
Ž roost site than wild ones. They also found that the zoo flocks of up to about 40
. individuals showed extreme locality fixation with the roosting sites as the center points
Ž .
of the territories Collias et al., 1966 . Therefore, the method of assigning birds to the small sample groups and to investigate their daytime area preferences based on their
roosting pattern seems justified.
4.4. The concept of subgroup Ž
Recent theories about the dynamics of aggression in large groups of hens Pagel and .
Dawkins, 1997 suggest that different strategies come into play at differing group sizes. Ž
. A hen has to come quite close in order to identify another hen Dawkins, 1995 , and in
Ž .
small groups - 8–10 birds this would result in dominance relationships based on individual recognition, whereas in larger groups of hens, a system depending on status
signalling rather than recognition of individuals would appear. Pagel and Dawkins suggest that the limiting factor is the cost and pay off for the fights it takes to establish a
peck order rather than the inability to recognize a large number of other birds individually. This theory would explain the aggression in large groups as being fights
over resources with little or no individual recognition, but recognition of signals of dominance and subordination. According to the theory subgroup formation in large
flocks is rather unlikely to occur as it is too costly. However, our results with flocks of about 500 birds suggest that subgroups might be formed at this flock size as we found
clear indications of the existence of home ranges and there was less aggression among familiar than among unfamiliar birds. The limit where hierarchy formation based on
individual recognition is no longer cost-effective has yet to be found. The contradicting results from the studies of large groups referred to in the introduction could have to do
with the fact that it may be difficult to discover signs of smaller groups in a large flock in a crowded intensive system. Furthermore, lights are often quite dim under these
conditions, which might cause difficulties for human observers in identifying individual
Ž .
animals as well as for the hens themselves D’Eath and Stone, 1999 . At which group-size subgroup formation occurs is also still unknown, as is the hen’s
Ž .
preferred group-size. Lindberg and Nicol 1996a for example, found that hens had a clear preference for large groups of 70 birds over small groups of four birds. They also
found that low-ranking individuals were more consistent in their choice of the larger group, suggesting that these hens could avoid persecutors more easily in large groups
despite the fact that there were more birds to peck them. Lindberg and Nicol concluded that a hen’s group size preference probably is influenced by its position in the hiearchy,
and also that space seems to play an important role. This may offer an explanation to the
Ž .
contradicting results from studies of smaller groups: McBride and Foenander 1962 found that groups of 80 birds did not mix, but had separate territories. Appleby and
Ž .
Jenner 1993 got much the same results when they used two groups of 40, whereas Ž
. Ž
. Widowski and Duncan 1995 and Keeling and Savenije 1995 could find no clear sign
of subgroup formation as defined by local preference or preference for individual birds, within groups of 60 and 70 birds, respectively. It could be that in all these groups the
birds could more or less recognize each other individually — as also suggested by
Ž .
McBride and Foenander 1962 — and that a further division into subgroups was
therefore not to be expected. Maybe the group-size limit of recognition must be well Ž
. over 100 birds, as Guhl 1953 found evidence of a peck-order in a flock of 96 hens.
Ž .
Furthermore, in single-sexed flocks of 400 laying hens, Craig and Guhl 1969 found a stronger tendency for individual birds to stay within a defended area as compared to
Ž .
flocks of 200 hens There were not sufficient data from flocks of 100 hens . In a flock of Ž
. about 1200 hens, McLean et al. 1986 found that the birds did not move freely over the
whole available area, but no thorough experiments were carried out to test the hypothe- Ž
. sis of subgroup formation. In a more recent study, however, Hughes et al. 1997
concluded that in flocks of 300 birds, there was a lack of social structure, as birds seemed to react similarly to ‘‘familiar’’ and ‘‘unfamiliar’’ birds. The authors used a
method rather similar to our own, with formation of sample groups. In assigning birds to ‘‘familiar’’ and ‘‘unfamiliar’’ groups, however, they caught the birds during the daytime
without, as it seems, any previous monitoring and they did not record the development of agonistic behaviour continuously over time. According to our study, birds mingle
more during the day than when choosing night-roost and therefore, selecting birds on the basis of night roosting site seems to give a better assessment of location constancy as
well as assignment of birds to the different categories.
There is certainly a need to further examine ‘‘subgroups’’ in large flocks of laying hens. Perhaps they vary with the situation and the time of day. In large flocks in
crowded environments, like those in the present study, the ‘‘subgroup’’ might perhaps be best defined as a group of acquainted birds in which the individual bird feels
reassured at night or when resting at any time, while during most of the daytime the birds, independent of their ‘‘subgroup’’, use more of the total available area. Males
might both strengthen the ‘‘reassurance’’ effects while resting by gathering and watch- ing over a group of females as well as during the day act more like ‘‘highway patrolers’’
that are spotting trouble and reacting quickly to it. In fact, in a crowded environment, as in the present study, the task of gathering and guarding is likely to be very hard. There
are simply too many hens. Therefore, the direct control of aggressiveness by mere presence seems to be the most probable way the ‘‘male effect’’ works.
5. Conclusions
