Ž .
objects Dawkins, 1968; Hogan, 1973; Rogers, 1995 . However, the range of test stimuli Ž
. has generally been small and a recent report
Huber-Eicher and Wechsler, 1998 suggests that the above propensity might not always translate into a preference. Indeed,
chicks showed more foraging behaviour when they had access to large polystyrene blocks or bundles of long-cut straw rather than to polystyrene beads or a layer of
Ž .
shredded straw, respectively Huber-Eicher and Wechsler, 1998 . Similarly, although Ž
feathers are considered to be particularly attractive stimuli for pecking Appleby et al., .
1992 , their attractiveness may depend on whether or not they are on the bird. Indeed, though gentle pecking at the feathers of other chicks appears as early as 3 days of age
Ž .
Blokhuis, personal communication , we found that young chicks pecked much more readily at string than at bunches of feathers collected from age-matched conspecifics
Ž .
Jones et al., 1997 . Neither is there consensus concerning chicks’ preferences for Ž
pecking at certain colours even when the class of stimulus, e.g., food, is the same see .
Experiment 2 . In view of these apparent inconsistencies we attempted to establish the birds’ pecking
preferences in a systematic manner by presenting pair-housed chicks with a choice of stimuli. Previous studies revealed that string was the most attractive of a number of
Ž stimuli presented to chicks of the ISA Brown laying strain Jones et al., 1997; Jones and
. Carmichael, 1999a . We now asked if this preference would generalize to include chicks
of another commercial laying strain and we assessed the effects of manipulating the component features of that type of stimulus eliciting the greatest interest in order to
identify its most attractive form. Therefore, in Experiment 1 we measured the pecking preferences of pair-housed Lohmann Brown chicks for lengths of string, chains or beads
Ž .
when these were presented consecutively one stimulus per day or simultaneously for 10 min on each of 3 consecutive days. Rationales for our choice of stimuli are given in
the introductions to each experimental section. Preference was defined as the tendency for the chicks to peck sooner and more often at one stimulus than at the others. Having
determined which stimulus elicited most interest we then varied its component features. Colour preferences were examined in Experiment 2; here the chicks were simultane-
Ž .
ously presented with five differently coloured white, yellow, blue, green or red
versions of the preferred stimulus. The effects of combining colours within a stimulus Ž
. thus arguably increasing complexity and of varying the size of the preferred device
were studied in Experiments 3 and 4, respectively. Experiment 5 examined chicks’ responses to bunches of string that either incorporated small silver beads or not.
Ž .
We focused on only one sex in the present study because the duration persistence of attention that chickens pay to specific stimuli is known to be sensitive to sex and to
Ž .
circulating levels of testosterone Andrew, 1972 . We chose females for the present study because the number of laying hens kept in industry far exceeds that of cockerels.
2. General methods
2.1. Animals, housing and husbandry Five batches of female Lohmann Brown chicks were obtained from a commercial
Ž .
supplier at 1 day of age one batch for each experiment . The numbers of chicks in each
batch varied according to experimental demands. Immediately upon receipt the chicks were allocated at random to pairs and then housed in wooden boxes measuring either
Ž .
36 = 38 = 30 cm or 72 = 38 = 30 cm length = breadth = height . The size of the boxes again reflected experimental demands. To obtain the smaller boxes we simply divided
the larger ones into two halves with a wooden partition. One bird from each pair was always marked on the back of the head with indelible ink to facilitate later identification.
Ž .
Each box rested on 85-cm-high shelves and their wire-mesh floors 1-cm grid were raised 2 cm off the shelving in order to allow the passage of excreta. Wire-mesh lids
Ž .
prevented the chicks from escaping. Food layer starter mash and water were provided ad libitum in white, semi-circular plastic hoppers suspended from the centre of one wall;
this was the central partition in those boxes that had been divided into two compart- ments and one of the long walls in the non-partitioned ones. These hoppers could be
removed and replaced remotely for maintenance purposes thus minimising visual contact with the attendant. They were replenished twice daily between 0900 to 0930 and 1630 to
1700 hours and they were always returned to the same location. The photoperiod ran from 0500 to 1900 hours and dull emitter heaters suspended above each box maintained
an ambient temperature of approximately 268C. Supplementary warmth was provided by convector heaters if required. The birds then remained undisturbed, apart from mainte-
Ž .
nance, until a pre-test habituation procedure began at 4 days of age see below . No chicks were sacrificed in this study; they were all returned to the Institute’s
rearing stock at the completion of each experiment. 2.2. Test procedure and data collection
In order to minimize the potential disturbance caused by observation at test, we exposed the birds to a sham test procedure when they were 4 days old. This involved
removing the lid from each box, moving the overhead heater to one side, and suspending Ž
. a micro camera see below over the box for 10 min.
Testing began at 0900 hours on the following day. The types of stimuli presented to the chicks and their positioning in the box varied according to the demands of each
experiment. However, they were always suspended for 10 min at approximately equi-distant positions from the top of one or more of the cage walls so that the bottom of
each stimulus was approximately 2 cm off the floor. The devices were never suspended from the wall bearing the food and water hoppers. The chicks’ responses to these
devices were then recorded on videotape during the 10 min observation period using a
Ž .
Panasonic Industrial Colour CCD micro camera WV-KS152 8 = 2 cm, length = breadth suspended from a gantry 50 cm above each box and connected to a video tape recorder.
The camera was always moved into place before the stimuli were presented. This Ž
. Ž
. procedure was repeated on each of 3 Experiment 1 or 5 Experiments 2–5 consecutive
days. Though the number of exposure days varied across experiments, test order was always randomized across boxes and across test days. When subsequently reviewing the
videotapes we only recorded the behaviour of the marked chicks, i.e., one per box, because pair-housed chicks generally stay close together and often engage in the same
activity. We measured the latencies from their introduction to the first peck at each of
the stimuli as well as the numbers of pecks and pecking bouts directed at each one. Bouts were recorded as discrete events if they were separated by a minimum of 5 s
Ž .
Jones and Carmichael, 1999a .
3. Experiment 1: preferences for pecking at different types of stimuli presented consecutively or simultaneously
In this experiment, the chicks were presented with three types of stimuli; these consisted of lengths of white string, white wooden beads strung together or chrome
chain. Apart from its practicability, we chose string for two other reasons. Firstly, at least one turkey farmer routinely provides baler twine as pecking material for his birds
Ž .
and reports substantial interest Parker, 1998 . Secondly, string was pecked much more Ž
. often than feathers, beads or baubles Jones et al., 1997; Jones and Carmichael, 1999a
by ISA Brown chicks. White or yellow string also elicited considerable interest in Ž
. individually caged adult hens of the same strain Jones and Carmichael, 1998 . By using
Lohmann Brown chicks in the present experiment we were able to determine if this preference for string generalized to include another strain of laying chickens. Lengths of
beads were selected as another stimulus because chicks peck readily at small spherical
Ž .
objects, including beads Dawkins, 1968; Rogers, 1995 . Although the beads used here were 0.8 cm in diameter and thereby difficult for the chicks to grip in their beaks, it was
thought that they might have borne some ‘‘super-stimulus’’ resemblance to grain or other seeds. Finally, chains were included because of anecdotal reports from farmers that
they are readily pecked by birds kept on the floor in industry.
It was conceivable that the chicks’ preferences might be more or less marked if they received a choice of stimuli to peck at rather than just one. Therefore, the stimuli were
presented singly on consecutive days to the chicks in half of the boxes whereas the other chicks received simultaneous exposure to all three devices.
3.1. Methods Seventy-two chicks were randomly allocated to 36 pairs and each pair was housed in
Ž .
Ž .
one half 38 = 36 = 30 cm of each of 18 wooden boxes see above . Ž
. Three pecking stimuli string, beads, chain
were used. First, the string device consisted of three strands of white polypropylene twine, each was 8 cm long and
approximately 0.3 cm wide. The strands were held firmly together at one end with a Ž
0.5-cm-wide strip of white plastic tape. Second, 10 matte white wooden beads each .
measuring 1 = 0.8 cm, width = depth were strung together with clear nylon line to form a device that was 8 cm long. The third stimulus consisted of a single 8 cm length of
chrome plated chain with 1-cm links; the chicks could easily grip the solid parts in their Ž
. beaks because they were just 1 mm thick. Thus, the lengths
8 cm and widths
Ž .
approximately 1 cm of each device were similar. Each stimulus was suspended from the top of a cage wall in a central position with a length of clear nylon monofilament
fishing line. Each of these devices moved freely if pecked, pushed or pulled, but because
of their rigidity, the beads and chain moved further and quicker when pecked than did the string.
Each pair of chicks was randomly assigned to one of two treatment groups. Thus, 18 Ž
. pairs received simultaneous exposure to each of the three stimuli string, beads, chain
for 10 min on each of 3 consecutive days. The positions of the stimuli were randomised across the 18 compartments and across exposures with the proviso that none was ever
presented on the same wall as the food and water hoppers. In the consecutive exposure treatment the stimuli were presented singly for similar periods on each of the 3 days to
the remaining 18 pairs. Here, the order of presentation was randomised within a balanced design, e.g., six pairs received string, beads and chain on days 1, 2 and 3,
respectively, while six received chain, beads and string on the corresponding days, etc. The wall from which the single stimulus was suspended was also determined at random
each day. The chicks’ responses were recorded on videotape for later analysis of the behaviour patterns described above. The scores recorded for each bird across all 3 days
were summed and then averaged to give a single value for each behaviour pattern for each chick. The data sets were non-normally distributed and were therefore transformed
Ž .
to square roots. The main effects of treatment stimulus and method of presentation Ž
. were examined using analysis of deviance Genstat 5.3 Committee, 1993 .
3.2. Results and discussion The Lohmann Brown chicks used in the present experiment pecked significantly
sooner and more often at the bunch of string than at either beads or chains regardless of whether these stimuli were presented singly or simultaneously on three consecutive days
Ž .
Table 1 . These findings are consistent with previous reports that string elicited more interest from ISA Brown chicks than other stimuli that we had thought likely to be
Ž attractive, e.g., baubles, beads and feathers Jones et al., 1997; Jones and Carmichael,
. 1999a . Thus, the strong attraction towards string is common to chicks of two different
laying strains. It is not considered likely to reflect differences in the chicks’ ability to Ž
. Ž
. manipulate the stimuli because they could grip both chain 1 mm and string 3 mm in
their beaks. The method of presentation was also influential in that the stimuli were pecked
significantly more when they were presented simultaneously rather than singly and this was particularly true for string. These observations may have important practical
implications. Firstly, they suggest that the presentation of multiple rather than single enrichment stimuli may be more effective in eliciting interest. Secondly, simultaneous
screening of selected pecking stimuli would greatly accelerate the identification of particularly attractive ones. Thirdly, simultaneous presentation of multiple devices
Ž .
allows assessment of potential conservatism in stimulus choice see below . The significant interactions found between the effects of stimulus type and the
method of presentation probably reflect the fact that some chicks switched preferences from chain to beads and vice versa across days. Interestingly, none switched their
preference from string. The fact that there was only one value per day for each measure precluded
meaningful statistical analysis of the evolution of pecking upon repeated presentation of
Table 1 Ž
. Responses meansstandard errors of pair-housed chicks to string, beads and chain devices when these were
presented singly or simultaneously for 10 min on 3 consecutive days and the results and probability values of an analysis of deviance of the effects of stimulus, method of presentation and their interaction
Method of Measure
Stimulus presentation
String Beads
Chain Ž .
Single Lat. peck s
437.457.2 567.927.9
584.915.1 Ž
. Pecks no
1.781.12 0.670.61
0.060.06 Ž
. Pecking bouts no
1.220.60 0.670.61
0.060.06 Ž .
Simultaneous Lat. peck s
262.232.5 462.534.2
543.222.0 Ž
. Pecks no
8.831.57 0.800.30
0.200.07 Ž
. Pecking bouts no
5.110.80 0.700.23
0.190.07 Measure
Effects of Ž . Ž
. Ž . Ž
. Ž
. Stimulus S
df s 2 Method M
df s1 S=M df s 2
2 2
2
Lat. peck x s 21.08
x s 0.10 x s 0.30
P - 0.0001 N.S.
N.S.
2 2
2
Pecks x s61.07
x s 42.61 x s14.14
P - 0.001 P - 0.001
P - 0.001
2 2
2
Pecking bouts x s 44.55
x s10.40 x s 21.08
P - 0.001 P - 0.001
P - 0.01 Ž .
Ž .
Lat.s latency to; s sseconds, no s number; df sdegrees of freedom; N.S.s not significant
Ž .
the stimuli. However, descriptive graphs Fig. 1a and b reveal that the latency to peck at string decreased sharply and the number of pecks increased progressively over the three
presentations. The practical implications of such apparent sensitization of response, rather than habituation, are covered in the General Discussion.
Ž . Ž .
Fig. 1. a The latencies to peck at and b the numbers of pecks delivered at each of the string, bead and chain devices when these were presented simultaneously for 10 min on each of the test days in Experiment 1.
4. Experiment 2: preferences for different colours of string
