Access to barrier perches improves behavior repertoire in broilers.

Abstract

Restriction of behavioral opportunities and uneven use of space are considerable welfare concerns in modern broiler production, particularly when birds are kept at high densities. We hypothesized that increased environmental complexity by provision of barrier perches would help address these issues by encouraging perching and enhancing use of the pen space across a range of stocking densities. 2,088 day-old broiler chicks were randomly assigned to one of the following barrier and density treatment combinations over four replications: simple barrier, complex barrier, or control (no barrier) and low (8 birds/m(2)), moderate (13 birds/m(2)), or high (18 birds/m(2)) density. Data were collected on focal birds via instantaneous scan sampling from 2 to 6 weeks of age. Mean estimates per pen for percent of observations seen performing each behavior, as well as percent of observations in the pen periphery vs. center, were quantified and submitted to an analysis of variance with week as the repeated measure. Barrier perches, density and age affected the behavioral time budget of broilers. Both simple and complex barrier perches effectively stimulated high perching rates. Aggression and disturbances were lower in both barrier treatments compared to controls (P<0.05). Increasing density to 18 birds/m(2) compared to the lower densities suppressed activity levels, with lower foraging (P<0.005), decreased perching (P<0.0001) and increased sitting (P = 0.001) earlier in the rearing period. Disturbances also increased at higher densities (P<0.05). Use of the central pen area was higher in simple barrier pens compared to controls (P<0.001), while increasing density above 8 birds/m(2) suppressed use of the central space (P<0.05). This work confirms some negative effects of increasing density and suggests that barrier perches have the potential to improve broiler welfare by encouraging activity (notably by providing accessible opportunities to perch), decreasing aggression and disturbances, and promoting more even distribution of birds throughout the pen space.

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Access to Barrier Perches Improves Behavior Repertoire
in Broilers
Beth A. Ventura1¤, Frank Siewerdt1, Inma Estevez1,2,3*
1Department of Animal and Avian Sciences, University of Maryland, College Park, Maryland, United States of America, 2IKERBASQUE, Basque Foundation for Science,
Alameda Urquijo, Spain, 3Neiker Tecnalia, Arkaute Agrifood Campus, Vitoria-Gasteiz, Spain
Abstract
Restriction of behavioral opportunities and uneven use of space are considerable welfare concerns in modern broiler
production, particularly when birds are kept at high densities. We hypothesized that increased environmental complexity by
provision of barrier perches would help address these issues by encouraging perching and enhancing use of the pen space
across a range of stocking densities. 2,088 day-old broiler chicks were randomly assigned to one of the following barrier and
density treatment combinations over four replications: simple barrier, complex barrier, or control (no barrier) and low (8
birds/m2), moderate (13 birds/m2), or high (18 birds/m2) density. Data were collected on focal birds via instantaneous scan
sampling from 2 to 6 weeks of age. Mean estimates per pen for percent of observations seen performing each behavior, as
well as percent of observations in the pen periphery vs. center, were quantified and submitted to an analysis of variance
with week as the repeated measure. Barrier perches, density and age affected the behavioral time budget of broilers. Both
simple and complex barrier perches effectively stimulated high perching rates. Aggression and disturbances were lower in
both barrier treatments compared to controls (P,0.05). Increasing density to 18 birds/m2compared to the lower densities
suppressed activity levels, with lower foraging (P,0.005), decreased perching (P,0.0001) and increased sitting (P=0.001)
earlier in the rearing period. Disturbances also increased at higher densities (P,0.05). Use of the central pen area was higher
in simple barrier pens compared to controls (P,0.001), while increasing density above 8 birds/m2suppressed use of the
central space (P,0.05). This work confirms some negative effects of increasing density and suggests that barrier perches
have the potential to improve broiler welfare by encouraging activity (notably by providing accessible opportunities to
perch), decreasing aggression and disturbances, and promoting more even distribution of birds throughout the pen space.
Citation: Ventura BA, Siewerdt F, Estevez I (2012) Access to Barrier Perches Improves Behavior Repertoire in Broilers. PLoS ONE 7(1): e29826. doi:10.1371/
journal.pone.0029826
Editor: Claudia Mettke-Hofmann, Liverpool John Moores University, United Kingdom
Received August 26, 2011; Accepted December 5, 2011; Published January 27, 2012
Copyright: ? 2012 Ventura et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was funded by the University of Maryland. The funders had no role in study design, data collection and analysis, decision to publish, or
preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: iestevez@neiker.net
¤ Current address: University of British Columbia Animal Welfare Program, Vancouver, British Columbia, Canada
Introduction
Broiler production has risen exponentially since the 1940s as
commercial operations have become vertically integrated and
confinement has grown increasingly intensive [1]. While such
expansion has undoubtedly resulted in significant economic
progress, it has also prompted concerns regarding repercussions
on poultry welfare. Behavioral restriction and uneven use of space,
especially in high density rearing scenarios, are of particular issue.
Broilers in large-scale production tend to be reared in
environments of low complexity, where required travel and
foraging effort is low and bird movement and activity is limited
[2,3,4]. It is in such contexts of low activity that restriction of
natural behavior is most likely to occur [5]. Compounding the
issue is broiler inactivity, potentially influenced by two factors:
lameness caused by increased growth rate and body weight [6] and
restricted opportunity for movement due to high stocking density
levels [2,7].
The effects of stocking broilers at high densities are reflected in a
reduction of behavioral expression. Locomotion and distance
traveled have consistently been reported to decline as density
increases [2,8,9], perhaps because more birds per unit area create a
barriereffectbyhamperingbirddispersionthroughoutthepen[2,7].
Birdsalsospend lesstime resting athigher densities duetoa resultant
increase in disturbances [8,10]. Further, increases in density appear
to have a suppressive effect on behaviors like scratching and walking,
though this may be due to indirect effects of increasing density – like
a decline in litter quality – rather than to a lack of space [11].
Adequate physical space is required for an animal to express the
full complement of behaviors intrinsic to its biology. However,
individual broilers in flocks do not utilize the entire space available
to them in experimental [9] or commercial facilities [12]. Rather,
they tend to cluster around the periphery, other objects or among
themselves in a reflection of anti-predation mechanisms [7,13,14].
Such uneven use of the pen can cause a host of welfare issues.
Aggregations around the periphery create an underused central
area, providing prime space for agonistic interactions [10,15].
Individuals entering or leaving such clusters disrupt the resting
intervals of birds already in the group [10]. Finally, as
temperatures increase reduced airflow may subject individuals
within these aggregations to heat stress [16,17].
Given these issues, we assert the importance of seeking avenues
to promote maximal use of space along with an increased range of
behavioral expression. Increasing the complexity of the environ-
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ment by adding enrichment can have a substantial impact on
broiler welfare [12,18,19], in part by providing substrate upon
which previously restricted behaviors can be expressed [20].
Perching serves as a good example in intensive broiler production,
as past attempts to stimulate perching in broilers have not yielded
promising results [21–23]. However, since higher perching rates
have been observed with application of wooden barriers [19], it is
likely that perching is still a motivated behavior in broilers.
Providing barrier perches – which are grounded lower than other
types of perches and hence more adapted to the heavier nature of
modern broilers – may provide enhanced opportunities to express
a wider variety of natural behaviors. Barrier perches may also be
expected to function similarly to cover panels [13] by attracting
birds away from the pen periphery in a way that achieves
improved bird dispersion throughout the pen space.
It is currently unknown how barrier perches may manifest these
effects when birds are stocked at industry-level densities. Our
objective was to explore the effects of providing barrier perches at
varying densities on the behavior repertoire and use of space in
broiler chickens.
Materials and Methods
Animals and experimental setup
A total of 2474 day-old, straight run broiler chicks (Ross 308)
were obtained from a commercial hatchery and placed into
experimental pens on day 1. Each experimental pen had an area
of 4.46 m2and was bedded with approximately 5 cm of wood
shavings. Feed and water were provided ad libitum. A large tubular
feeder and one line of nipple drinkers were positioned along one
side of each pen before chick placement. Feed was also provided in
two shallow feed trays during the first two weeks. The feeding
program consisted of a standard three-phase commercial diet.
Feeders and drinkers were partially blocked in order to maintain
equal resource access per bird regardless of density treatment. The
lighting program used was 24L:0D from day 0–2 and 14L:10D for
the remainder of the experiment. Temperature and ventilation
practices were standard. Further management details are provided
elsewhere [24].
On day 1, ten birds per pen (360 birds total) were randomly
designated as focal birds and tagged on both sides of the neck for
identification using the Swiftack Poultry Identification System
(Heartland Animal Health, Inc., Fair Play, MO). This tagging
protocol allows for swift and permanent identification of
individuals without negatively affecting behavior or welfare
[13,21,23].
The birds were randomly divided into 36 groups and assigned to
one of three barrier treatments at one of three stocking densities
(363 factorial). Barrier-density combinations were replicated four
times. Barrier treatments were: simple barrier, complex barrier, and
no barriers (control). Simple barrier treatment pens contained three
wooden barriersmeasuring 100 cm615 cm64 cm
height6width). Complex barrier treatment pens also contained
three barriers of the same dimensions as those used in the simple
barrier treatment, but two of the barriers had three additional
‘arms’ attached to one side, creating an ‘E’ shape when viewed from
above. Each arm measured 20 cm615 cm64 cm (length6height6
width). The bases of all barriers were bracketed for stability. Once
placed in the pens containing 5 cm of wood shavings, the effective
height of all barriers was 10 cm. All barriers were placed in a
staggered setup in two rows between the food and water sources.
See [24] for a schematic layout of the experimental pens.
Original experimental stocking densities were: low, moderate,
and high, corresponding to 10 birds/m2(45 birds/pen), 15 birds/
(length6
m2(67 birds/pen) and 20 birds/m2(90 birds/pen), respectively.
An unexpectedly high 7% mortality rate during the first week was
attributed to poor chick quality, as the effect of treatment on first
week mortality was nonsignificant. New lower stocking densities
were calculated to 8, 13, and 18 birds/m2(36, 58, and 80 birds/
pen, respectively) and birds were redistributed on day 7 for the
following reason: some pens had exceedingly high mortality rates
to the extent that existing bird counts within those pens did not
satisfy even the new lower density requirements. In these cases,
care was taken to ensure that any birds added to pens to meet the
required number had come from pens with the same treatment
conditions. This research protocol (R-08-01) was approved by the
Institutional Animal Care and Use Committee at the University of
Maryland.
Observations and data collection
Behavioral and spatial data were collected on focal birds via
instantaneous scan sampling [25]. Recorded behaviors included:
feeding, drinking, foraging, aggression, disturbances, standing,
sitting, walking, running, perching, preening, dust bathing,
flapping, and other (adapted from [10,18], see Table 1). Focal
birds’ locations in the pen were recorded simultaneously. Each of
the 36 pens was visited in a random order; this cycle was repeated
twice per day, four times per week from weeks 2 to 6, producing a
total of 1440 pen scans for the study. Week 1 observations were
omitted due to the early high mortality rate. Data collection was
performed between 8:00 h and 13:00 h to reduce variability
attributable to diurnal behavior patterns [26]. Observations were
conducted by the same researcher and lasted approximately 2–
5 minutes per pen, depending on how quickly focal birds were
located. Intervals between scans lasted only as long as it took the
researcher to move to the next pen for observation. Data were
recorded on a tablet PC (Toshiba, Inc.) using the Chickitizer
program, (v.4. University of Maryland, College Park, Maryland,
USA). This software allows each bird’s location to be digitized into
an XY coordinate system and recorded in conjunction with the
identity and behavior of each particular focal bird at that point in
time. Data collection was facilitated with the use of a numbered
7610 grid coordinate system that was marked on all pen walls.
Each cell in the grid measured approximately 26.14624.40 cm.
Use of space was quantified by calculating the proportion of
observations in the central vs. peripheral areas of the pen. The
periphery was defined as the area within 25 cm of the pen walls
(coinciding with approximately 1 cell width in the grid) and the
remaining area was considered the central area [13]. Relative
areas of the center and periphery were standardized by dividing
the percent observations in either zone by the area of that zone
before proceeding with analysis.
Statistical analysis
Data that met analysis of variance (ANOVA) assumptions were
analyzed using a model with barrier and density as fixed factors in
a 3 by 3 factorial arrangement and week of age as the repeated
measure unless otherwise stated. Analyses were performed using
Statistical Analysis System v 9.1.3 [27] and significance was set at
a=0.05. P values above this threshold indicate non-significance
and are not reported here.
Behavioral scans.
The proportion of observations that birds
were seen performing each behavior each day was determined for
each focal bird and averaged across focal birds and days to obtain
weekly pen means for analysis. Residuals for the behaviors feed,
drink, sit, stand, walk, and preen met normality assumptions so
these variables were analyzed with an ANOVA with week as the
repeated measure and pen designated as the experimental unit.
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The perch behavior was analyzed with the same model after
control pens were removed from the data set since it was not
possible to observe perching in the control treatment. Data on the
variables forage, aggress, disturb, and run were averaged across
weeks to obtain overall pen means for the entire study period,
which were then analyzed under a similar model except that
repeated measures were not included. The flapping, dust bathing,
and other behaviors occurred so infrequently that they were not
included in the analysis. Tukey’s HSD was used for all means
comparisons [28].
Space use.
The percentage of observed locations in the
center vs. periphery was calculated for each focal bird and then
averaged across focals within each pen to obtain weekly percent
means per pen for analysis. As percent of observations in the
peripheryvs. center is complementary,
observations in the central area is reported here. Mean percent
of observations in the central area was analyzed with an ANOVA
with repeated measures over weeks.
only percentof
Results
Interactions between factors were not significant unless
otherwise stated.
Appetitive behaviors
Stocking density did not affect the proportion of time birds were
observed feeding or drinking. However, feeding and drinking were
affected by both age (feeding: F4,108=21.55, P,0.0001 and
drinking: F4,108=8.23, P,0.0001, Table 2) and barrier (feeding:
F2,27=9.83, P,0.001 and drinking: F2,27=8.89, P=0.001,
Table 3). Feeding was observed less often in the complex barrier
treatment versus the control pens (P,0.001) and was highest
during the second week, generally declining thereafter (P,0.0001).
Drinking was less frequently observed in birds in the simple barrier
pens compared to controls (P,0.001) and was lowest during the
second week of age (P,0.05). In contrast, the proportion of
observations of foraging was not affected by barrier treatment but
was affected by density (F2,26=8.90, P=0.001; Table 4) such that
the proportion of time that broilers spent foraging was significantly
less for 18 birds/m2compared to 8 or 13 birds/m2(P,0.0001 for
both comparisons).
Activity
Barrier treatment did not affect the proportion of time that birds
were observed sitting, though there was a significant interaction
between density and age (F8,108=3.57, P=0.001; Figure 1),
mostly due to a high proportion of sitting during week 5 in the 18
birds/m2treatment compared to the 8 birds/m2treatment
(P=0.015). In contrast, the proportion of observations of standing
was not affected by density treatment but was by barrier such that
birds in control pens spent more time standing compared to
individuals in simple and complex barrier pens (F2,27=15.68,
P,0.0001; Table 3). Age effect was also present: birds stood least
frequently during week 2 and most frequently during week 4
(F4,108=26.19, P,0.0001; Table 2).
Proportion of walking was highest at the lowest density
(F2,27=6.45, P=0.005; Table
(F4,108=8.23, P,0.0001; Table 2) but was not affected by barrier
treatment. Contrary to the effect on walking, the proportion of
time that birds were observed running was not affected by density,
though barrier treatment had an effect (F2,26=4.17, P=0.027;
4) and atyoungerages
Table 1. Experimental behavioral ethogram.
Appetitive Behaviors
FeedingBird is located next to feeder and has its beak inside the feeder.
DrinkingBird’s head is raised toward nipple drinkers and is either attempting to or is currently contacting its beak with the drinker.
Foraging Bird is pecking or scratching at the ground.
Activity Behaviors
Sitting Bird has ceased locomotion and its breast is in contact with the ground. Eyes may or may not be closed.
StandingBird maintains upright position on motionless, extended legs.
WalkingRelatively low-speed displacement of bird on the ground in which the propulsive force is derived from the action of the legs.
RunningHigher speed displacement of bird on the ground in which the propulsive force is derived from the action of the legs.
PerchingBird’s feet are grasping the barrier and bird is not locomoting. Breast of bird may or may not be in contact with barrier.
Dust bathing Bird is lying on the ground and tossing dirt onto its back/wings by ruffling and shaking its feathers.
FlappingBird is in an upright position and extends its wings repeatedly.
PreeningBird is using its beak to peck, stroke or comb plumage.
Aggression1
Chase One bird runs at least three steps after another bird.
Fight Two birds are standing facing each other with heads and necks raised to the same level. One bird delivers more than two
vigorous kicks at opponent. Pecks may or may not be observed.
LeapTwo birds face each other; one or both jump without extending legs toward other bird.
PeckFace-to-face encounter in which one bird raises its head and directs vigorous pecks toward another bird.
Standoff Two birds facing each other with heads at same level for more than two seconds.
ThreatBird stands with raised feathers and erect neck while opponent holds its head at lowered level.
DisturbanceAnother bird makes physical contact with resting focal bird, causing it to readjust itself or stand.
Other Any behavior not belonging to the previous categories was recorded under this label.
1Observations in the categories ‘‘chase,’’ ‘‘fight,’’ ‘‘leap,’’ ‘‘peck,’’ ‘‘standoff,’’ and ‘‘threat’’ were recorded and analyzed under the ‘‘aggression’’ label.
doi:10.1371/journal.pone.0029826.t001
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Table 3) such that birds reared with simple barriers were observed
running less often than birds in the control treatment (P=0.022).
On the other hand, perching frequency was not affected by
barrier type, though a clear density by age interaction effect was
detected (F8,72=5.09, P,0.0001; Figure 2). The general trend for
all density treatments was for perching to rise during the first few
weeks of observation, peak at week 4, and decline thereafter.
However, perching in the 18 birds/m2treatment declined earlier
than perching in the 8 birds/m2pens (between 4 and 5 weeks of
age, P=0.041).
The proportion of observed preening was not influenced by
either barrier or density treatment but was affected by age
(F4,108=13.44, P,0.0001, Table 2), with a peak in preening
observed at 3 weeks.
Aggressionand Disturbances.
aggressive interactions was unaffected by density. However, birds
in the simple and particularly complex barrier treatments
experiencedloweraggression
(F2,26=12.30, P=0.0002; Table 3). Rate of disturbances was
affected by an interaction between barrier and density treatment
(F4,26=3.28, P=0.027; Figure 3). Disturbance frequency was
highest at all densities for the control treatment compared to both
barrier treatments and generally increased as density rose.
The proportion of
ascomparedto controls
Use of space.
distribution (F2,27=8.46, P=0.001; Table 3) such that use of the
central area was more frequent when simple barriers were added
to pens (P,0.0001 compared to control). Birds’ occupation of the
central area was also affected by density (F2,27=5.70, P=0.009;
Table 4) and was greatest in the 8 birds/m2treatment compared
to the 13 birds/m2(P=0.016) and 18 birds/m2treatment
(P=0.021). Use of space was not affected by age.
The presence of barriers affected bird
Discussion
Increasing environmental complexity in the form of barrier
perches had clear benefits for broilers, not only by providing
behavioral opportunities in the form of perching, but also by
controlling aggressive interactions and reducing disturbances,
especially at higher rearing densities.
In contrast with previous studies for modern broiler strains [21–
23], we found that birds in either simple or complex barrier
treatments allocated a substantial portion of time toward perching.
Perching was observed nearly one-quarter of the time in the lowest
density treatment during the fourth week of age, demonstrating
that broilers have retained the ability and motivation to perch so
long as they are provided with suitable environmental enrichment
[20]. It is likely that the low height of the barrier perches used in
this study translated into greater accessibility and thus more
frequent use. Nevertheless, the decline in perching frequency with
age and density was similar to the abovementioned studies.
In addition to perching, general activity – shown in high levels
of walking and a reduction in sitting – peaked in week four. These
results are consistent with earlier research [19,21,23]. The lower
proportion of standing found in this study compared to prior work
[19] can probably be explained by the higher rates of perching
through the growth period. The consistency of results across
studies related to the decline in activity with age suggests that rapid
growth rates may influence broiler activity [5,29,30].
Although behaviors such as foraging, sitting and preening were
unaffected by barrier treatment, barriers did influence appetitive
behaviors. Lower feeding frequencies were observed in the
complex barrier treatment compared to controls, in contrast to
the lack of effects reported on wooden barriers in the past [19].
The additional arms of the complex barriers may explain this
discrepancy, as resource access may be limited when barrier
structure becomes too intricate. Differences in resource accessi-
bility may also explain why drinking was more frequent in the
control vs. simple barrier treatment, a finding that confirms the
results of earlier barrier work [19]. However, given the lack of
Table 2. Behaviors1(mean %) affected by age.
Age (weeks)Age effect2
Behavior23456F valueP value
Feeding14.360.6a
9.960.6b
7.860.6bc
8.260.6bc
7.460.6c
21.55
,0.0001
Drinking 4.560.6b
6.760.5a
8.460.6a
7.160.5a
8.460.5a
8.23
,0.0001
Standing5.360.7d
8.760.7c
14.360.7a
10.360.7bc
12.060.7ab
26.19
,0.0001
Walking6.260.5a
5.960.5ab
7.260.5a
4.260.5b
4.260.5b
8.23
,0.0001
Preening 2.960.4b
6.360.4a
4.060.4b
3.460.4b
2.760.4b
13.44
,0.0001
1Values are LSM 6 SEM. ‘‘Foraging,’’ ‘‘running,’’ ‘‘aggression’’ and ‘‘disturbance’’ data were pooled over weeks. ‘‘Sitting’’ and ‘‘perching’’ were affected by age6density
(Figs. 1 & 2). ‘‘Central area use’’ was not affected by age (F4,108=1.00).
2Mixed model repeated measures ANOVA, df=4,108.
aMeans within a row with different superscripts are significantly different (P,0.05) after Tukey’s comparison.
doi:10.1371/journal.pone.0029826.t002
Table 3. Behaviors1(mean %) affected by barrier treatment.
Barrier treatment
Barrier treatment
effect2
BehaviorsControlSimpleComplexF valueP value
Feeding11.060.4a
9.560.5ab
8.060.4b
9.83
,0.0001
Drinking8.360.4a
5.860.4b
7.060.4ab
8.89 0.001
Standing12.560.5a
9.060.5b
8.960.5b
15.68
,0.0001
Running1.060.1a
0.660.1b
0.760.2ab
4.170.027
Aggression 0.960.1a
0.360.1b
0.0260.1b
12.30 0.0002
Central area use34.060.8b
38.860.8a
36.460.8ab8.460.001
1Values are LSM 6 SEM. ‘‘Foraging’’ (F2,26=0.38), ‘‘sitting’’ (F2,27=0.46),
‘‘walking’’ (F2,27=1.68), and ‘‘preening’’ (F2,27=2.54) were not affected by
barrier treatment. ‘‘Perching’’ (F1,18=0.41) was not affected by barrier type.
‘‘Disturbance’’ was affected by barrier6density (Fig. 3).
2Mixed model repeated measures ANOVA, df=2,27, with the exception of
running with df=2,26 as running means were pooled across weeks.
aMeans within a row with different superscripts are significantly different
(P,0.05) after Tukey’s comparison.
doi:10.1371/journal.pone.0029826.t003
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differences in feed conversion or final body weights reported in our
companion study [24], it seems that birds may have adjusted their
feeding strategies to meet the demands of navigating through their
environment; thus, these results should not prompt much concern.
In addition to encouraging desirable behaviors, our findings
suggest that barrier perches may have been an effective tool to
minimize behaviors that have a strong negative impact on the
welfare and performance of broiler chickens. Both aggressive
interactions and disturbances were at least halved when barriers
were added, and in one instance (complex barrier treatment),
aggression was almost eradicated (See Table 3). It was recently
demonstrated that providing visual barriers (tin sheets and straw
bales) reduces aggression in breeding rink-necked pheasants,
which could be explained by a number of factors including
reduction in visual horizon, increased escape opportunities, or
more even bird distribution in the pen [31]. In our study, the
decrease in disturbances and aggression in the enriched environ-
ments may be explained by how the barriers affected birds’ spatial
distributions: their attraction as perching furniture along with the
protection offered by the barrier edges may have dispersed resting
areas throughout the pen [13]. This is supported by the fact that
use of the central pen area increased when barriers were added to
the pens (though not significantly so for the complex treatment) in
comparison with control pens. Further, since aggression between
broilers occurs mainly within the open areas of an enclosed
environment [10,15], barrier perches may have reduced the
occurrence of such displays by breaking up this open space. In
support of this theory, running (a behavior often performed in
conjunction with aggressive displays) occurred most often in
control pens, where birds did not have to navigate among barriers.
In this study we found a clear increase in disturbance frequency
with higher densities, especially in the increase from 13 to 18
birds/m2. Similar effects have been previously observed [8,10,32]
and can be explained by more chickens searching for resting
locations along the wall [7,10]. The welfare consequences of high
disturbance rates are important, as disturbances interrupt resting
time [10] and compromise body integrity due to a higher
incidence of scratches inflicted by birds traveling through the
resting group [33,34]. We provided clear evidence that distur-
bances can be effectively managed by adding barrier perches, as
rates with either barrier were lower than controls across all
experimental densities. By reducing potential competition for
Table 4. Behaviors1(mean %) affected by density treatment.
Density treatmentDensity treatment effect
Behaviors8 birds/m2
13 birds/m2
18 birds/m2
F valueP value
Foraging 2.760.3a
2.660.3a
1.360.3b
8.900.001
Walking 6.560.4a
5.160.4b
4.960.4b
6.450.005
Central area use38.660.8a
35.260.8b
35.360.8b
5.70 0.009
1Values are LSM 6 SEM. ‘‘Feeding’’ (F2,27=1.21), ‘‘drinking’’ (F2,27=2.93), ‘‘standing’’ (F2,27=1.29), ‘‘running’’ (F2,26=0.37), ‘‘preening’’ (F2,27=2.63) and ‘‘aggression’’
(F2,26=1.71) were not affected by density. ‘‘Sitting’’ and ‘‘perching’’ were affected by age6density (Figs. 1 & 2) and ‘‘disturbance’’ by barrier6density (Fig. 3).
2Mixed model repeated measures ANOVA, df=2,27, with the exception of foraging with df=2,26 as foraging means were pooled across weeks.
aMeans within a row with different superscripts are significantly different (P,0.05) after Tukey’s comparison.
doi:10.1371/journal.pone.0029826.t004
Figure 1. Density by age interaction effect of mean percent sitting (LSM ± SEM).
doi:10.1371/journal.pone.0029826.g001
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prime resting locations around peripheral walls, barrier perches
may have mitigated the negative repercussions of high density on
disturbance rate.
In addition to the adverse effects on disturbances, high densities
discouraged active behaviors and decreased use of the pen central
space. The reduction in foraging and walking and the increase in
sitting as density increased from 13 to 18 birds/m2, especially as
birds aged, may relate to a reduction in floor space and subsequent
opportunity to walk and forage [8,34]. More specifically, the
observed decline could be due to how density constrained the
birds’ movement in the available space [9] or simply by the fact
that the allotted space became inadequate for birds to perform
active locomotive behaviors as they grew [34]. We have shown
elsewhere that footpad lesions are more serious at higher densities
[24], so it is also possible that lesions (or other lameness issues)
contributed to more frequent sitting earlier in life. In respect to the
observed effects on foraging, birds may have been discouraged as
the litter quality deteriorated and became more compacted in the
high-density environments [35].
The decline in perch use at higher densities in this study
contrasts with earlier work with perches and broilers [23]. As birds
did use the barrier perches more frequently overall, it is possible
Figure 2. Density by age interaction effect on mean percent perching (LSM ± SEM).
doi:10.1371/journal.pone.0029826.g002
Figure 3. Barrier by density interaction effect on mean percent disturbances (LSM ± SEM).
doi:10.1371/journal.pone.0029826.g003
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that the total available perching space in our study limited the
opportunity for some birds to perch at higher densities,
particularly as they grew larger in size. Accounting for the types
and number of barriers in the pens, and if each bird occupied
approximately 15 cm of perch space, this would allow about 6.7
birds to simultaneously occupy each simple barrier perch (20 birds
total in a simple barrier treatment pen), and 10.7 birds on a
complex barrier perch (28 birds total in a complex barrier
treatment pen). We did not observe these levels of saturation;
therefore, additional barrier space may not have reduced the
burden of high density more than what we found, though it is
difficult to make this conclusion without further research. It is
possible that high density could have influenced the results in
another way, perhaps simply by hindering access to the barriers.
In summary, in this study we provided further evidence of the
detrimental effects of increasing density, as attested by suppression
of activity levels, increased disturbances and decreased use of
perches and central areas. We also provide evidence that barrier
perches, and more specifically, simple barrier perches, can be an
effective tool to improve broiler welfare by (1) encouraging a
broader behavioral repertoire that translated into increased
activity levels and decreased aggression and disturbances and by
(2) promoting improved use of space by increasing bird dispersion
for resting. Based on the results of this and our companion study
[24], we conclude that creating a more complex environment by
using simple barrier perches is an advantageous, cheap strategy for
producers to improve broiler health and welfare.
Acknowledgments
The authors would like to thank the staff and most especially Mark
Spicknall at the University of Maryland Applied Poultry Research facility
(Upper Marlboro, MD) for their assistance during the research phase of
this study. We are also indebted to Erin Leone, Kathleen Baczynski, Taylor
Callicrate, Avanti Mallapur and W. Ray Stricklin (Department of Animal
and Avian Sciences, University of Maryland, College Park) for their
support during various stages of this project.
Author Contributions
Conceived and designed the experiments: IE BV. Performed the
experiments: BV IE. Analyzed the data: BV FS IE. Contributed
reagents/materials/analysis tools: IE BV FS. Wrote the paper: BV IE FS.
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