Preening cups increase apparent wet preening behaviors, but have no impact on other behaviors, body condition, growth, or body morphometrics of grow-out Pekin ducks

Abstract

Preening cups may be a form of open water that would allow ducks to express preening behaviors. We set out to test the hypothesis that preening cups would not have detrimental effects on ducks or their environment. Control pens (N = 6, 65 ducks/pen) had nipple lines while experimental pens (N = 6, 65 ducks/pen) had the same nipple line plus one preening cup (PC). Body weights of 30 ducks per pen, and body condition scores on 50 ducks per pen were recorded weekly. On d 18 and 43, 5 ducks per pen were euthanized and their spleens, Bursas, liver, and uropygial glands were weighed. Behavior data were collected using scan sampling with video being recorded for 72 continuous hours at 4 different ages: 25 d, 30 d, 36 d, and 40 d. Body morphometrics were analyzed by 2-way ANOVA with repeated measures. Body condition scoring was analyzed by Pearson's chi-square. The GLIMMIX procedure (SAS 9.4) was used for behavioral analyses to examine treatment differences in the proportion of ducks performing dry preening, wet preening, eating, drinking, standing, and laying down. Feather pecking, feather picking, preening conspecifics (also known as allopreening), dunking head, and drinking from preening cup were analyzed using PROC LOGISTIC with the Firth bias correction for quasi-complete separation and odds ratios were calculated. More PC ducks housed with PC performed wet preening compared to control ducks (25 d: F_1,26 = 6.90, P = 0.0143; 30, 36, and 40 d; F_1,78 = 24.53, P < 0.0001). Ducks in the PC group were also more likely to lay down compared to controls (25 d: F_1,33 = 4.95, P = 0.0330). No differences were observed for any other behavior, body condition score, body weight or morphometrics at any age. Although ducks in the preening cup group showed an increase in wet preening, our data suggest that open water is not necessary to maintain feather condition or uropygial gland size.

INTRODUCTION

The presence of open water sources in commercial duck barns has been a global and controversial topic in the poultry industry. Studies have found that open water access allows for natural preening behaviors and improved production (O'Driscoll and Broom, 2012), while other studies found that open water access led to poor environmental conditions that resulted in decreased production and increased mortality in commercial Pekin ducks (Lowman et al., 2016; Schenk et al., 2016). Preening cups have been suggested as an alternative to open water to allow ducks to express preening behaviors as they are semiopen, or even shower/misters, that allows ducks to dunk their heads, but not their bodies, in order to preen more effectively (Campbell et al., 2022). Figure 1 illustrates the preening cups housed with ducks. Preening cups are new design of a semiopen water source that may allow for natural preening behaviors, however the impacts of preening cups on behavior and production have not been evaluated scientifically. In industry, preening cups are typically placed in commercial barns at 1 cup per 1,000 to 2,000 ducks (G. S. Fraley, personal communications with stakeholders). However, it is not known if a smaller number of ducks to preening cups would further improve the use of this enrichment and prevent unwanted behaviors or produce undesirable conditions that could negatively impact welfare and preening.

Figure 1.

Photographs of ducks housed with preening cups at Purdue ASREC. (A) Ducks placed with preening cups around d 18. (B) Ducks with preening cups at approximately d 25.

In poultry science, preening behaviors are divided into 2 designations not made in ornithological literature, dry preening, and wet preening, for domestic ducks (Jones and Dawkins, 2010; Mi et al., 2020). Dry preening occurs when a duck nibbles or strokes its feathers without the use of water and wet preening is when the duck collects water in its bill or places its head in open water and distributes it over the feathers, followed by nibbling or stroking of the feathers (Jones and Dawkins, 2010). In the United States, the majority of commercial farms provide water in nipple water lines, which do not enable ducks to dunk their heads in water (Chen et al., 2021). There has been much debate about whether or not ducks need open water access in order to wet preen. Rice et al. (2014) found no significant differences in the percentage of birds who preened themselves under nipple lines as opposed to away from the nipple lines in a commercial barn. Further, those ducks also had excellent body condition scores, suggesting that they were able to preen effectively while being housed with standard nipple lines (Rice et al., 2014). However, other studies showed that Pekin ducks housed with nipple lines alone showed an increase in feather cleanliness scores and nostril scores (thus worse) compared to Pekin ducks housed with a bath or a trough (O'Driscoll and Broom, 2011). Comparing 3 other types of open water sources (narrow, intermediate, and wide troughs), Liste et al. (2012a) reported that out of the 3 open water sources, ducks housed with wide troughs had the highest foot pad dermatitis scores although they were considered to have good health overall. Litter in the pen with ducks who were housed with a water trough and 2 standard nipple lines had increased litter moisture, compared to ducks housed with 4 standard nipple lines (Lowman et al., 2016). The group with the trough in that study showed higher levels of total bacterial counts, suggesting an increased risk for poor health. A study by Schenk et al. (2016) showed that ducks housed with water troughs used a considerably greater volume of water compared to ducks housed in barns with standard nipple lines, and ducks with water troughs showed worse body condition scores and increased mortality; likely due to increased bacterial contamination associated with the troughs. The combination of environmental contamination, increased lameness and mortality and the vastly increased water waste suggests the use of nipple lines is a preferable alternative for ducks housed in US management systems (Schenk et al., 2016). However, water systems may also impact the expression of unwanted behaviors in ducks, such as feather picking and feather pecking.

Pekin ducks show both feather picking and feather pecking behaviors. Feather pecking is the act of damaging the skin and feathers of a conspecific in the same barn/pen, while feather picking is the act of automutilation (Savory, 1995; Blokhuis and Wiepkema, 1998). Small amounts of feather picking are viewed as beneficial, in the form of preening or removal of juvenile down but can become harmful under certain conditions (Colton and Fraley, 2014). The causes or stimuli for excessive feather picking and pecking are unknown, but one potential stimulus is thought to be the lack of suitable environmental enrichment. Environmental enrichment is defined as the stimulation of the brain by enhancing an individual or animal's social or physical surroundings (Mellen and Sevenich MacPhee, 2001). Preening cups (PC) used in our study are not only thought of as an alternative to open water access, but also as a form of environmental enrichment that will stimulate the ducks by enhancing their normal environment, thus decrease unwanted behaviors. The purpose of our study was to evaluate the effects of preening cups on body condition scores, behavior, and production. We hypothesized that preening cups will be a form of environmental enrichment that increases preening behaviors and decreases unwanted behaviors by providing a resource to express natural behaviors. We also hypothesized that the ducks with the preening cups would have significantly larger uropygial glands compared to ducks housed with a standard nipple line alone. Our results suggest that even at a low number of ducks per preening cup (65 ducks/cup in our study), preening cups do not adversely affect Pekin duck health, welfare, or performance. However, it is still unclear as to whether preening cups provide an outlet for natural behaviors to the extent of preventing or reducing unwanted behaviors.

MATERIALS AND METHODS

Animals

All procedures were approved by Purdue's Institutional Animal Care and Use Committee (PACUC # 2109002194). A total of 260 grow-out Pekin ducks were obtained from Culver Duck, Inc. (Middlebury, IN) on the day-of-hatch and brought to the Purdue University Animal Sciences Research and Education Center (ASREC) farm. They were housed in 11.5 m² floor pens with 0.2 m² per duck, approximately 65 ducks per pen. They were allowed ad lib access to water and feed as recommended (Chen et al., 2021). They were fed industry standard starter feed for the first 10 d of life and grower feed for the remainder of life per industry recommendations (Chen et al., 2021). All feed and water were provided ad libitum. The ducks were placed in the single rooms with an 18:6 light cycle with lights turning on at 3:00 am and turning off at 9:00 pm, temperature of 20°C to 22°C. Brooder temperatures during the first 10 d of life were maintained as recommended (Chen et al., 2021). Water nipple lines (5 ducks per nipple) were placed over a pit covered with raised plastic flooring, and the remaining area of the pens were covered with pine shavings and added to or replaced as necessary at the same time for all pens. The ducks were grown-out until 43 d of age. We utilized 2 rooms with 2 pens per room, with one room as the control with nipple lines only, and one room with preening cups (placed on d 18; “IP bell” from IMPEX Watering Solutions, Gainesville, GA) and a nipple line. Three repetitions were run resulting in N = 6 pens per water environment. Based on previous, anecdotal, observations in commercial barns, ducks vocalize and produce other noises when they utilize the preening cups. Thus, both treatments could not be represented within the same room because those vocalizations and noises could have impacted the control ducks’ behaviors in the same room. Water lines (5 ducks per nipple, ∼50 mL/min flow rate) were placed over a pit covered with raised plastic flooring and the remaining area of the pens was covered with pine shavings which were added to, or replaced as necessary, simultaneously for all pens. The preening cups were placed on d 18 also over the pit as per industry practices (G. S. Fraley, conversations with stakeholders). At the end of the experiment, ducks were either necropsied for organ morphometric analyses, or brought to the Purdue Butcher Block on d 43 for processing.

Morphometrics and Body Condition Scores

Ducks were weighed, and body condition scores were assessed on a weekly basis on 50 ducks per pen using a rubric previously published by Karcher et al. (2013). Foot pad quality, eyes, nostrils, feather cleanliness and feather quality were assessed. Before placement of the preening cups on d 18, 5 ducks per pen were euthanized using FatalPlus (pentobarbital, 390 mg/mL/kg) and their spleens, Bursa of Fabricius, livers, and uropygial glands were weighed (final N = 10/treatment). Preening cups (IMPEX Watering Solutions, Gainesville, GA) were placed on d 18 as per industry standard. The preening cups were placed at the appropriate height to prevent the ducks from climbing in but allowed the ducks to dunk their heads. Each preening cup was connected to the main water supply and was self-filling. Five ducks per pen were euthanized on d 43, 23-days postpreening cup placement using FatalPlus and their spleens, Bursa of Fabricius, livers, and uropygial glands were weighed (final N = 30/treatment). Ammonia levels, temperature, and humidity were also recorded at the level of the ducks’ heads in 3 places per pen weekly with the use of an ammonia meter and a digital thermometer.

Behavior Analyses

WYZE Cam V3 cameras were placed overhead, across each pen, and each camera was able to visualize its entire, respective, pen. Videos were recorded for 72 continuous hours at 25, 30, 36, and 40 d of age. Videos were analyzed by scan sampling and behavior events were recorded using JWatcher software along with Excel. We analyzed the first 15 min/h in each pen for am (3:30, 4:30, 5:30) and pm (14:30, 15:30, and 16:30). These times were chosen based upon preliminary examination of the videos that indicated these to be peak hours of activity and are similar to time frames reported previously for behavioral analyses of ducks (Rice et al., 2014). We analyzed each minute during the 15-min intervals by counting how many ducks were performing each behavior and then divided that number by the number of ducks in the pen, resulting in a proportion for each behavior.

The analyzed behaviors included dry preening, wet preening, preening conspecifics (also known as allopreening), drinking from nipple line, drinking from preening cup, eating, feather picking, feather pecking, standing, laying down, and dunking head in preening cup. The ethogram is provided in Table 1 and was adapted from Liste et al. (2012b) and Mahmoud et al. (2020). Wet preening has many different definitions in poultry literature. Jones and Dawkins (2010) describe wet preening as nibbling or stroking of the feathers with the use of water in the bills; while Waitt et al. (2009) describes wet preening as nibbling at the feathers while applying water either directly with the bill or after tossing water over the body. However, we were unable to observe water in the bills of the ducks on the videos to confirm actual wet preening. Therefore, we created a 1-duck length quadrant around the nipple lines and the preening cups. We defined the ducks’ behavior as wet preening if they were preening inside of those quadrants immediately following an interaction with water, and dry preening if they were preening outside of those quadrants. There were 5 observers for this study and interobserver reliability was verified by Fleiss’ κ = 0.98.

Table 1.

Ethogram used for the behavior data. Adapted from Liste et al. (2012b) and Mahmoud et al. (2020).

Behavior	Description
Feather pecking	Directed at another duck, involves removal or attempted removal of another's feather. Differentiable from preening in that the receiving party will be distressed and make efforts to move away from the attacker.
Drinking from preening cup	Dipping beak in water and immediately pulling head back line to attain water from the preening cup
Eating	Involves the active consumption of food from a designated food source.
Feather picking	Involves rapid pulling and removal of own feathers. Feathers from any area can be targeted, but its most often the back and wing feathers that are pulled.
Dunking head in preening cup	Rapidly ducking head in water with no clear purpose. Dunking may precede wet preening. Differs from preening/drinking in that the duck does not pull their head back.
Laying down	The duck will be in ventral recumbency, either awake or asleep, but not engaged in another activity. Wings will typically be more relaxed than those of standing ducks and may be stretched out on the side of the duck.
Drinking from nipple line	Standing or sitting directly under the nipple line with their neck extended, reaching 1 nipple and actively attaining water from it.
Allopreening	Grooming of another duck which may or may not be reciprocated by the other party. The duck performing the service may groom any area of the receiving duck, however the aforementioned areas are the most typical.
Wet preening	Manipulation of the feathers with the aid of water. The duck will target many areas of its body, with the most common being the chest, back, and wings.
Dry preening	Manipulation of feathers without the use of water. The most common areas selected for preening are the chest, back and wings.
Standing	Characterized by the duck passively resting in one place on both feet without being engaged in another activity. The wings are folded to the body and the head is most often held in a relaxed posture. Occasionally, the duck will tuck its’ head under its wing while standing.

Behavior Analyses

Behavior at 25 d of age was only recorded in replicates 1 and 2; therefore, these data were analyzed separately to examine differences due to treatment, time and their interaction. Data for 30, 36, and 40 d of age were analyzed for replicates 1 and 3 to compare the effects of treatment, age, time and their interactions. The GLIMMIX procedure (SAS 9.4) was used for all analyses to examine treatment differences in the proportion of ducks performing dry preening, wet preening, eating, drinking, standing, and laying down. Pen nested in room was included as a random effect and time was included as a repeated measure. Pairwise comparisons were examined using Tukey's test. Data were examined for normality using studentized residual plots.

Feather pecking, feather picking, allopreening, dunking head in preening cup and drinking from preening cup were analyzed using PROC LOGISTIC (SAS 9.4) with the Firth bias correction for quasi-complete separation and odds ratios were calculated with p ≤ 0.05 considered significant. Dunking head in preening cup and drinking from preening cup were analyzed with PROC LOGISTIC (SAS 9.4) for just the PC group, as the control group did not have a preening cup.

Statistical Analyses

Morphometrics and Body Condition Scores

Data were analyzed by SAS JMP (v.15, SAS Institute, Raleigh, NC). The pen was the statistical unit for all data. Body morphometrics were analyzed by 2-way ANOVA with repeated measures. Body condition scoring was analyzed by Pearson's chi-square as data were not normally distributed. A p ≤ 0.05 considered significant.

RESULTS

Morphometrics and Body Condition Scores

No significant differences were observed on either d 18 or 42 for relative spleen, Bursa of Fabricius, body, uropygial gland, liver weights, or growth rates (P > 0.1) from ducks housed with preening cups compared to those housed with nipple lines alone. Data are illustrated in Figure 2, Figure 3 and Table 2.

Figure 2.

Preening cups do not affect body weights of ducks. No significant differences were observed in body weights of ducks housed with nipple lines or controls at any age.

Figure 3.

Body morphometrics data. Ducks were weighed on a weekly basis. Ducks were weighed and euthanized on d 18, before preening cup placement, and d 43 (A), after preening cup placement and their spleens (B), Bursa of Fabricius (C), liver (D), and uropygial glands (E) were weighed. We observed no significant differences for any morphometrics.

Table 2.

Body and organ morphometric data.

Organ	Control		Preening cup
	D 18	D 41	D 18	D 43
Body weight1	1.14 ± 0.021	3.65 ± 0.052	1.18 ± 0.020	3.66 ± 0.070
Spleen2	0.11 ± 0.006	0.08 ± 0.003	0.10 ± 0.005	0.07 ± 0.003
Liver2	5.86 ± 0.114	3.63 ± 0.074	5.6 ± 0.119	3.53 ± 0.095
Bursa of Fabriscius2	0.16 ± 0.006	0.10 ± 0.004	0.16 ± 0.005	0.10 ± 0.004
Uropygial gland2	0.28 ± 0.007	0.16 ± 0.005	0.29 ± 0.009	0.17 ± 0.006

¹Mean ± standard error of the mean in kilograms.

²Relative organ weight as percent of body weight in grams.

No significant differences were observed between the 2 groups for any body condition score. Though not significant, we did see an increase in feather pecking in the PC group (4 out of 6 pens compared to 2 out of 6 pens in controls). Body condition score results are illustrated in Figure 4 and Table 3. No clear reasons for the outbreak of feather pecking were apparent in our study. Our lab's experience is that feather pecking outbreaks in a controlled academic setting are extremely rare. There were no environmental, health, or physical signs to explain the outbreak. No other body condition score variables showed differences between the treatment groups. There were no significant differences between the groups for ammonia levels, temperature, or humidity between the 2 groups (data not shown).

Figure 4.

Body condition scores. BCS were taken weekly on 30 ducks per pen per week. We observed no significant differences between treatments for any body condition score (A, eye score; B, nostril score; C, feather cleanliness; D, feather quality; and E, foot pad scores). Note the range of the Y-axes; all birds in this study showed exceptional body condition scores.

Table 3.

Average body condition scores.

Week of age	Eye1		Nostril2		Feather cleanliness2		Feather quality1		Foot pad1
	Control	PC	Control	PC	Control	PC	Control	PC	Control	PC
1	0.060 ± 0.008	0.010 ± 0.008	0.058 ± 0.016	0.035 ± 0.12	0.024 ± 0.009	0.036 ± 0.028	0.011 ± 0.005	0.012 ± 0.005	0.187 ± 0.054	0.113 ± 0.030
2	0.005 ± 0.005	0.000 ± 0.000	0.140 ± 0.099	0.076 ± 0.047	0.005 ± 0.005	0.096 ± 0.041	0.013 ± 0.009	0.016 ± 0.007	0.214 ± 0.092	0.267 ± 0.050
3	0.000 ± 0.000	0.000 ± 0.000	0.124 ± 0.067	0.214 ± 0.125	0.097 ± 0.035	0.054 ± 0.024	0.106 ± 0.024	0.046 ± 0.016	0.034 ± 0.017	0.075 ± 0.057
4	0.000 ± 0.000	0.015 ± 0.009	0.100 ± 0.032	0.172 ± 0.061	0.185 ± 0.057	0.161 ± 0.060	0.413 ± 0.170	0.330 ± 0.140	0.068 ± 0.028	0.032 ± 0.025
5	0.044 ± 0.019	0.038 ± 0.011	0.118 ± 0.027	0.095 ± 0.020	0.265 ± 0.074	0.201 ± 0.059	0.668 ± 0.124	0.872 ± 0.170	0.194 ± 0.054	0.148 ± 0.041

¹0 to 2 scoring range, with higher numbers indicating poorer score.

²0 to 1 scoring range, with higher numbers indicating poorer score.

Behavior Analyses D 25

Treatment Effect

The average proportion of ducks wet preening was significantly influenced by treatment (F_1,26 = 6.90, P = 0.0143). More ducks in the PC group (0.74603 ± 0.15083) were wet preening than ducks in the control group (P = 0.0143). The average proportion of ducks drinking from the nipple line was significantly influenced by treatment (F_1,33 = 114.47, P < 0.0001). More ducks in the control group (5.4461 ± 0.5807) drank from the nipple line than the ducks in the PC group (P < 0.0001). The average proportion of ducks laying down was significantly influenced by treatment (F_1,33 = 4.95, P = 0.0330). Fewer ducks in the control group (−0.05083 ± 0.02284) laid down than the PC group (P = 0.0330).

The proportion of ducks allopreening was significantly influenced by treatment (Wald χ² = 6.0751, P = 0.0137). Ducks in the control group were 0.0638 times less likely to not allopreen than ducks in the PC group (95% CL: 0.00716–0.5692, P = 0.0137). There were no significant differences between treatments for dry preening, feather pecking, eating, standing, laying down, dunking head in PC or drinking from PC. Feather picking was not observed for this time point.

Time Effect

There were significant differences between time for dry preening (F_5,33 = 2.61, P = 0.0426), but post hoc analyses did not reveal any significant differences. There was a significant difference between time for drinking from the nipple line (F_5,33 = 3.72, P = 0.0089) where ducks were more likely to be drinking at 3:30 am (4.6862 ± 0.7651) when compared to 5:30 am (P = 0.00168) and 2:30 pm (P = 0.0155). There was a significant difference between time for laying down (F_5,33 = 2.89, P = 0.0284), where more ducks were laying down at 5:30 am (4.4624 ± 0.04001) when compared to 3:30 am (P = 0.0174). There were no significant differences between time for wet preening, allopreening, feather picking, feather pecking, eating, standing, dunking head in PC or drinking from PC.

Interaction Effect

There was a significant interaction between time and treatment for drinking (F_5,33 = 3.01, P = 0.0239). Ducks in the control group were drinking from the nipple line more than the PC group at 2:30 pm (4.6622 ± 1.2202, P = 0.235). Fewer ducks in the control group were drinking from the nipple line at 2:30 pm (−4.3752 ± 1.2202) than at 3:30 pm (P = 0.0419). More ducks in the control group were drinking from the nipple line than the PC group at 3:30 pm (5.3860 ± 1.2202, P = 0.0049). Fewer ducks in the control group were drinking from the nipple line at 4:30 pm than 3:30 am (-5.8240 ± 1.2202, P = 0.0018). D 25 behavioral data are provided in Tables 4 and 5.

Table 4.

Proportion means, and standard errors of behaviors performed by each treatment on d 25.

	Means ± SE d 25⁎
Behavior	Control	PC
Dry preening	6.0093 ± 1.4291	5.1529 ± 1.4291
Wet preening	0.34581 ± 0.07332	0.74603 ± 0.1508
Eating	0.9520 ± 0.1062	0.8283 ± 0.1062
Drinking	5.4461 ± 0.5807	0.116 ± 0.5807
Standing	5.7416 ± 1.0644	5.5971 ± 1.0644
Laying down	4.3929 ± 0.03285	4.4437 ± 0.03285

^⁎Bolded characters denote P < 0.05.

Table 5.

Odds ratio of behaviors between control and PC groups on d 25.

Control vs. PC (d 25)
Behavior	Odd ratio estimate (lower–upper), P value
Feather pecking	1.4375 (0.3037–6.8047), P = 0.6473
Feather picking	N/O
Allopreening	0.0638 (0.00716–0.5692), P = 0.0137

N/O = no specific behavior observed.

Behavior Analyses D 30, 36, and 40

Treatment Effect

The average proportion of ducks wet preening was significantly influenced by treatment (F_1,78 = 24.53, P < 0.0001). More ducks in the PC group (0.70753 ± 0.08981) were wet preening than ducks in the control group (P < 0.0001). The average proportion of ducks drinking from the nipple line was significantly influenced by treatment (F_1,66 = 66.07, P < 0.0001). Ducks in the control group (1.4582 ± 0.1527) drank from the nipple line more than the ducks in the PC group (P < 0.0001). No significant differences between treatments for dry preening, allopreening, feather pecking, feather picking, eating, standing, or lying down.

Time Effect

There was a significant difference due to time for dry preening (F_5,98 = 9.83, P < 0.0001) where more ducks were dry preening at 3:30 am (11.6288 ± 0.8064) compared to all other time points (P < 0.0001). There was a significant difference among time points for drinking from nipple line (F_5,66 = 3.07, P = 0.0150) where at 3:30 am (1.2439 ± 0.2649) more ducks were drinking from the nipple line compared to 5:30 am (P = 0.00195) and 3:30 pm (P = 0.0261). There was also a significant time effect for laying down (F_5,98 = 3.97, P = 0.0025) where fewer ducks were laying down at 3:30 am (58.1853 ± 3.4923) than at 5:30 am (P = 0.0011), 2:30 pm (P = 0.0178) and 3:30 pm (P = 0.0116).

The average proportion of ducks in the PC group dunking their head in the PC was significantly influenced by time (Wald χ² = 12.7083, P = 0.0263). Ducks at 3:30 am were 6.2913 times more likely to not be dunking their heads in the PC than ducks at 2:30 pm (95% CL: 1.4430–27.4285, P = 0.0144). Ducks at 4:30 am were 8.4443 times more likely to not be dunking their heads in the PC than ducks at 2:30 pm (95% CL: 1.8815–37.8999, P = 0.0054). Ducks at 5:30 am were 8.1428 times more likely to not be dunking their heads in the PC than ducks at 2:30 pm (95% CL: 1.8213–36.4051, P = 0.0061). Ducks at 2:30 pm were less likely to not be dunking their heads in the preening cup than ducks at 3:30 pm (95% CL: 0.0465–0.9507, P = 0.0294) and 4:30 pm (95% CL: 0.0183–0.3980, P = 0.0017).

Age Effect

There were significant differences among ages for dry preening (F_2,28 = 12.32, P < 0.0001), where more ducks were dry preening at d 40 (9.3359 ± 0.6762) when compared to d 30 (P < 0.0001) and d 36 (P = 0.0044). There was also a significant difference between ages for wet preening (F_2,78 = 4.78, P = 0.0110) where more ducks at 40 d (0.64065 ± 0.09595) of age were wet preening compared to ducks at 30 d of age (P = 0.008). There was a significant difference among ages for standing (F_2,96 = 7.76, P = 0.0008) where fewer ducks at 30 d of age (5.3235 ± 0.7719) were standing than ducks at 36 (P = 0.0164) and 40 (P = 0.0008) d of age. There was a significant age effect for laying down (F_2,98 = 10.79, P < 0.0001) where ducks at 30 (79.4487 ± 2.2476) d of age laid down more than ducks at 36 (P = 0.0351) and 40 (P < 0.0001) d of age.

The average proportion of ducks feather pecking was significantly influenced by age (Wald χ² = 31.4170, P < 0.0001) where ducks at 30 d of age were 7.7850 times more likely to not feather peck than ducks at 36 and 40 d of age (95% CL: 2.9840–20.3108, P < 0.0001). The average proportion of ducks allopreening was significantly influenced by age (Wald χ² = 10.7612, P = 0.0046). Ducks at 30 d of age were 0.2743 times less likely to not be allopreening than ducks at 36 and 40 d of age (95% CL: 0.0905–0.8314, P = 0.0222). When looking at just the PC group, the average proportion of ducks drinking from the PC was significantly influenced by age (Wald χ² = 7.3079, P = 0.0259). Ducks at 30 d of age were 0.3431 times less likely to not drink from the preening cup than ducks at 36 and 40 d of age (95% CL: 0.1255–0.9378, P = 0.0370). The average proportion of ducks in the PC group dunking their head in the PC was significantly influenced by age (Wald χ² = 16.2618, P = 0.0003). Ducks at 30 d of age were 3.3374 times more likely to not be dunking their heads in the PC than ducks at 36 and 40 d of age (95% CL: 1.1972–9.3039, P = 0.0212).

Interaction Effect

There was a significant age x treatment interaction for dry preening (F_2,98 = 5.94, P = 0.0037) where fewer ducks in the control group were dry preening at d 30 than at d 40 (−2.7531 ± 0.9398, P = 0.0471) and d 36 than at d 40 (−3.6406 ± 0.9398, P = 0.0026). Fewer ducks in the PC group were dry preening on d 30 than on d 36 (−2.9917 ± 0.8409, P = 0.0074) and d 40 (−3.4573 ± 0.8409, P = 0.0011). These data are illustrated in Tables 6 and 7.

Table 6.

Proportion means, and standard errors of behaviors performed by each treatment on d 30, 36, and 40.

	Means ± SE d 30, 36, and 40⁎
Behavior	Control	PC
Dry preening	7.6893 ± 0.6114	7.5438 ± 0.5952
Wet preening	0.34991 ± 0.0477	0.70753 ± 0.0898
Eating	0.62337 ± 0.0765	0.69852 ± 0.0746
Drinking	4.3485 ± 0.6670	0.7052 ± 0.1368
Standing	9.0292 ± 1.1006	8.6566 ± 0.9406
Laying down	72.1341 ± 1.9438	70.6063 ± 1.8063

^⁎Bolded characters denote P < 0.05.

Table 7.

Odds ratio of behaviors between control and PC groups on d 30, 36, and 40.

Control vs. PC (d 30, 36, and 40)
Behavior	Odd ratio estimate (lower − upper), P value
Feather pecking	0.4930 (0.2114–1.1498), P = 0.1017
Feather picking	0.000038 (0−5.01E78) P = 0.9170
Allopreening	1.3683 (0.5478–3.4175), P = 0.5019

DISCUSSION

The purpose of our study was to evaluate whether preening cups placed with 65 ducks per pen when compared to industry of 1,000 to 2,000 ducks per pen would alter preening behaviors compared to ducks housed with nipple lines alone. To accomplish this goal, grow-out Pekin ducks were placed in pens with either nipple lines alone or with a combination of nipple line and a preening cup. We observed no significant differences in body morphometrics or body condition scores between the 2 groups. Significant differences were observed for wet preening, where more ducks in the PC group wet preened than the control ducks. Ducks did have an apparent preference for interactions with the preening cups rather than the nipple lines, which would explain the increase in wet preening. The ducks with preening cups spent significantly less time interacting with the nipple lines compared to the controls, suggesting that the preening cup was a more efficient, or a preferred, vehicle for the ducks to access water. Although the preference has not been specifically reported for ducks, when housed with a nipple line and a bowl, broilers drank from the bowl longer than they drank from the nipple line, although they did not have a preference between the 2, based upon first choice (Houldcraft et al., 2018). Although the preening cups may present a preferred water access for the ducks, the presence of the preening cups may not mitigate unwanted behaviors.

According to Liste et al. (2012b), the act of preening is when a duck nibbles at or strokes feathers with the use of their bill to maintain plumage condition. This behavior allows ducks to remove dirt, debris, and parasites from their feathers (Schenk et al., 2016). The uropygial gland, also referred to as the “preening gland,” improves the efficacy of this behavior. The uropygial gland is a holocrine gland that is located at the base of the tail and produces oil that is distributed throughout the feathers by the duck's bill (Moreno-Rueda, 2017). Hypothesized functions of the uropygial gland and oil include plumage maintenance, defense against ectoparasites, and water proofing (Moreno-Rueda, 2017). The uropygial gland size can change the properties of the oil secreted, which affects the bird's health (Moreno-Rueda, 2016), and the size can also be affected by environmental factors (Chiale et al., 2014). The presence of preening cups could be considered an environmental factor, so one of our hypotheses was that the ducks with the preening cups would have significantly larger uropygial glands compared to ducks housed with a standard nipple line alone. Previous studies have suggested that ducks raised without open water sources are not able to preen and thus show greatly reduced uropygial gland size (Mi et al., 2020). However, we found no differences in uropygial gland size in ducks with preening cups compared to nipple lines. Our observations suggest that semiopen water is either not necessary to maintain feather condition and uropygial gland size, or it is insufficient to stimulate uropygial gland hypertrophy. Previous studies mentioned above that did see a difference in uropygial gland size, also had an extremely large number of ducks per nipple. This situation likely set up a competitive access to the nipples with inadequate hydration leading to the reduction in uropygial gland size and in preening. Other studies have shown that by taking away a duck's open water access, the relative weight of the uropygial gland decreases in size (Mi et al., 2020). When looking at different species of birds, Montalti and Salibián (2000) found that there was no correlation between the birds’ gland size and the birds’ exposure to water. Comparing aquatic birds vs. land birds, their relative preening gland sizes are comparable, even considering their vastly different environments (Montalti and Salibián, 2000). The extent to which water availability influences domesticated Pekin duck uropygial gland size in commercial barns requires further investigation. A study in the Mallard suggests that preening gland oil is critical for normal egg production and for the health of hens and their offspring (Giraudeau et al., 2010). Thus the importance of preening in ducks is clear, how one water source improves that ability over other types of water sources is definitely not clear.

When ducks use water while preening, it is referred to as “wet preening” in poultry science, however, ducks also exhibit preening behavior when they do not have access to open water. Previous studies suggested that nipple water lines were not sufficient for preening behavior of ducks (Jones et al., 2009; Waitt et al., 2009; Jones and Dawkins, 2010); however, these studies did not investigate US nipple line management systems, as UK nipple line management systems have many more ducks per nipple than in the US. This difference in nipple density could elicit the effect that the nipples themselves could become a defensible resource in a high enough density. Other studies have also criticized water lines for not allowing wet preening behavior and decreased cleanliness of duck eyes and feathers (Jones and Dawkins, 2010). O'Driscoll and Broom (2012) reported that ducks who are offered an open water resource that provides full body access to the water allows for more water-related preening behaviors that promote improvements in feather hygiene. Other studies like Schenk et al. (2016) looked at bacterial contamination and water quality in troughs used by Pekin ducks. They found that the ducks who were housed with water troughs had poorer body condition scores and increased mortality. Preening cups have been suggested as an alternative to open water access, as they allow the ducks to only dunk their head into the water, and thus not defecate in and contaminate the water.

Our study observed no significant differences between the 2 groups for any body condition score. However, there was an outbreak of feather pecking in 4 of the treatment pens as shown by the increased (though not significant) feather quality score. Feather pecking is the act of pecking at the feathers of a conspecific, which can cause damage and lead to disease in the duck. Since there were no clear reasons underlying the outbreak, we can conclude that although the preening cups may be a form of environmental enrichment, they were not able to provide sufficient stimuli to prevent feather pecking. Although we did not see a significant treatment effect for feather pecking, we did see a significant age effect, where ducks were less likely to perform feather pecking when they were 30 d of age than when they were 36 and 40 d of age. Dong et al. (2021) found that Pekin ducks increased feather pecking from 20 to 22 d to 27 to 29 d, and then declined from 27 to 29 to 34 to 36 d, contradicting what we observed. The role of preening cups in preventing pecking behavior warrants further investigation because pecking and picking behaviors are influenced by the type of resource provided. The increased feather pecking observed in our study with the preening cup groups again suggests that at the density used in our study, preening cups may become a defensible resource—however, that needs to be more fully examined. Colton and Fraley (2014) found decreased feather pecking in ducks housed with non–water-based environmental stimuli. When laying hen chicks were housed at a low stocking density with different types of enrichment, such as pecking stones and blocks, feather pecking behavior decreased (Zepp at al., 2018). Other studies have shown that enrichment did not affect feather pecking in laying hens (Savory, 1995) nor in turkeys (Duggan et al., 2014). In this study we observed that ducks in the PC group performed wet preening more than the ducks in the control group. However, we saw no difference for body condition scores or the size of the uropygial gland between groups. It is possible that our definition of wet preening was insufficient. Alternatively, and more likely given the similarity in uropygial gland sizes and body condition scores, nipple lines and preening cups provide equal ability to wet preen. Another possibility is that dry preening is as sufficient to maintain feather condition as wet preening. This possibility is further bolstered by the fact that the senior author and several colleagues evaluated ducks representing many 10s of thousands of ducks housed with nipple lines alone, all of which had exceptional body condition scores (Fraley et al., 2013; Karcher et al., 2013).

In summary, we saw no significant differences for body condition scores, organ weights or growth rates, suggesting that preening cups do not adversely affect ducks’ health or productivity. We did see an increase in wet preening with the PC ducks, but that did not affect their body condition scores or their uropygial gland size. As our study shows, the presence of preening cups does not adversely affect the ducks or their environment when there is a low number of ducks per preening cup, and when the preening cups are placed over a pit. Installing preening cups over litter may present additional challenges. It still remains unclear as to whether the presence of preening cups will prevent unwanted behaviors such as feather pecking. Future research should increase the number of ducks to preening cups to better match that of a commercial setting, as well as to measure how preferences in preening cup usage translate to water usage. Lastly, future research should also look into whether or not ducks develop a hierarchy, to see if there are dominant ducks who do not allow the other ducks to utilize the preening cup, as we observed ducks who performed feather pecking around the preening cup, but there were no significant differences between treatments, only age. Preening cups may provide environmental enrichment; however, more studies need to be undertaken to fully evaluate their value. In particular, more studies on microbe contamination and biosecurity associated with open water sources are required given the increased likelihood of Highly Pathogenic Avian Influenza outbreaks.