Effect of Environmental Complexity and Stocking Density on Fear and Anxiety in Broiler Chickens

Abstract

Barren housing and high stocking densities may contribute to negative affective states in broiler chickens, reducing their welfare. We investigated the effects of environmental complexity and stocking density on broilers' attention bias (measure of anxiety) and tonic immobility (measure of fear). In Experiment 1, individual birds were tested for attention bias (n = 60) and in Experiment 2, groups of three birds were tested (n = 144). Tonic immobility testing was performed on days 12 and 26 (n = 36) in Experiment 1, and on day 19 (n = 72) in Experiment 2. In Experiment 1, no differences were observed in the attention bias test. In Experiment 2, birds from high-complexity pens began feeding faster and more birds resumed feeding than from low-complexity pens following playback of an alarm call, suggesting that birds housed in the complex environment were less anxious. Furthermore, birds housed in high-density or high-complexity pens had shorter tonic immobility durations on day 12 compared to day 26 in Experiment 1. In Experiment 2, birds from high-density pens had shorter tonic immobility durations than birds housed in low-density pens, which is contrary to expectations. Our results suggest that birds at 3 weeks of age were less fearful under high stocking density conditions than low density conditions. In addition, results indicated that the complex environment improved welfare of broilers through reduced anxiety.

Keywords: affective state; animal welfare; anxiety; attention bias; broiler chicken; environmental complexity; fear; stocking density; tonic immobility.

Figure 1

(a) High-complexity pen with four functional spaces for “feeding”, “comfort”, “resting”, and “exploration”. The feeding space contained four feeders () and pecking stones, the “comfort” space included a sand dust bath (), the resting space contained three perches (), and the exploration space contained varying pairs of enrichment objects. The feeding, comfort, and resting spaces each contained a water line with three nipple drinkers (). (b) Low-complexity (control) pen, containing four feeders and three water lines.

Figure 2

Photograph of the perch design in high-complexity pens in (a) Experiment 1 (n = 3/pen) and (b) Experiment 2 (n = 3/pen).

Figure 3

Diagram of the attention bias (AB) arena used in Experiments 1 and 2. A familiar feeder (exact same as provided in pens) was placed in the center of the arena and wood shavings were provided as litter.

Figure 4

Least squares mean estimates (s ± SEM) for latency to begin feeding (n = 144) and resume feeding (n = 78) for broiler chickens kept in high-complexity, low-complexity, high-density, and low-density treatments in Experiment 2 at 4 and 5 weeks of age (days 32, 33, and 38). The timer was reset to zero after the second alarm call was played to record latency to resume feeding. * p < 0.05.

Figure 5

Least squares mean estimates (s ± SEM) for tonic immobility duration (0–300 s) for broiler chickens (n = 71) kept in high-complexity, low-complexity, high-density, and low-density treatments in Experiment 2 on day 19 of age. * p < 0.05.