Struggling behavior in shackled male and female broiler chickens

Anecdotal evidence suggests that the struggling behavior of shackled broiler chickens may be positively related to compression of the shank and the probable associated discomfort: birds with large shanks tend to struggle more violently than do those with smaller shanks. Males are generally heavier and have thicker shanks than females. Therefore, we tested the hypothesis that, because the leg gaps of shackles are fixed in size, male broilers would struggle more than females. At 42 d of age, 264 floor-reared broilers were cooped in groups of 12 (six males and six females) and were transported from the university farm to the abattoir. Eighty of these served as test birds (n = 40/sex) and were shackled on a moving processing line with a bird of randomly selected sex on either side. Upon shackling, the latencies to struggle, numbers of struggling bouts, and total time spent struggling were recorded during a 1-min test period. Subsequently, the BW and circumference of the right shank (CRS) of each test bird were measured. Male birds were heavier and had thicker shanks than females (both P < 0.0001); they also struggled sooner (P < 0.01) and longer (P < 0.008). When data from males and females were pooled, CRS was negatively correlated with latency to struggle (r = -0.30; P < 0.006) and positively associated with SB (r = 0.23; P < 0.04) and total time spent struggling (r = 0.23; P < 0.04). However, there were no detectable correlations within sex. Body weight was not significantly correlated with any of the struggling behavior measures. Although other gender-related factors may be influential, an interpretation of our findings based on sex differences in CRS seems the most parsimonious. We conclude that use of shackles of fixed leg-gap size may contribute to increased struggling behavior in male broilers.