The reference method is the six-point scale of Kestin et al. (1992), which runs from 0 for a bird that walks normally to 5 for a bird that cannot walk. Garner et al. (2002) tightened the category definitions to improve agreement between observers, and Webster et al. (2008) proposed a three-point version for use in the field. The six-point scale is the one adopted in the Welfare Quality protocol for broilers (Welfare Quality, 2009). All of these require an observer to make each bird walk and to judge it against a verbal description. Training is needed, scores drift between assessors and over time, and only a small sample can be scored in a house of many thousands of birds.

Three lines of automation have been explored. The first treats the flock as a moving texture. Dawkins et al. (2009) computed optical flow from overhead video and found that the statistical distribution of flow across the image was related to the gait scores of the flock, without any bird being identified. The second measures activity. Aydin et al. (2010) showed that image-based activity differed between groups of birds with different gait scores, and Silvera et al. (2017) related the output of a commercial camera system to gait score in commercial flocks. The third follows individuals: Aydin (2017) derived step and speed variables from images of single birds, and kinematic studies have quantified how the gait of lame birds differs in velocity and stride characteristics (Caplen et al., 2012).

Flock-level methods are robust and cheap but say nothing about which birds are affected or how severely. Individual-level methods describe gait directly but have mostly been developed in corridors or pens with one bird in view. The procedure proposed here takes a middle course. It follows individual birds for short bouts in the ordinary house and reports a summary for the flock.

Cameras are mounted on the roof structure looking straight down at areas of open litter between feeder and drinker lines, where birds must walk to reach resources. A vertical view avoids most occlusion of one bird by another and keeps the scale nearly constant across the image. Each camera is calibrated for lens distortion with a planar target, following the standard method of Zhang (2000), and a target of known size laid on the litter gives the conversion from pixels to distance. Because litter depth changes and cameras are knocked during cleaning, the scale check is repeated weekly.

Broiler activity varies with the lighting programme, feeding and the presence of staff. Segments are therefore recorded at fixed times in the light phase, away from routine inspections, and for long enough to yield an adequate number of walking bouts. A short period following the daily walk-through is recorded as well, since birds made to move by a passing person provide bouts comparable to those elicited in manual scoring.

Birds are located in each frame with a convolutional object detector of the single-stage kind introduced by Redmon et al. (2016), trained on annotated frames from the house type and ages concerned. Detections are linked from frame to frame into tracks. White birds on pale litter at high density are a hard case, and tracks that merge or break are frequent late in the cycle. The procedure does not attempt to repair them. Any track in which two detections overlap or the identity is uncertain is discarded, on the principle that fewer clean bouts are preferable to many doubtful ones. Where finer description is wanted, markerless pose estimation can place points on the head, body centre and tail (Mathis et al., 2018).

Most of what a broiler does is not walking. A bout is accepted when a tracked bird moves from rest, travels at least a set number of body lengths and does so without sharp turns. The thresholds are fixed before recording begins and expressed in body lengths, so that they scale with the growth of the bird.

Four features are computed for each bout, chosen because each corresponds to something a human scorer looks for in the scale of Kestin et al. (1992) and has support in the kinematic literature (Caplen et al., 2012). They are walking speed in body lengths per second, the distance covered before the bird stops, the side-to-side sway of the body centre about the line of travel, and whether the bout ends with the bird sitting down. Speed and distance fall, and sway and sitting increase, as gait worsens. Body length measured in the image serves to normalise the features and gives a rough check on growth.

The features are standardised and combined into one value per bout, and the daily index is the proportion of bouts beyond a chosen cut-off together with the median. Gait deteriorates with age in every flock, so the index is read against a reference course for the genotype and house, built from earlier flocks. An alert is raised when the index departs from that course for several consecutive days. The weights, the cut-off and the alert rule are all quantities to be estimated in a validation study and cannot be stated in advance.