Affective trajectories: Are hens influenced by positive and negative changes in their living conditions?

Most studies of the effects of housing and husbandry on animals' affective states and welfare investigate the impact of stable living conditions, comparing for example, animals living in enriched environments with those living in non-enriched ones. Changes in living conditions, including from more to less enriched environments, have also been found to have effects on measures of affective state and welfare in some species. But these studies have not investigated whether it is the trajectory of change that has affected the animals (e.g., worsening conditions), or simply the nature of their final environment (e.g., non-enriched). Here, we hypothesised that laying hens living in worsening conditions across a six-week period (gradually moving from preferred to non-preferred living conditions; "Trajectory to Non-Preferred", TNP, n = 30), would show evidence of more negative affective states and poorer welfare than those living continuously in non-preferred conditions for the same duration ("Stable Non-Preferred", SNP, n = 30). We also hypothesised that hens living in improving conditions (gradually moving from non-preferred to preferred living conditions; "Trajectory to Preferred", TP, n = 30), would show evidence of more positive affective states and better welfare than those living continuously in preferred conditions ("Stable Preferred", SP, n = 30). The preferred living condition provided extensive resources and intermittent rewarding events (such as the delivery of food treats) known to be valued and preferred by most hens, while the non-preferred living condition provided just basic resources and intermittent aversive events (e.g., loud noises). The hens' affective states and welfare were measured using home-pen behavioural observations, body condition assessments, physiological stress measures (e.g., blood corticosterone, glucose, etc.), physical challenge tests, and judgement bias tests. A number of differences between hens in the trajectory and stable living conditions were found: TP hens were lighter, showed more foraging behaviour and less standing alert and head-shaking than SP hens, while TNP hens showed more head-shaking, mild feather pecking and aggressive attacking of pen mates than SNP hens. However, some of these differences failed to reach significance following Benjamini adjustments for multiple testing. The groups also did not differ in their judgement biases (measured in a sub-sample of 12 hens per experimental group), response to physical challenges, or measures of physiological stress. We conclude that the hens in the present study showed some evidence of responsiveness to 'affective trajectories' in their living conditions, but no definitive effects on their affective states and welfare.