Simple SummaryThere are many factors that can cause stress in poultry, but one of the most significant is high temperatures in the environment, which can make production difficult. It is important to assess the impact of heat stress (HS) on breeder ducks and their offspring (F1generation) to identify biomarkers that can measure the response of poultry to HS effectively. This approach can reveal both physical and genetic characteristics that can serve as useful markers to determine the susceptibility or tolerance of different poultry breeds to HS. The results of our study indicated that exposing the parents to HS resulted in heightened Hypothalamic–Pituitary–Adrenal (HPA) axis and fear responses in their offspring. We concluded that the maternal hormones that are deposited into eggs during HS may influence the early programming of the brain, leading to alterations in the HPA and behavioral response in the F1generation.AbstractThe effects of HS on the welfare of poultry have been reported to have a transgenerational effect on phenotype plasticity. The goal of our experiment was to determine whether parental exposure to HS would impair the performance, HPA axis response, or behavior of their offspring. We treated adult drakes and hens (n = 80 ducks/treatment) at peak lay with HS or the control temperature for 3 weeks and incubated eggs collected from the last 3 days of the experiment. We utilized 76 ducklings/parental treatment group: control (CON-F1) and HS (HS-F1). Weekly data for body weights, body condition scores (BCSs), and novel object test (NOT) were collected. At 3 weeks of age, the ducks (n = 6/treatment) were subjected to adrenocorticotropic hormone (ACTH/cosyntropin, 0.0625 mg/kg) challenge or vehicle as the control. Blood samples were collected at 0, 1, 2, 3, and 4 h relative to treatment for serum glucocorticoid and heterophil-to-lymphocyte ratio (HLR) analyses. All injected birds were euthanized with pentobarbital on the second day relative to ACTH administration, and the spleen and bursa were removed and weighed immediately. Duck level analyses were completed using one- or two-way ANOVA as appropriate. BCSs were analyzed using a chi-squared test. The HS-F1ducks had a lower hatch weight (p< 0.05) compared with the CON-F1ducks but no significant difference in growth rates during the 5-week period. NOT (n = 4) analyses showed that the HS-F1ducks had a greater fear response (p< 0.001) compared with the CON-F1ducks. Similarly, an ACTH stimulation test showed that the HS-F1ducks had significantly (p< 0.05) heightened corticosterone and HLR responses compared with the CON-F1ducks. The HS-F1ducks showed altered baseline and ACTH-stimulated levels of cortisol compared with the controls. Our data suggest that parental exposure to HS impacts the HPA response and fearfulness of the F1generation in Pekin ducks.