1. Previous studies have shown that more yolk corticosterone is found in the eggs of random bred Japanese quail hens implanted with corticosterone during egg formation; both unstressed and stressed quail hens selected for exaggerated (high stress) rather than reduced (low stress) plasma corticosterone response to brief restraint deposit more corticosterone into their egg yolks. The length of egg incubation is also known to be shorter in eggs laid by high than low stress hens.

2. Here we investigated the interactive effects of quail stress line (low vs. high stress) with maternal corticosterone treatment (empty implant controls vs. corticosterone-implants) during egg formation on length of egg incubation.

3. Mean (±SEM) length of egg incubation for high stress control eggs (397⋅3 ± 0⋅4 h) was similarly shorter (by about 4.5 h) than that found for low stress control eggs (392⋅8 ± 0⋅2 h). In addition, on average, the incubation length of eggs laid by corticosterone-implanted hens (392⋅9 ± 0⋅5 h) was nearly 3 h shorter than that found for eggs laid by control hens (395⋅8 ± 0⋅2 h) regardless of stress line.

4. Line × hen-implant treatment effects on mean (±SEM) length of egg incubation partitioned in rank order as follows: low stress control (397⋅8 ± 0⋅5 h) > low stress corticosterone-implant (395⋅9 ± 0⋅7 h) > high stress control (393⋅8 ± 0⋅3 h) > high stress corticosterone-implant (391⋅2 ± 0⋅4 h).

5. Our original contention that selection for exaggerated adrenocortical responsiveness is associated with a reduction in the length of egg incubation was supported. Because maternal stress-induced elevations of yolk B are known to occur, the present findings of further shortenings of the hatching times of eggs of corticosterone-treated hens of both stress lines are also important to the poultry industry because they warn producers that unless stress in hens during egg formation is minimised, abbreviated egg incubation periods may result beyond the effects that a hen's genetic predisposition to adrenocortical stress responsiveness has on the length of egg incubation.