The length of the embryonic period varies both among and within species and can affect the individual phenotype in many
ways, both physiologically and behaviorally. In chickens, the hatch window may last 24–48 hours (up to 10% of the
incubation time), and studies have shown that incubation length may affect post-hatch growth and physiology. However,
little is known about effects on behavior. We therefore investigated how behavior variation correlates with hatching time in
the early life of chickens. We also measured egg weight and egg weight loss in relation to hatching time, as well as posthatch
growth. For females, there was a negative correlation between hatch time and body weight from day 4 and
throughout the experiment. For males, such a correlation was only observed when testing all hatched males up until day 10.
The birds were exposed to a number of behavioral tests, and a principal components analysis was performed on the
variables, resulting in four components. For the largest component, termed ‘‘Passivity’’, a tendency of a difference was
found between early and middle male hatchers. Furthermore, a significant difference between early and middle male
hatchers was found in the second component, termed ‘‘Response to novelty’’. In a spatial learning test, late hatchers tended
to learn slower. The behavior of females was not significantly affected by hatching time in any of these tests. This study is
among the first to demonstrate a link between time of hatching and early behavior in a precocial species like the chicken,
and may help shedding light on the evolutionary trade-offs between incubation length and post-hatch traits. The results
may also be relevant from a perspective of stress coping and therefore also for animal welfare and productivity in the
chicken industry. The mechanisms linking hatching time with post-hatch phenotype remain to be investigated