Considerable variation in pre-laying behaviour of domestic hens (Gallus gcillus
domesticus) is observed within individuals, between individuals and between different
husbandry systems. The contributions o f internal and external factors to this variation
were considered. Part o f this variation is in relative amounts o f searching and nesting,
so criteria were defined to separate pre-laying behaviour into a searching and sitting
The main internal factors considered related to lag and position o f eggs in sequences.
The duration o f the sitting phase and the total time spent in the nest box was related to
lag. The duration o f pre-laying behaviour was longer for the first egg o f a sequence
than for the other eggs. This was mainly due to a longer searching phase in which hens
performed more nest examinations. The duration of the sitting phase was longer for the
last egg o f a sequence than for other eggs; this may have been related to a longer lag
found for the last egg.
Effects o f competing behavioural tendencies on pre-laying behaviour involve both
internal and external factors. A series o f experiments was conducted to investigate the
performance o f pre-laying behaviour while altering the availability o f food and
motivation to feed (length of food deprivation). Hens always interrupted their prelaying behaviour in order to feed when food was presented. Length o f deprivation did
not influence the duration o f feeding or pre-laying behaviour, that is, even "satiated"
hens stopped their pre-laying behaviour and fed. However, the delay in oviposition was
found to be greater when food was presented in the later, rather than the earlier, stages
of pre-laying behaviour. The duration o f pre-laying behaviour and o f the searching
phase was longer if hens were food deprived than if food was available. These findings
suggests that the expression o f pre-laying behaviour is determined by the tendency to
perform pre-laying behaviour competing with other behavioural tendencies.
External factors examined included the effects o f conditions that facilitate searching
and nesting behaviour on pre-laying behaviour. Hens provided with an unlittered nest
box showed an extended pre-laying behaviour and searching phase, more searching
behaviour and nest examinations and more nest entries o f a shorter duration than when
provided with a littered nest box. These results suggested that in the absence o f a
suitable nest site, hens delayed, and showed an incomplete transition from searching to
nesting behaviour. Providing an exploratory walkway to facilitate searching behaviour
resulted in the searching phase starting earlier than expected, and in the occurence of
more searching behaviour and nest examinations during this time. Environments that
facilitate searching behaviour may provide external cues that allow the motivation to
perform pre-laying behaviour to be expressed earlier than in barren environments.
The strength o f hens' motivation to reach a nest box was assessed with the aid of
aversive stimuli. Hens were required to pass through an empty corridor, or past a
dominant, subordinate or unfamiliar hen to reach a nest box. Hens delayed their
approach to the nest box and made more attempts to find alternative routes to the nest
box when required to pass a dominant or unfamiliar hen. Thus social factors were
found to influence access to the nest site and pre-laying behaviour. Hens appeared to
be only weakly motivated to reach the nest site during the searching phase; motivation
to reach the nest site increased near the start o f the sitting phase.
The effects o f social interactions on access to a nest site and on the pre-laying
behaviour of hens in small groups was investigated. Evidence o f competition for the
nest site was found when more than one hen was showing pre-laying behaviour.
Subordinate hens walked more in the last hour before oviposition and sat less in the
last 25 minutes when other hens were also showing pre-laying behaviour than when no
other hens were in the pre-laying phase. Dominant hens in the pre-laying phase
remained nearer the nest when other hens were showing pre-laying behaviour than
when none were doing so. Thus social interactions during the pre-laying phase result in
variation in pre-laying behaviour in both directions; subordinate hens do not settle into
the expected nesting phase whereas dominant hens stay nearer the nest.
A motivational theory o f pre-laying behaviour is proposed in which pre-laying
behaviour is controlled by an interaction between the tendencies to perform searching
and nesting behaviour. The tendency to start searching behaviour is influenced by
internal factors, competition between motivational systems and external cues for
exploration. The tendency to start nesting behaviour is influenced by the availability o f
a suitable nest site and social factors. It is suggested that a certain amount o f
behavioural priming is normally required before oviposition can occur. The
implications o f this model for the welfare of laying hens is discussed.