Changes in behaviour and prolactin secretion in bantam hens (gallus domesticus) during extended incubation

Abstract

The function of incubation behaviour in birds is to maintain the eggs at the temperature required for embryo development and hatching. During incubation, single-sex incubators such as the domestic hen, leave their nests each day for a limited number of short periods (recesses) to carry out body maintenance activities. Most activity during a recess is food orientated. Incubation behaviour in the hen is maintained by increased concentrations of plasma prolactin and is normally terminated by the hatching of the eggs. At this time concentrations of plasma prolactin decrease. Hens sitting on infertile eggs will continue to incubate for 12 weeks or more, well beyond the normal 3-week incubation period. The aim of this study was to identify the factors responsible for the termination of prolonged incubation in the bantam hen, a form of the domestic hen in which broodiness can be readily induced. The possibility that prolonged incubation terminates because of an excessive loss in body weight was discounted because during a period of 10 weeks prolonged incubation body weight fell during the first 3-4 weeks and thereafter remained constant. The hypothesis investigated was that during prolonged incubation, a fall in the concentration of plasma prolactin results in a decrease in motivation to nest, which in turn predisposes the bird to nest desertion.
Concentrations of prolactin were measured in 10bantam hens incubating for ten weeks. During the first three weeks, the concentration of prolactin was higher than during weeks 8-10. An analysis of week-to-week changes in the concentrations of plasma prolactin showed that a significant decrease occurred between 4 and 5 weeks of incubation. An assessment was made of possible changes in the neural mechanisms controlling prolactin secretion during prolonged incubation. This involved the removal of nests for 48 hours from hens which had been incubating for 2 or 10 weeks. Nest removal caused concentrations of plasma prolactin to fall but after nest return, prolactin concentrations increased again in those birds which renested. The increase in prolactin in renesting birds which had been incubating for 10 weeks was less robust than in those which had been incubating for 2 weeks.
Changes in motivation to incubate during prolonged incubation were assessed in three ways. The first was indirect, and involved an analysis of behaviour, particularly during the first recess of the day. A direct assessment was made using an operant conditioning system in which the incubating hens were required to "work" for access to their nests by pushing through a weighted door. An additional assessment was made by comparing the readiness of hens at 2 and 10 weeks of incubation to renest after 48 hours nest-deprivation.
Behavioural analyses showed that during 10 weeks prolonged incubation, there was no change in the duration of the first recess of the day, although the birds began this recess earlier in the day during the course of extended incubation. A change in behaviours within the recess was observed from 5-6 weeks incubation onwards, with predominantly intense and dominant feeding behaviour progressively giving way to less intense feeding asssociated with foraging behaviours. Hens sitting on their nests showed no change in response to a threatening stimulus during prolonged incubation.
In the operant conditioning test, compared with 2-4 weeks of incubation, after 8-10 weeks incubation, the hens took longer to begin to pushing through the door, and increased the time spent pushing, to gain access to their nests. After 2 and 10 weeks incubation, 9 of 11 and 3 of 10 hens respectively, resumed incubation after 48 hours nest deprivation. Motivation to renest was found to depend on the functional activity of the neural system involved in prolactin release rather than concentrations of plasma prolactin at the time of testing.
It is suggested that after prolonged incubation a moderate decrease in circulating prolactin may be responsible for the resurgence of foraging-related behaviours which are typical of nonincubating birds. As these behaviours become established, they may result in an increase in the frequency of recesses which predisposes the hen to nest desertion. This suggestion assigns to prolactin a major role in the regulation of feeding-orientated behaviours. An analysis of changes
in plasma prolactin during incubation in birds with different diets and incubation strategies is consistent with this hypothesis.