THE ETHOLOGICAL ANALYSIS OF INCUBATION BEHAVIOUR
Authors: Baerends G. P. 
Published: April 2008

Summary.An integrative survey of behaviour mechanisms in birds, known to contribute to securing the proper conditions for the development of eggs during incubation, has been attempted. Our ideas of these mechanisms, schematically represented in Fig. 1, are mainly based on work with the Herring Gull, but, where possible, are compared with those on other birds.In addition to incubation (=sitting on eggs) several other activities of the fixed‐pattern type can be observed in a bird on its nest. Through statistical analysis of quantitative observations groups of these activities can be proved to have causal factors in common; consequently these groups are distinguished as separate systems (instincts). Different instincts which share common causal factors have been placed under the same superordinate instinct. A study of the biological significance of behaviour elements usually shows separate instincts to serve different functions. It should be stressed that distinguishing these instincts is of temporary value, helpful at this stage of the analysis as a shorthand description, but does not solve at all the problem of what exactly determines the grouping of behaviour elements. Inhibitive as well as stimulative relations between instincts have been found.Incubation, with the appetitive patterns of egg‐retrieving and settling, belongs to one system: the incubation instinct. It is argued that this system has inhibitive relations with the instincts serving preening, escape, and nest‐building, although with the latter it could also be shown to have causal factors in common.The degree to which the incubation instinct is activated depends—apart from other factors not mentioned here—on the weather and on the total value of the stimulus situation “nest with eggs in territory”. The latter situation seems to have a priming effect on the incubation instinct as well as a releasing effect on the activities egg‐retrieving and settling. The analysis of the mechanisms for evaluating this situation has been most thoroughly investigated for the releasing effect of the egg. In the Herring Gull this effect is the greater the more numerous, the smaller, the darker and the more contrasting the speckles are, the more the colour matches a light greenish tint and the greater the size of the egg. However, characters of shape are of hardly any importance. It seems likely that at least part of these “differential preferences” are due, not to the receptors, but to more centrally situated nervous mechanisms.Whereas an intact nest with eggs of high value mainly stimulates the incubation instinct, nest disturbances or eggs of low value also activate the tendency to leave the nest (escape instinct). This simultaneous activation of two conflicting tendencies is considered to cause an increase of preening and building, the rate between the frequencies of each depending on the relative strength of the tendencies.Reactions to disturbances at nest or clutch not only occur before the bird settles on the nest, but also during brooding. Incubation could be shown to continue effectively only when the proper feed‐back stimuli are received by the tactile‐ and thermo‐receptors of the brood patches; otherwise the appetitive activities of incubation set in, often mixed with (displacement) building and preening. Also under these circumstances building and preening are thought to arise in consequence of a conflict, as the abnormal tactile and temperature stimuli must stimulate the tendency to escape. Moreover, deviations of the egg temperature from normal lead to the performance of behaviour patterns serving the temperature regulation of the bird, which are also subordinate, together with the chemical heat‐regulation, to the thermo‐regulatory centre of the hypothalamus.