Nest humidity is a function of how water vapour is retained within the nest. Previous analysis suggested that in general, nest humidity was largely unaffected by ambient humidity but there was no consideration of the type of nest involved. This study examined data collected from the literature to test the hypothesis that the relationship between nest and ambient humidity is a function of the type of nest. Data for water vapour pressure for the egg (PE), nest (PN) and ambient air (PA) were collected for 48 examples of nests from 44 species of bird from a range of Orders. Initial egg mass and water vapour conductance of the eggshell was also recorded for each species. Analysis showed that birds on scrape nests are able to raise PN above PA by on average 5 Torr irrespective of PA. By contrast, PN in cup nests was unaffected by PA, such that at low values nests are well insulated against water vapour loss. The balance between the proportion of water vapour lost from the egg to the ambient air that is controlled by the nest varied with the type of nest and PA. For eggs in scrape nests mass specific water vapour conductance showed a positive relationship with PA. By contrast, for eggs in cup nests the relationship was negative and under conditions of low PA the high PN means that mass specific shell conductance values have to be high so as to ensure the appropriate rate of weight loss from the egg. Whilst birds do not control nest humidity directly the inter-relationship between weight loss from an egg, its initial mass, its shell conductance, and the incubation period can now be extended to include the water vapour conductance of the nest wall
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