THE THERMAL SIGNIFICANCE OF THE NEST OF THE SOCIABLE WEAVER PHILETAIRUS SOCIUS: SUMMER OBSERVATIONS

Extending a previous study of the thermal significance of the nest of the Sociable Weaver carried out during the winter, we measured temperature and humidity in the matrix and chambers of a large nest of this species in the Kalahari Gemsbok National Park, South Africa, during the austral summer of December 1973. Air temperatures outside the nest ranged from 16 to 33.5°C but temperatures in occupied chambers varied over a range of only 7 or 8°C and remained well within the zone of thermal neutrality for a passerine bird of this size. Compared to outside air temperatures, those within the nest matrix were lower during the day and higher at night. Thus, the nest ameliorates the effects of external temperatures and allows maintenance inside the chambers of a range of temperature favourable to the birds. In winter we found up to five roosting adults per chamber, with some chambers left empty. In the same nest in summer we found no more than two adults per chamber but virtually all chambers were occupied. The principal mechanism for maintaining chambers within the zone of minimal energetic cost is changes in the number of birds in the nest chambers at night. Humidity inside the occupied and unoccupied chambers was somewhat higher in the former but always less than that of outside air in both cases. Air movement through the desiccated nest materials causes uptake by these materials of most of the water vapour introduced by the birds, and this moisture is dissipated to the outside during the day so that the nest remains dry. The highly social and colonial habits of the birds and their year-round occupancy and maintenance of the nest favour a system of opportunistic breeding that may be initiated by rainfall at any season. Larger nests provide the most favourable environment for energy conservation and successful reproduction. Even the largest nests, however, do not prevent predation during the warm season by snakes such as the Cape Cobra, which may consume all the eggs and young in all the chambers of a large nest. The effects of such heavy predation may be offset by the birds' capability for breeding during times too cold for reptile activity. It seems likely that in smaller nests such as those on telephone poles, lack of predation would favour summer breeding while thermal problems would limit breeding success in winter. In larger nests, breeding success may be lower in summer because of predation and higher in winter when reptile predation is lacking and thermal problems are minimized by the nest structure. The large nest not only makes possible the success of the Sociable Weaver in desert areas, but the nest could only exist in such areas and the species' range is thereby restricted. Higher humidity and heavier rainfall would cause fermentation within the nest mass, loss of its thermoregulatory advantages, and ultimately its decomposition and destruction. Therefore, the unique nesting system of the Sociable Weaver appears to be initially self-reinforcing and ultimately self-limiting.