Some of the biotelemetry systems used in previous studies were made and tested in nests at the Wildfowl Trust, Slimbridge. All proved to be unsuitable for long term studies of the avian nest microclimate. Drift in the calibration of these systems and the limited potential - a maximum of 4 sensors per egg - were the principal faults. A novel multichannel biotelemetry system, based on pulse position modulation, was developed. This had the potential to monitor continuously 9 sensors throughout the entire incubation period. The sensors measured temperature at 5 points on an "egg shell", relative humidity and the temperature at the humidity probe, light intensity and egg attitude. All were contained together with the radio transmitter and batteries in a fibreglass shell having the same shape, size and, if need be, cryptic markings as the egg of the species under study. The completed 'radio egg' was made to be of the same weight as that of one of the bird's under study. A low-power UHF transponder relayed the signals from the egg to the data logging system. The novel system was used for a total of 150 days in 3 nests of the Black swans, Cygnus atratus, 1 of the Whooper swan, Cygnus cygnus, and 2 of the Barnacle geese, Branta leucopis, at the Wild Fowl Trust, Slimbridge. The nest of a Peregrine falcon, Falco peregrinus, was also studied in S.W. Scotland. Features common to all the nests were: (1) a slow increase in nest temperature at the beginning of incubation, (2) a temperature gradient away from the brood patch of the bird, (3) a reduction in the absolute vapour pressure of the nest as incubation proceeded - this was attributed to the drying out of nesting material and was concurrent with the slow temperature rise, and (4) the eggs rested mainly with their long axes within 45° of the horizontal plane. An interplay between ambient conditions and the nest microclimate was not apparent in analysis of the data. A diurnal rhythm in the nest temperature, vapour pressure and rate of egg turning was associated with bird behaviour.
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