HUMIDITY LEVELS IN PIED FLYCATCHER NESTS MEASURED USING CAPSULE HYGROMETERS

We used plastic capsules that were perforated at one end and filled wi th silica gel to measure the water vapor pressure (P-N) in nests of Pi ed Flycatchers (Ficedula hypoleuca) during 1992 And 1993. Nest humidit y measured daily for 7 to 16 consecutive days of incubation in 31 nest s averaged 2.1 kPa or 15.7 torr (range 1.8-2.5 kPa). Humidity varied g reatly from one nest box to another and within individual nests, but d id not affect the rate of egg water loss or hatching success. Nest hum idity often was significantly correlated with variations in ambient hu midity. Large clutches had significantly lower average P-N than small ones. In general, nest humidity was low at the beginning of incubation , rose during midincubation, and remained elevated thereafter.