Thermoregulatory and Behavioral Responses during Incubation of Free-Living Female Pied Flycatchers Ficedula hypoleuca

The changes in body temperature ( T b), respiratory frequency (${\rm f}_{{\rm R}}$) and oxygen consumption (M) of two free-living Pied Flycatcher females were recorded during the incubation period. T b followed a marked diurnal rhythm, with maximum temperatures around noon or just afterwards and minimum temperatures during the night. Upon entering the nest T b decreased (phase 1) to a minimum level (phase 2) and then increased (phase 3) prior to the female's next period-off. Phases 1-2 and part of phase 3 coincided with the warming up of the eggs to an incubation temperature of approx. 35-37°C. The duration of the female's periods-on was negatively related to the egg temperature at the start of incubation and positively to the metabolic rate. Respiratory frequency was high at the onset of the periods-on, declined thereafter, and finally levelled off; ${\rm f}_{{\rm R}}$ tended to increase again shortly before the female left the nest. Experimental maintenance of the eggs at incubation temperature during the female's periods-off resulted in an immediate increase in T b when she resettled on the eggs, and a marked shortening of the periods-on. Conversely, when the eggs instead were cooled down during the female's periods-off, the following periods-on were considerably lengthened and M correspondingly increased. The long-lasting nightly period-on (mean length 7.0 h) showed the same three phases as the daytime periods-on. Mean T b during the nightly phase 2, characterized by a relatively stable level of T b and lasting on average 4.8 h, was negatively correlated both with the air temperature and the female's metabolism. In the middle of the night, following sudden, experimental changes in nest box temperature, the incubating female's M was immediately adjusted so as to maintain egg temperature at an appropriate level. According to the present results, female attentiveness seems to be basically regulated by an internal rhythm, modified by the ambient temperature. Our data do not support the hypothesis that the incubating female breaks off her incubation periods when the eggs have reached the optimal incubation temperature level.