A TEST OF THE HARM TO OFFSPRING HYPOTHESIS IN EXOTHERMIC PIED FLYCATCHER (FICEDULA-HYPOLEUCA) NESTLINGS

Field experiments using pied flycatchers supported the harm to offspri ng hypothesis. A stuffed sparrowhawk was placed on the nestbox with yo ung exothermic nestlings. The nestling cloacal temperature at the star t of the trial, and the time elapsing between the removal of the sparr owhawk model and the entry into the nest of at least one parent were m easured. The females returned to the nest before males in all cases. P aired comparison of trials, taken in each nest during normal, and expe rimentally decreased nestling body temperature, showed that females en tered the nest sooner when body temperature was reduced as predicted f rom the harm to offspring hypothesis.