SYNCHRONIZATION BY LOW-AMPLITUDE LIGHT-DARK CYCLES OF 24-HOUR PINEAL AND PLASMA MELATONIN RHYTHMS OF HATCHLING EUROPEAN STARLINGS (STURNUS-VULGARIS)

In young European starlings, as in other avian species, high-amplitude 24-hr rhythms in plasma and pineal melatonin are already present arou nd the time of hatching. In chickens this rhythmicity results at least partly from the light sensitivity of the melatonin-producing and -sec reting system. In contrast to the chicken, the starling is a hole-nest ing bird, and it seemed questionable whether the low light intensities in the nest are sufficient to synchronize perinatal melatonin rhythms . We therefore exposed starling eggs to light cycles roughly simulatin g those measured in nest-boxes, i.e., an 11-hr phase of complete darkn ess and a 13-hr phase consisting of 15 min of dim light (10 lux) alter nating with 30 min of darkness. For one group the photophase lasted fr om 0600 to 1900 hr; for the other group the photophase lasted from 180 0 to 0700 hr. In approximately 10-hr-old hatchlings of both groups, pl asma and pineal melatonin concentrations were high during the dark pha se and low during the light phase. We conclude that perinatal low-ampl itude light intensity changes of the kind experienced by hatching star lings in the field are sufficient for synchronizing the melatonin-prod ucing and -secreting system in the pineal and possibly other organs.