Circadian control of migratory restlessness and the effects of exogenous melatonin in the brambling, Fringilla montifringilla

Circadian pacemakers control both "daytime" activity and nocturnal restless ness of migratory birds, and the daily rhythm of melatonin release from the pineal has been suggested to be involved in the control of migratory activ ity. To study the phase relations between the two activity components durin g entrainment and when free running, locomotor activity of bramblings (Frin gilla montifringilla) was recorded continuously under a 12:12 "cool light" to "warm light" cycle (CL:WL, ca. 5000 K and ca. 2500 K, respectively) or b lue light to red light cycle (BL:RL, maxima at 440 and 650 nm, respectively ) at different irradiance ratios. Migratory activity was expressed primaril y during the WL or RL phase of the light cycles. Under free-running conditi ons, the circadian periods tau correlated with the phase relations between day and night (migratory) activity components during preceding entrainment. Bramblings with migratory activity had significantly longer tau at constan t light intensity than the same individuals without migratory activity. Bir ds with migratory activity reentrained faster after a 6h phase shift of the CL:WL cycle than birds without migratory activity. When exogenous melatoni n was given in the drinking water (200 mu g/mL 1% ethanol or 0.86 mM) to br amblings exposed to 12:12 CL:WL cycles with constant irradiance, the amount s of activity, which were initially higher during the WL phase of the light cycle, were suppressed to similar low levels during both light phases. The systematic changes in the amounts of activity during melatonin treatment w ere not correlated with consistent changes in entrainment status. The data support the hypothesis that changes in the amplitude and level of the daily melatonin cycle are involved in regulating migratory restlessness, by eith er allowing or inhibiting nocturnal activity.