Twenty-four hour melatonin profiles in a nocturnally migrating bird during and between migratory seasons

The garden warbler Sylvia borin is a long-distance migrating bird that travels exclusively at night. During the migratory seasons caged warblers develop intense nocturnal activity which may become even more intense than that during the day. It is demonstrated that in spite of dramatic seasonal changes in the 24-h pattern of locomotor activity measured in caged garden warblers, the corresponding pattern of plasma melatonin changed only very little. As in other species melatonin levels were generally low during the day (below 50 pg/ml) and high at night (350 to 650 pg/ml). A slight reduction in the nocturnal melatonin peak (from 650 pg/ml to between 350 and 400 pg/ml) observed during the autumn and spring migratory seasons, was possibly due to an increased light perception of the birds moving around while being exposed to dim night lights. In general the results show that the 24-h plasma melatonin profiles of these birds are rather robust and that locomotor activity does not depend in a simple and direct way on plasma melatonin levels.