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The purpose of the research was to find a time frame for the beginning, duration of maximum intensity and end of nocturnal bird migration. The research was carried out using photo registration on a radar screen during the spring and autumn seasons of 1999-2001. Examination of the average length of daylight at the beginning of migration and the onset of civil twilight yielded a high correlation factor for both spring and autumn. The results showed that, on average, nocturnal migration began at the onset of civil twilight, that is, half an hour after sunset. The time elapsing between the onset of migration and the maximum concentration of migratory birds averaged about 70 minutes in both spring and autumn. Nocturnal migration usually came to an end within the one and a half to two hours after sunrise. We ascertained the seasonal time shift for the onset of nocturnal migration corresponding to the seasonal time shift related to the approach of darkness. The average times of the beginning, maximum intensity and end of nocturnal migration were found to be related to photoperiodic factors.

Temporal Characteristics of Night Bird Migration Above Central Israel — A Radar Study