Circannual and Circadian Contributions to the Timing of Avian Migration

Abstract

Only a few phenomena in the living world occur as predictably with regard to both time of day and time of year as the migrations of birds. Humans living along the major migratory pathways have exploited for ages the opportunity of enriching their diet with valuable animal protein provided by migrant birds. Because the times of passage are exactly known, preparations for hunting can be made well in advance. The daily timing of migrations, e.g., during the warm midday hours in raptors or at night in many songbirds or waders, is conspicuous and common knowledge to many people. The annual migration times are even more obvious. For the agricultural society of the Kelabits on the highlands of Borneo, the times of passage of certain key migrants provide a calendar that helps people plant crops at the appropriate times: when the first yellow wagtails (Motacilla flava) arrive in September, farmers begin to prepare the fields so that the rice can be planted in the months of the brown shrike (Lanius cristatus), October/November, and of the Japanese sparrow hawk (Accipiter gullaris), November/December (Smythies 1960). Thus, migrants provide important cues about time of year in a region in which reliable seasonal information is scarce. In Europe, the arrivals of popular birds in spring are highly emotional events celebrated in numerous poems and folk songs.