Factors affecting roost departure and entry by wintering merlins

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Abstract
Merlins (Falco columbarius) overwintering in Saskatoon, Saskatchewan, depended upon mature spruce trees, planted in residential areas of the city, for roosting sites. Variation in entry times at the roost by radio-tagged Merlins was accounted for by sunset time, total daily solar radiation, and temperature. To a lesser extent the variation in departure times was explained by sunrise time and temperature. Merlins left the roost significantly earlier as the days became shorter in the period leading up to the winter solstice, but returned at the same time relative to sunset resulting in a longer active phase than that provided by natural day length.

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