The Effects of Weather and Lunar Cycle on Nocturnal Migration of Landbirds at Southeast Farallon Island, California

Abstract

Proximal climatic and lunar effects on amval and departure of nocturnal migrant landbirds at Southeast Farallon Island (SEFI), California, were examined using multivariate and univariate statistics. Predictive models including date, weather and lunar variables were developed for both spring and fall, which accounted for 33-40°/o of variation in amval totals and 18-2 1% of vanation in departure proportions. Seasonal, regonal, and taxonomic variation in weatherand lunar-migration relationships were assessed and, along with comparisons of amval and departure patterns, used to differentiate proximal effects on amval to SEFI, from widespread effects resulting in increased migration volume over California. Low wind speeds, low to moderate visibility, full cloud cover and lack of fog viere proximal effects increasing amval to SEFI, while low wind speeds, low but rising barometric pressure, clear and clearing skies, high visibility, and decreased moonlight (in fall) resulted in increased departure proportions and, presumably, caused higher migration volume over the region. Effects of wind direction and air temperature, although related to synoptic weather-migration relationships, generally had obscure or minimal direct influences on amval and departure at SEFI. Departure proportion in spring increased with decreased departure proportion the day before, but few other delay effects between weather variables and amval or departure were found. Seasonal, regional, and taxonomic variation in departure effects were relatively small suggesting that selection of weather-migration strategies has evolved convergently in a diverse group of migrants flying over a region the size of California.