This study is the first quantitative assessment of fine-scale patterns of migration along the lower Gulf coast of Texas. I used radar technology to monitor bird migration over three years for two coastal sites in autumn (15 August to 17 November) of 2007–2009 and spring (15 March to 1 June) of 2008–2010. I quantified migration timing, magnitude, and flight altitudes for over 14 million targets and flight directions for over 800,000 targets in 16,360 hours of radar operation across 785 day/nights. Migration patterns of birds differed greatly between seasons. Autumn migration was more protracted than spring with 46% higher passage rates that were mainly diurnal. Spring migration was concentrated within a four-week period and movements were predominantly nocturnal. I found flight altitudes to be 10–33% higher in spring than autumn and nocturnal movements to be at 10–28% higher altitudes than diurnal movements. I found that migration in autumn was directed along the Texas coastline for both sites, whereas migration was focused north/northwesterly in spring regardless of shoreline orientation. I also investigated the influence of weather on nocturnal migrating birds at two coastal sites. Wind direction was the strongest predictor of mean migration traffic rates within different altitudes and occurred in all top models. This effect was consistent for autumn and spring. Birds migrated with following winds in autumn, and in following and opposing winds in spring. I also found wind speed to be a significant factor on mean migration traffic rates. Bird passage was highest in low wind speeds, and decreased as wind speed increased. My study shows that the lower Texas Gulf coast acts as a significant migration corridor concentrating millions of birds between breeding and wintering ranges. This baseline information on fine-scale bird migration patterns along the lower Texas coast will enhance the ability of state and federal coastal managers to make sound recommendations for future development to reduce negative impacts to migratory birds.
July 13, 2015
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