Fall Migration in Coastal Louisiana and the Evolution of Migration Patterns in the Gulf Region.

摘要

Passerine nocturnal migration was observed with radar and portable ceilometer on 34 nights during fall, 1969, at Lake Charles in southwestern Louisiana. Weather patterns over the Gulf in fall are generally similar to those of late spring and summer and are usually characterized by southerly winds. These conditions, which favor large-scale trans-Gulf migrations in spring, are opposed to such flights in fall. The daily weather patterns observed during this study were grouped into five basic types. Passerine migrants at Lake Charles flew with the wind, regardless of its direction. Because of the frequency of southerly winds, 'reverse' migrations were common. However, a strong net flow of birds in a southwesterly direction resulted from prevailing northeasterly winds and the occurrence of disproportionately large migrations when air flow was favorable for movement toward wintering areas. Downwind flight assures that small landbirds will not embark on long overwater flights in unfavorable winds, but at the same time allows them to take advantage of northerly post-frontal winds when they occur. (Author)