Using acoustic monitoring of avian flight calls for phenological study

Background/Question/Methods Avian populations and their distributions are influenced by many factors and are constantly in flux. Detecting changes involves interpreting field data, and the precision in determining changes depends on the accuracy of field survey methods and the proportion of a targeted population surveyed. Some species are easier to survey than others and different survey methods have inherent strengths and weaknesses. Confidence in determining a change increases when independent survey methods corroborate a trend. This presentation discusses an emerging methodology that uses electronic technology to document flight vocalizations of birds in nocturnal migration. Most species of North America's migrant landbirds make their transcontinental migration flights at night, and many species give contact vocalizations while they fly. In eastern North America, thousands of avian flight calls can be recorded from a single acoustic monitoring station operating nightly over the course of a migration season. The question is, can systematic monitoring of such calling provide information on avian population and distributional changes? Current investigations are focused on characterizing existing flight calling patterns in different regions of eastern North America and efforts continue in facilitating acoustic monitoring of avian nocturnal flight calls for broader public participation. Results/Conclusions Dickcissel (Spiza americana) flight calls are commonly recorded at night in south Texas during spring while rarely in Florida. Conversely, Bobolink (Dolichonyx orizivorous) flight calls are commonly recorded at night in Florida during spring while rarely in south Texas. These patterns correspond with known migration routes based on decades of field observations. However, multiple years of Dickcissel night flight call monitoring in south Texas have documented widely varying numbers of calls and migration timing from year to year. Assessment of population and migration timing changes using acoustic monitoring is complicated near coastal regions because of weather factors that may steer a portion of an actively migrating population over water or cause coastal concentration dynamics. Artificial light, especially in conjunction with low cloud cover, is another variable that has been shown to affect the rate of flight calling. Sorting out the potential influence of such variables from population change and phenological signals in flight call data will need long-term monitoring and efforts to correlate such acoustic data with known population and migration timing changes determined by other survey methods.