Evaluation of methods to estimate nocturnal bird migration activity: A comparison of radar and nocturnal flight call monitoring in the American West

Abstract

Bird migration involves the movements of billions of individuals but is difficult to study because it occurs primarily at night. We sought to improve our understanding of the methods available to study migration, particularly in understudied regions of western North America. We evaluated 2 methods: weather radar and nocturnal flight call monitoring. We analyzed variability in estimates of migration activity from each method, how estimates relate, and identified factors associated with variation in this relationship. We collected radar and flight call data from southern New Mexico in western North America during the fall migration of 2021 and 2022. Similar studies have occurred in eastern North America, but it is unknown if regional variability alters the relationship between estimates from each method. We found that estimates were positively related across a season, but relationships were variable among nights. Also, we discovered that the strength of the association between methods varied across sites, indicating that local factors may influence acoustic sampling. We determined that variation in acoustic estimates of migration activity was associated with cloud cover, crosswind, date, migrant height, migrant speed, moon illumination, tailwind, and time of night. For radar, we found crosswind, date, migrant height, migrant speed, tailwind, and time of night to be associated with variations in estimates of migration activity. Overall, our findings support those of previous studies from eastern North America and demonstrate that, despite regional differences, estimates from each method are also correlated in western North America. Our findings provide new insight into factors associated with variation in estimates of migration activity from 2 widely used methods and an improved understanding of factors that impact migration behavior.