FLIGHT PATHS AND CHANGING PATTERNS: TRACKING INDIVIDUAL BIRDS TO UNDERSTAND AERIAL HABITAT SELECTION AND MIGRATION STRATEGIES

Abstract

Aerial migrants such as birds must contend with dynamic atmospheric conditions that have the capacity to greatly assist or impede their movements. Understanding habitat selection aloft provides powerful insights into how birds complete their biannual journeys, both about how conditions shape behaviors and how birds may respond to future conditions. The earliest studies of flight behavior were estimated by using weather radar, with radar studies continuing to provide observations of the system-wide migration patterns. However, species and individual level data of flight behavior (i.e. altitudinal profiles of a species, departure and arrival behaviors) and migration strategies is logistically challenging to collect but would facilitate empirical hypothesis testing about aerial habitat selection and flight strategies. Fortunately, the recent production of more affordable and lightweight multi-sensor data-logging devices that can collect high resolution spatiotemporal data is helping to bridge the gap between data collection and research questions regarding aerial habitat. My dissertation uses data collected on individuals by the most recent tracking technology to address gaps in knowledge about the relationships between animal movement and environmental conditions in the air. In Chapter 1, we reanalyze historical movement data collected by light-level geolocators, a type of tag that estimates location by the timing of sunrise and sunset, to ask questions about how life history traits and atmospheric conditions affect decisions to depart rest and refueling areas during migration. Because birds are expected to be limited in time and energy during migration, departure from rest and refueling areas, or stopover areas as they are more commonly known, has received much study, but with inconclusive evidence of driving atmospheric factors as well as life history traits. We find that large temperature changes, average migration distance, windspeed, and interaction of body size and surface pressure had positive effects on departure, that is the individual was more likely to depart under those conditions, and that if it was fall migration, the time since arrival to departure, the interaction of the time since arrival and temperature at departure had negative relationships with departure. In Chapter 2, I use high resolution GPS tracking data from Long-billed Curlews (Numenius americana) to compare how current migration strategies, including departure time, migration duration, and migration speed may have changed in the last decade. Though migration distances haven’t changed, we find that southbound departure was delayed for both sexes, but an overall faster migration pace has led to an earlier arrival to the wintering grounds especially for males. While male departure dates in spring show little change, female departures were delayed. Spring arrival time to the breeding grounds, however, appears unchanged for both sexes. We also described flight bouts and found that Long-billed Curlews completed their ~2500 km migration usually in several bouts. In both seasons, the first bout was generally the longest, however southbound first bouts often covered 50% of the total flight distance compared to in northbound migration where the first bout covered only 25% of the total flight distance. Faster groundspeeds were significant for longer distances covered in a flight bout. In Chapter 3, I investigate flight speeds and aerial habitat selection of Long-billed Curlews during migration using high resolution GPS tracking data. We found strong differences between diel and seasonal cycles for flight speeds and altitudes and that temperature, wind support, and boundary layer height had the strongest effects on altitude selection. For example, while Long-billed Curlews often reach heights above 5000 meters above sea level during prolonged flights that extended into daylight hours, these prolonged diurnal flights only happened in the fall season.