PROLONGED SPRING MIGRATION IN THE RED-EYED VIREO (VIREO OLIVACEUS)

We used archival geolocators to track the migration of Red-eyed Vireos (Vireo olivaceus), abundant forest songbirds with significantly increasing breeding-population trends, to identify important stopover and wintering regions. All individuals from a single breeding site (n = 10) wintered in northwestern South America, an extensively forested region, and in spring used a consistent route, crossing the Gulf of Mexico from the Yucatan to Louisiana. Their spring migration rate (146 km day(-1)) was slower than that of most other songbirds tracked with geolocators from South America (>280 km day(-1)). Red-eyed Vireos had an unexpectedly prolonged stopover (mean +/- SD =18.6 +/- 4.9 days) in Colombia soon after the onset of spring migration, and we suggest that this area may provide important fruit resources for fueling subsequent, more rapid, migration. The total duration of spring migration averaged 45.9 +/- 4.6 days, but individuals covered the journey of similar to 6,600 km in an average of only 13 days of flight. Males arrived at the breeding site over a 15-day period, and arrival date was significantly correlated with departure date from the wintering site in South America (r = 0.81, P = 0.002), which is surprising, considering the prolonged and variable durations of stopovers en route. Even more intriguing, fall arrival date in South America was significantly correlated with individual departure in spring, which suggests that some birds are on year-round early-versus-late schedules.