Meeting Abstract

S3-1.3  Friday, Jan. 4  Radar evidence for dispersed groups of migrating vertebrates at night LARKIN, Ronald P.; Illinois Natural History Survey r-larkin@uiuc.edu

Several kinds of lore and indirect evidence from previous studies suggest that nocturnally migrating vertebrates (perhaps bats but mostly avian, hereafter, �birds�) sometimes fly in widely dispersed flocks. The observations include stationary and scanning radars, recordings of flight calls, and moonwatching. Direct observations of such groups have been lacking. This paper presents novel tracking radar data on nocturnal aggregations. Statistical analysis of short, straight, detailed flight paths strongly supported the hypothesis (p<0.002) that birds flying closer than 200 to 300 m together fly parallel (in the same direction at the same speed) much more often than similar birds flying apart. This inference was strengthened by comparisons among different segments of the same track (a control for tracking accuracy and bird steadiness of flight) and comparisons with small nocturnal insects tracked by the identical method (a partial control for wind effects in local atmospheric structure). Additional, weaker evidence suggested that birds flying together may have more similar wing beats than expected by chance. Possible functions include not only mutual benefits on the ground during migratory stopover (habitat use, avoidance of predators, and social feeding) but also in-flight sharing of orientation information. The tracking radar method did not provide good information on species composition, seasonality, three-dimensional group structure, cohesiveness of flocks during long-distance migration, or functions of groups at night.