Competition and cooperation among chimney swifts at roost entry.

Chimney swifts (Chaetura pelagica) are highly aerial, small, insectivorous birds well known for roosting en masse in chimneys during their autumn migration. These roosting events require hundreds to thousands of birds to enter a small opening (here 0.64 m) within a short amount of time (15-30 min). Thus, these entry events pose a complex navigational and behavioral challenge as birds identify their entry route, cooperate with other birds present to form an entry flock, and compete with other birds at the time of chimney entry. We used six synchronized cameras to capture and reconstruct the 3D flight trajectories of swifts before and during chimney entry. Navigation into the chimney is consistent with use of a relative retinal expansion velocity cue, which results in an entry/non-entry decision point about 1.5 m above the chimney, or 0.4 s at typical entry speeds. Entries were highly clustered with 91 of 136 entries occurring within 1 s of another entry. We observed both synchronous (entry within 0.2 s) and sequential entry behavior (entry separated by ~0.4 s). Birds entering the chimney flew in close proximity to other birds (median minimum distance 0.51 m; 1.7 wingspans). In cases where two birds appeared to attempt a near-simultaneous entry, the bird either slightly to the rear or with a velocity vector bringing it closer to the current position of the other bird tended to make an avoidance maneuver and abandon its entry attempt. Overall, these results show how groups of animals execute complex landing and collision avoidance maneuvers in a natural setting without central control authority.