Radar reveals the nocturnal flights of breeding Grey Gulls Leucophaeus modestus in the Atacama Desert, Chile

Capsule: Grey Gulls Leucophaeus modestus breed in the Atacama Desert, Chile, and perform nocturnal commuting flights between their inland nesting sites and the coast in order to forage. Some aspects of the gullś flight patterns and routes remain unknown. Aim: Determine the patterns and the routes of the commuting flights of Grey Gulls breeding in a remote colony in the Atacama Desert. Methods: Using a modified marine radar we tracked the departure and arrival times, and direction of Grey Gulls. Furthermore, we managed to ground truth the information obtained from the radar by tracking two incubating gulls with GPS tags. Results: Birds left the colony in two waves: first between 21:00 and 22:00 at the onset of nautical twilight, and a second, larger wave between 02:00 and 05:00 (peak at 03:15) before sunrise. Birds returned to the colony through the whole night with a major peak at 23:00, two hours after sunset. Tracking data confirmed the travel times and directions obtained from the radar. Conclusion: Grey Gulls adjust their departure time from the colony and from the coast to fly only in the dark. Most birds left just after nautical twilight and flew at a constant speed to the coast. By knowing the arrival time, the peak of bird passage and their inbound flying direction, it is possible to infer the location of the departure place on the coast. By identifying consistent inbound flight patterns with radar it is possible to detect unknown colonies.