The roles of head movements in the search and capture strategy of a tern (Aves, Laridae)

Gull-billed terns (Gelochelidon nilotica) were video-filmed while searching for and capturing fiddler crabs. Search consists of a vertical head nystag mus, with fast upward flicks and downward slow phases made at the angular s peed of the substrate in the approximate direction of the bill. The bill po ints down at about 60 degrees during hunting, but is brought up to 15 degre es from time to time, which brings the visual streak into line with the hor izon; 45 degrees roll movements of the head are consistent with alternation between the use of the temporal and central foveas to view the same object . When a crab has been detected the nystagmus is suspended, and the tern tr acks the crab continuously as it manoeuvres into a catching position. This may involve tucking the head under the body so that the bill is 45 degrees behind the vertical, and flying up and backwards for some metres, straighte ning up the head at the same time.