Short Communication Optimisation of the Flight Speed of the Little, Common and Sandwich Tern

Studies of the mechanical performance of birds during flight are rarely accomplished with wild birds because accurate tracking of flight paths is necessary. In this study, we present data from three species of tern as they flew along a straight flight path on the north Norfolk coast, UK. Measurement of wind speed and the time taken to fly a set distance allowed direct calculation of the airspeed. The airspeeds were compared with the maximum range speed, Vmi, and the minimum power speed, Vmp, values predicted from aerodynamic models generated with data from, for example, observations made in wind tunnels (Tucker, 1969) and from both radar and double theodolite tracking systems (Tucker and Schmidt-Koenig, 1971). Data for the little tern, Sterna albifrons, the common tern, Sterna hirundo, and the sandwich tern, Sterna sandvicensis, were recorded over a period of 7 days at the end of July 1991. Observations were made at two sites at Blakeney Point, and at Scolt Head Island. The shoreline at these locations was very straight and offered clear views in both directions. Terns flew a straight course along the coast, and a 40 m segment of their flight was measured as they flew in front of a set of marker posts, aimed like gunsights, on the beach. Recordings were made at each end of the 40 m course for both the time that the terns passed (on synchronised stopclocks) and for the number of wingbeats counted during the course. Because of the nature of this technique, terns measured flying on a slightly oblique path will bias the calculated airspeeds to be too low; it is estimated that this will introduce an error of not more than 2 %. The calculated airspeeds were most sensitive to timing errors, and it is estimated that these can produce deviations of up to 3 %; any bias due to anticipation of the terns' arrival may have been duplicated at both ends of the course, and thus may have been removed when the two absolute times were subtracted. The wind speed was measured with an anemometer at a height of 1.5 m, and the wind direction was measured with a magnetic compass and a piece of freely blowing cotton. The wind speed profile was measured up to a height of 4.5m for two wind speeds, 6.2ms" and 9.4ms" (at 1.5 m); it was estimated that there was greater than 95 % confidence for the wind speed measurements for all