Optimal climbing flight in migrating birds: predictions and observations of knots and turnstones

Abstract

Abstract. A prediction of optimal climb rate up to the cruising altitude for birds setting out on a migration flight is developed. The argument is based on aerodynamic flight theory and minimization of energy expended during the whole flight. Predicted climb rates for knots, Calidris canutus, and turnstones, Arenaria interpres, are compared with observational data of these species when departing on spring migration from north-west Iceland. The data agree reasonably well with the predicted values, but the birds failed to show an expected negative relationship between airspeed and climb rate. It is argued, on the basis of measurements by weather balloons, that observations are likely to have been affected by turbulent air as a result of orographic wind effects at the hilly study site. Shorebirds seem to be able to locate and take advantage of lift while climbing. The test of the predictions should be regarded as provisional and future studies are required to refute or give support to the model.