Flight speeds of central place foragers: female Lapland longspurs feeding nestlings

According to a recent model (Norberg 1981), birds feeding nestlings should fly faster than V_mr, the speed which minimizes the cost of transport over the ground. The flight speeds of female Lapland longspurs (Calcarius lapponicus) feeding nestlings were not significantly different from the predicted V_mr under various wind conditions. They were, however, significantly higher than the speed that minimizes the rate of energy expenditure (i.e. minimum power speed, V_mp) and significantly lower than the optimal speed (V_opt) predicted from Norberg's model. Thus, maximizing the distance travelled per unit energy expenditure appears to be the most important energetic factor affecting their flight speeds. We suggest that there may be no benefit to increasing air speed above V_mr when brood size is smaller than the maximum number that parents can successfully nourish, or when travel time makes up a small proportion of the total foraging time.