The speed of birds in flapping flight is a behavioural attribute that, when interpreted in the light of optimization theory, may provide important implications about the limitations in time, energy and safety that affect birds in different situations. This study is an evaluation and review of optimal flight speeds of birds, based on foraging, migration and flight mechanical theory. Flight in different ecological contexts is considered, such as foraging flight, food transportation flight, migration flight and aerial display flight. Relevant optimization criteria and immediate currencies are identified for these flight situations, permitting the derivation of optimal flight speeds. Foraging birds are expected to maximize foraging gain ratio (the ratio of gross energy intake rate to the cost of foraging in excess of the resting metabolism) when energy minimization is of imminent importance or when they are constrained by a metabolic ceiling. In other circumstances they are expected to maximize the net energy intake rate. Generally, optimal flight speeds are faster in the latter than in the former case. Thus when the foraging gain ratio is maximized the optimal flight speed between foraging patches is V
mr
(speed of minimum energy cost per unit of distance flown), whereas it is faster than this, to a variable degree depending on the quality of and distance between patches, when net energy intake rate is maximized. Birds should adapt their flight speed differently when transporting food or migrating as compared with flying in pure foraging situations. Cost of transport (energy/distance) or resulting speed of transport or of migration (distance/time) are the immediate currencies relevant for predicting optimal flight speeds depending on whether birds in food transportation flights are metabolically constrained or not and whether migrating birds are energy- or time-selected. Optimal flight speeds for maximizing the resulting speed of transport or of migration exceed V
mr
to an increasing degree with an increasing rate of food/energy gain. Still other optimization criteria apply to further flight situations that are reviewed, and, in addition, flight speed is expected to vary with wind, load, altitude, climb rate and flock size. Optimal flight speed theory provides a possibility to use flight speed measurements of birds in widely different situations for obtaining insights about crucial time and energy limitations.
Downy woodpecker foraging behavior: foraging by expectation and energy intake rate
