Flight and Reproductive Biology

The Thrust-Lift Anomaly

A truly oceanic lifestyle requires a bird to travel long distances between its nest and sparsely distributed patches of prey. The ability to travel such distances has been achieved only in the auks and petrels, two groups with strongly contrasting styles of flight. The petrels, particularly the albatrosses, exhibit an unusual degree of energetic efficiency while the auks seem to expend energy flagrantly to cover large distances at speed. Their success suggests that there might be unsuspected benefits to aerial speed.

Marbled Murrelet

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Surprisingly, if a bird is able to fly very fast, it is also able to carry relatively heavy objects at little extra cost. Typically speed in birds is achieved by very rapid wing beats that consume a great deal of energy. Consequently, a fast-flying bird must have a food source that reliably provides adequate energy. In auks the energy comes from the oily fishes and crustaceans on which they prey.

Fast flight requires that birds devote an inordinate amount of energy to thrust compared to lift. As a result, increases in load (or flight uphill) that represent a change in the amount of energy devoted to lift, represent relatively small increases in the total energy cost of flight.


Slow Flight	Fast Flight

The effect of this anomaly is so strong that an auk can carry an egg that is far larger for their body size than that of any land bird.


Black-footed Albatross	Xantu's Murrelet

Auk eggs are even larger than those carried by petrels of similar size.

see Data Appendix

Most auks cannot deliver loads of food to their young that are as large as those delivered by petrels. Perhaps the auk’s habit of carrying prey crossways in the beak increases the energy cost of flight by increasing drag. Petrels carry food for their young internally as does the Cassin’s Auklet, one of the few auks to deliver food loads comparable to those of petrels.