Data from: High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins

Flight is a key adaptive trait. Despite its advantages, flight has been
lost in several groups of birds, notably among seabirds, where
flightlessness has evolved independently in at least five lineages. One
hypothesis for the loss of flight among seabirds is that animals moving
between different media face tradeoffs between maximizing function in one
medium relative to the other. In particular, biomechanical models of
energy costs during flying and diving suggest that a wing designed for
optimal diving performance should lead to enormous energy costs when
flying in air. Costs of flying and diving have been measured in
free-living animals that use their wings to fly or to propel their dives,
but not both. Animals that both fly and dive might approach the functional
boundary between flight and nonflight. We show that flight costs for
thick-billed murres (Uria lomvia), which are wing-propelled divers, and
pelagic cormorants (Phalacrocorax pelagicus) (foot-propelled divers), are
the highest recorded for vertebrates. Dive costs are high for cormorants
and low for murres, but the latter are still higher than for flightless
wing-propelled diving birds (penguins). For murres, flight costs were
higher than predicted from biomechanical modeling, and the oxygen
consumption rate during dives decreased with depth at a faster rate than
estimated biomechanical costs. These results strongly support the
hypothesis that function constrains form in diving birds, and that
optimizing wing shape and form for wing-propelled diving leads to such
high flight costs that flying ceases to be an option in larger
wing-propelled diving seabirds, including penguins.