Table 1 Descriptions of the locomotory modes included in this study; descriptions compiled from published sources [9, 10, 44–46]

Forelimb propulsion

Use of forelimbs to produce lift and/or propulsion. This includes all birds that fly, as well as wing-propelled divers, such as penguins and auks. This is to avoid the simple dichotomy of “flighted” and “flightless” birds, as wing-propelled diving requires strong pectoral muscles. Species coded as 0 in this category would be flightless birds that do not engage in wing-propelled diving or flight.

Aerial flight

Flight in air regardless of lifestyle (flightless wing-propelled divers are excluded).

Sustained flight

Flight that can be sustained over medium to long distances.

Terrestriality

Terrestrial locomotion and lifestyle (terrestrial, coded as 'present') as opposed to an alternative lifestyle (i.e., aquatic, arboreal; coded as 'absent'). Although this category does not explicitly pertain to locomotion, it is relevant such that a terrestrial lifestyle influences the manner in which birds takeoff and land, which may influence both flight style and sternum shape.

Cursoriality

Running as a frequent form of locomotion.

Burst flight

Flight that involves explosive burst of flapping in takeoff to escape predator. Seen in Galliformes, Tinamiformes, and some Charadriiformes. This category is often conflated with short escape flights of birds with poor flight capabilities; however, the distinction between the two is crucial, as this specific burst flight behavior is not present in all poorly flying birds and may influence sternal morphology.

Wing-propelled diving

Locomotion involving swimming or diving underwater using primarily forelimb propulsion. This includes flightless wing-propelled divers, such as penguins and the great auk, as well as flying birds that can also dive using wing propulsion, such as dippers, diving petrels, and boobies.

Foot-propelled swimming and diving

Locomotion involving swimming or diving underwater using primarily hindlimb propulsion. This includes surface swimmers, like ducks, and underwater divers like loons and grebes. Some birds in this category are also wing-propelled divers.

Continuous flapping

Type of flight that primarily involves flapping continuously without interspersed gliding or soaring.

Flap-gliding

Flight that involves bursts of wingbeats that are interspersed with short periods of gliding (sometimes referred to as “undulating flight”). This does not refer to glides used when landing.

Soaring

Flight involving gliding either using thermal updrafts (“thermal soaring”), ridge lifts, or wind gradients (“dynamic soaring”).

Scansoriality

Locomotion involving climbing tree trunks and branches.

Intermittent bounding

Flight that involves bursts of wingbeats interspersed with short periods with the wings folded against the body.

Maneuverability

Flight that is highly maneuverable with fast and frequent aerial turns.