When birds need to get out of a tight spot quickly, smaller birds fair better than larger ones. Brandon Jackson and Kenneth Dial from The University of Montana explain that bird manoeuvrability decreases as size increases. The mechanism behind this scaling effect isn't clear; however, it has been suggested that ‘burst performances’ may be limited by the amount of power the flight muscles can produce and that this power output is limited by frequency of the bird's wing beat. Curious to find out if this is true, Jackson and Dial measured the wing beat frequency and muscular mechanical power produced by the pectoral muscles of members of the crow family ranging in size from a 69 g gray jay to a 0.89 kg common raven to find out how they varied with size (p. 452).

Plotting the maximum muscle-mass-specific power against the birds' masses, Jackson and Dial calculated the gradient of the graph and found a weak scaling relationship between the birds' maximum muscle-mass-specific power and their masses, with the smaller birds producing slightly higher maximum muscle-mass-specific powers than the larger birds. And when they plotted the birds' wingbeat frequencies against the body masses, they found a stronger scaling relationship, with the smaller birds beating their wings much faster then the larger birds. However, plotting the muscle strain from the pectoral muscles against the birds' masses, Jackson and Dial found that the relationship switched: this time, the largest birds produced the largest strains. This led them to conclude that the birds' muscle power was ‘not limited solely by wingbeat frequency’ and they speculate that large birds may benefit from having longer muscle shortening durations, thus maintaining average muscle stresses for longer, resulting in the positive scaling relationship between muscle strain and body mass that they found.


B. E.
K. P.
Scaling of mechanical power output during burst escape flight in the Corvidae
J. Exp. Biol.