"Allometry of the Duration of Flight Feather Molt in Birds" von Sievert Rohwer et al. ist in "PLoS Biology" (Bd. 7, 16. Juni 2009, DOI: 10.1371/journal.pbio.1000132.s001)
Birds: moulting limits body size!
With a wingspan of up to 240 centimeters and about 15 kilograms, the swan is one of the largest living flying birds. Most of the feathered animals, however, are significantly smaller. According to a recent study, one reason for the limited size is moulting.
According to the researchers, feathers do not grow back quickly enough as they grow taller. They wear out before they can be replaced by new ones:
https://sciencev1.orf.at/science/news/156030
Abstract
We used allometric scaling to explain why the regular replacement of the primary flight feathers requires disproportionately more time for large birds. Primary growth rate scales to mass (M) as M0.171, whereas the summed length of the primaries scales almost twice as fast (M0.316). The ratio of length (mm) to rate (mm/day), which would be the time needed to replace all the primaries one by one, increases as the 0.14 power of mass (M0.316/M0.171 = M0.145), illustrating why the time required to replace the primaries is so important to life history evolution in large birds. Smaller birds generally replace all their flight feathers annually, but larger birds that fly while renewing their primaries often extend the primary molt over two or more years. Most flying birds exhibit one of three fundamentally different modes of primary replacement, and the size distributions of birds associated with these replacement modes suggest that birds that replace their primaries in a single wave of molt cannot approach the size of the largest flying birds without first transitioning to a more complex mode of primary replacement. Finally, we propose two models that could account for the 1/6 power allometry between feather growth rate and body mass, both based on a length-to-surface relationship that transforms the linear, cylindrical growing region responsible for producing feather tissue into an essentially two-dimensional structure. These allometric relationships offer a general explanation for flight feather replacement requiring disproportionately more time for large birds.
Author Summary
The pace of life varies with body size and is generally slower among larger organisms. Larger size creates opportunities but also establishes constraints on time-dependent processes. Flying birds depend on large wing feathers that deteriorate over time and must be replaced through molting. The lengths of flight feathers increase as the 1/3 power of body mass, as one expects for a length-to-volume ratio. However, feather growth rate increases as only the 1/6 power of body mass, possibly because a two-dimensional feather is produced by a one-dimensional growing region. The longer time required to grow a longer feather constrains the way in which birds molt, because partially grown feathers reduce flight efficiency. Small birds quickly replace their flight feathers, often growing several feathers at a time in each wing. Larger species either prolong molt over two or more years, adopt complex patterns of multiple feather replacement to minimize gaps in the flight surface, or, among species that do not rely on flight for feeding, simultaneously molt all their flight feathers. We speculate that the extinct 70-kg raptor, Argentavis magnificens, must have undergone such a simultaneous molt, living off fat reserves for the duration.
https://journals.plos.org/plosbiology/a ... io.1000132
BTW: In the bearded vulture (Gypaetus barbatus) our biggest bird with a wingspan of 2.9 meters , it takes 40 days for a large feather to regrow!
EDIT: By the way: the feathers show growth markings, similar to those of the trees with the annual rings.
In the case of the feathers, there are day markings