Figure 1.
Relationships between mass and flight feather growth rate, length of flight feathers, and completeness of flight feather molts.
(A) Allometric relationships between log10 mass (g) and log10 primary growth rate (mm/d) and log10 summed primary length (mm). Arrows indicate the estimated time required to replace all primaries if they are grown one feather at a time for birds of 10 g, 10 kg, and for the extinct teratorn that weighed 70 kg. (B) Same as (A), except primary length is the longest primary. Arrows indicate the estimated time required to replace all the flight feathers in a simultaneous replacement of the primaries. (C) Fraction of individuals (n = 20 for most species, see Table S1) showing complete replacement of their primaries in their last molt, plotted against log10 mass for 77 species that fly while molting (black dots and loess curve), and the same relationship for 17 species that replace their flight feathers simultaneously (open circles).
Figure 2.
Body size distributions associated with different patterns of primary replacement; dark feathers are points of molt initiation.
(A) Size distribution for all flying birds (wing shows primary and secondary flight feathers). (B) Size distribution for species with simple molts, characterized by a single wave of primary replacement. (C) Size distribution for species with complex molts that feature either (1) multiple waves of primary replacement proceeding distally through the primaries, or (2) division of the primaries into two molt series with feathers replaced in opposite directions. (D) Size distribution for species that replace their primaries (and secondaries) simultaneously (wing shows lost primaries and secondaries depicted by dotted lines). Most species with simple molts (B) fail to approach 15 kg, the approximate upper size limit of birds set by the power available for flight because such large birds would require too much time to replace their flight feathers. But species with complex (C) and, especially, simultaneous primary replacement (D) reach the upper size limit of birds set by the power required for flight because these modes of primary replacement require less time than simple molts.
Figure 3.
Deviations from the growth rate regression in Figure 1, plotted separately for species that fly while molting and for species that replace their wing quills simultaneously.
The latter grow their primaries no slower than birds that fly while molting, suggesting that follicular constraints on the rate of feather synthesis, rather than energetic costs, limit the rate at which flight feathers grow.
Table 1.
Data used to generate the allometric equations of Figure 1A and 1B, and sources for the data on primary growth rate; primary lengths are from museum specimens.