Sievert Rohwer, retired from the U and the Burke, had us eating out of his hand on December 2nd with his inside view of molt and what we don’t know about it! Now, thanks to his work, we know that feather regrowth can’t be hurried, that all feathers grow at the same rate in most species, and that main differences between molt strategies have to do with how many feathers are molted at the same time: Simple (one at a time), Complex (various strategies including stepwise), and Simultaneous, with most feathers molted together. But each feather takes the same amount of time, whether all at once or one at a time. An extreme example of this is that it would take Argentavis, the giant prehistoric bird, almost 2 years to molt each primary one at a time, but it would take 7 months to molt 3 at a time, which could have been doable in a beast that size (like the giant prehistoric penguins).
We also know that breeding and molting in larger birds might not happen in the same year due to the cost to the bird. Studies on the breeding grounds of Laysan Albatrosses have determined that 20% of the population fails to appear each year to breed, but that they come back to breed the next year, so they spend some away time recovering and molting. However, Laysan males might attempt to breed even before they’re fit in order to maintain their pair bond in a female-centric society. Their pattern appears to be alternate years of major molts and then smaller molts, with P6-P7 being the key.
During molt, Western Kingbirds, Black-crowned Night Herons, and Double-crested Cormorants have been studied, but few other species. Much research remains to be done and could keep graduate students busy for many years. In the case of Laysan and Black-footed Albatrosses, some years ago, researchers were aboard trawlers that used drift nets in a study to determine how dangerous these nets were to these threatened species, which is why these two species have been studied, but no other albatross species (and yes, drift nets were determined to be a menace). The recovered albatross specimens from this once-in-a-lifetime study have been keeping researchers busy ever since.
Some interesting birds demonstrate seemingly “chaotic” molt, including herons, cuckoos, and kingfishers. In a study done at the U on specimens of cuckoos shipped in from many museums and schools, “transilient” molt was found, where blocks of feathers will molt, each block separated by a node, but each individual feather surrounded by non-molting feathers on each side. This is not well understood or well observed yet except in Cuckoos, especially the Common Cuckoo, and of course they don’t molt on our continent.
An interesting factoid is that if a feather is cut or traumatized, even though the keratin is “dead,” some sensor, perhaps in the filoplume, responds and starts an immediate molt and regrowth of that feather. Another interesting item was growth bands on feathers!! Yes, like trees!! Light days and dark nights show up in daily bands on young feathers. And many more fascinating tidbits, like those Limpkins and their supposed proximal molt pattern.
Two main points: (1) all of this applies just to primaries, which are the easiest to study; and (2) we’re still learning the “rules” of molt.
Thanks, Dr. Rohwer! We will be looking forward to having you back and hearing your work on migration!
Photos below – Click to enlarge:
