The specimen provides the first unequivocal evidence of immature feathers in the Mesozoic fossil record.
A small piece of Burmese amber preserving feathers interpreted as belonging to a juvenile enantiornithine bird: (A) amber with the dorsal surface of the feather cluster exposed; (B) ventral surface exposed; (C) close up of the ventral surface (region marked in B); (D) close up of the ventral surface region marked in (C); (E) close up of the ventral surface region marked in (D); (F) close up of the dorsal surface marked in (A, larger rectangle); (G) close up of the dorsal surface marked in (A, smaller rectangle). Dotted lines indicate desiccation surfaces. Scale bars – 0.5 mm in (A, B, D and F), 0.1 mm in (C); 0.3 mm in (E); and 0.2 mm in (G). Anatomical abbreviations: ipl – immature plumaceous feather; ipn – immature pennaceous feather; ks – keratinous sheath; pf – probable filamentous ‘protofeathers.’ Image credit: O’Connor et al., doi: 10.1016/j.cretres.2023.105572.
Today, baby birds are on a spectrum in terms of how developed they are when they’re born and how much help they need from their parents.
Altricial birds hatch naked and helpless; their lack of feathers means that their parents can more efficiently transmit body heat directly to the babies’ skin.
Precocial species, on the other hand, are born with feathers and are fairly self-sufficient.
All baby birds go through successive molts — periods when they lose the feathers they have and grow in a new set of feathers, before eventually reaching their adult plumage.
Molting takes a lot of energy, and losing a lot of feathers at once can make it hard for a bird to keep itself warm.
As a result, precocial chicks tend to molt slowly, so that they keep a steady supply of feathers, while altricial chicks that can rely on their parents for food and warmth undergo a simultaneous molt, losing all their feathers at roughly the same time.
Hypothetical molt cycle in juvenile enantiornithine birds: (A) hatchling bird with sparse natal body plumage; (B) rapid molt; (C) juvenile with juvenal plumage including fully developed rachis dominated feathers. Image credit: O’Connor et al., doi: 10.1016/j.cretres.2023.105572.
The cluster of immature feathers preserved a piece of amber from the Hukawng Valley in Kachin Province in northeastern Myanmar is the first definitive fossil evidence of juvenile molting.
The 99-million-year-old specimen reveals a baby bird whose life history doesn’t match any birds alive today.
“This specimen shows a totally bizarre combination of precocial and altricial characteristics,” said Dr. Jingmai O’Connor, a researcher at the Field Museum of Natural History.
“All the body feathers are basically at the exact same stage in development, so this means that all the feathers started growing simultaneously, or near simultaneously.”
However, this bird was almost certainly part of a now-extinct group called Enantiornithes, which were highly precocial.
The authors hypothesize that the pressures of being a precocial baby bird that had to keep itself warm, while undergoing a rapid molt, might have been a factor in the ultimate doom of Enantiornithes.
“Enantiornithines were the most diverse group of birds in the Cretaceous, but they went extinct along with all the other non-avian dinosaurs,” Dr. O’Connor said.
“When the asteroid hit, global temperatures would have plummeted and resources would have become scarce, so not only would these birds have even higher energy demands to stay warm, but they didn’t have the resources to meet them.”
A paper on the findings was published in the journal Cretaceous Research.
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Jingmai O’Connor et al. 2023. Immature feathers preserved in Burmite provide evidence of rapid molting in enantiornithines. Cretaceous Research 149: 105572; doi: 10.1016/j.cretres.2023.105572
