What were hindwings for?
After discussing pterosaur hindwings I feel its only fair to talk about the hindwings that we know actually existed. First conceptualised by William Beebe in 1915 based on bird embryo membrane vestiges, domestic breeds with hindwings and long leg feathers on Archaeopteryx specimens, this hypothesis would only further be vindicated in the passing decades, most notably with the discovery of Microraptor. In total, it seems hindwings occured multiple times in avian evolution: in microraptorans, Pedopenna (which according to Hartman et al 2019 might be a scansoriopterygid), anchiornithids, Archaeopteryx, omnivoropterygids and at least two enantiornitheans, with the long leg feathers of some modern raptors arguably counting as hindwings as well.
While hindwings have at times been brought up as being ancestral to birds and possibly Maniraptora as a whole, recent findings seem to imply that they have evolved independently multiple times. Microraptorans are nested deeply among Deinonychosauria, usually as dromaeosaurids, yet they are the only members of this group to have hindwings, with many well preserved dromaeosaurid and troodontid fossils lacking them. They might be ancestral to Avialae, especially if you interpret either Archaeopteryx or anchiornithids as closer to crown-Aves than to each other, but they’re clearly not ancestral any further than this: scansoriopterygids, the two enantiornitheans and modern predatory birds are all deeply nested among clades with no hindwings.
Therefore, while they might possibly have something to do with the acquisition of avian flight, the more likely explanation is that they evolved independently multiple times to fill the same biomechanical role.
Many studies have been made about Microraptor‘s exceptionally well-developed hindwings, including positing scenarios where it glided in a biplane model; however, several recent studies not only find this animal to be a poor glider but also more likely a burst or generalist flyer like a chicken or a crow respectively, rendering this biplane model unlikely (Agnolín 2013, Dececchi 2016, Pei 2020, Dececchi 2020). So, its unlikely that the hindwings were utilised in this bizarre fashion.
To answer the question of what hindwings were likely used for, we probably have to turn to living analogues. Modern raptors like falcons and booted eagles have long leg feathers, in many ways similar to the condition seen in Archaeopteryx, Pedopenna, omnivoropterygids and enantiornitheans (see cladogram above). No bird of prey uses the hindwings as a secondary lift generator most of the time, as they fly with the legs tucked against the body. However, they do seem to provide a pivotal role in shock absorption, being extended as the legs lunge forward.
Many of the extinct paravians with hindwings are predatory to some extent (microraptorans, Archaeopteryx, likely the enantiornitheans and anchiornithids), so the analogy to modern raptors is not a bad idea. Some are not (omnivoropterygids, possibly Pedopenna) but similar pressures towards shock absorption would make sense since these were still arboreal animals that likely landed against branches multiple times.
In most cases, the “hindwings” are seldomly more complex than the long feathers of modern birds of prey. Only microraptorans and anchiornithids have large airfoils mirroring the true wings; this could imply a more unique role, but personally I don’t see how they would differ significantly from the function in other flying dinosaurs. As noted previously, Microraptor was not an agile sky-meister like a Cretaceous peregrine falcon, just something that either flew ‘normally’ like a parrot or flapped for short distances like a turkey. Its hindwings were exaggerated but most likely not functionally any different from those of other dinosaurs, perhaps even a developmental quirk like those of modern domestic pigeon and chicken breeds.
To conclude, I don’t they they particularly hint at the development of bird flight, since they evolved multiple times, but the possibility that they are ancestral to Avialae at least is worth investigating.
References
Welker, Robert Henry (1975), Natural Man: The Life of William Beebe, Bloomington, Indiana: Indiana University Press, ISBN 978–0–253–33975–1
Sankar Chatterjee, R. Templin, BIPLANE WING PLANFORM AND THE FLIGHT PERFORMANCE OF MICRORAPTOR GUI, September 2005, Journal of Vertebrate Paleontology 25(3):43A-44A
Hartman, Scott; Mortimer, Mickey; Wahl, William R.; Lomax, Dean R.; Lippincott, Jessica; Lovelace, David M. (2019). “A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight”. PeerJ. 7: e7247. doi:10.7717/peerj.7247. PMC 6626525. PMID 31333906.
Federico L. Agnolín & Fernando E. Novas (2013). “Avian ancestors. A review of the phylogenetic relationships of the theropods Unenlagiidae, Microraptoria, Anchiornis and Scansoriopterygidae”. SpringerBriefs in Earth System Sciences: 1–96. doi:10.1007/978–94–007–5637–3. ISBN 978–94–007–5636–6. S2CID 199493087.
Dececchi, T.A.; Larsson, H.C.E.; Habib, M.B. (2016). “The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents”. PeerJ. 4: e2159. doi:10.7717/peerj.2159. PMC 4941780. PMID 27441115.
Pei, Rui; Pittman, Michael; Goloboff, Pablo A.; Dececchi, T. Alexander; Habib, Michael B.; Kaye, Thomas G.; Larsson, Hans C.E.; Norell, Mark A.; Brusatte, Stephen L.; Xu, Xing (2020). “Potential for Powered Flight Neared by Most Close Avialan Relatives, but Few Crossed Its Thresholds”. Current Biology. 30 (20): 4033–4046.e8. doi:10.1016/j.cub.2020.06.105. PMID 32763170.
Dececchi, T. Alexander; Larsson, Hans C. E.; Pittman, Michael; Habib, Michael B. (2020). “High flyer or high fashion? A comparison of flight potential among small-bodied paravians” (PDF). Bulletin of the American Museum of Natural History. 440: 295–320.
