– INTERROGATIONS ABOUT THE EVOLUTION OF FLIGHTLESS BIRDS –
The popular ideas we have about the way birds became flightless can seem straightforward, but the actual evolutionary pathway is rather complex. The stories about how evolution crafted biodiversity are fascinating, but they are long stories punctuated by exceptions and oddities. To illustrate some interesting facts and unsolved mysteries, “the true story of flightless birds” will be divided in several parts, presented in different articles.
Part 1 – A diversity of ultimate causes*
– “What permitted flightlessness?” –
Flight is a very successful evolutionary strategy. It helps to escape predators, disperse into new regions offering a better climate or resources, and finding mates more easily. Flight has evolved in three different animal classes: insects, mammals (bats) and birds. Yet its usefulness, some species have found more successful to get rid of flight! Flightlessness has evolved, independently, in a multitude of species, families and orders.
In insects for example, flightlessness is found in flies, moths, bees, wasps, ants, grasshoppers, crickets and beetles. On the contrary, no bats have been found to be entirely flightless, although two extinct and two living species are/were able to live part of their lives walking on the ground. The reasons why no bat ever become flightless and the possibility for bats to become flightless remain unknown.
In birds, complete flightlessness occurs, as well as all parts of the continuum between flying (volant) and flightless. Flying is not binary, and many species are rather “semi-flightless” birds. When birds are reluctant to take off and are only able to fly weakly over short distances, when they mostly flap, jump and glide between branches, it is hard to decide whether they are capable of flying or not. Generally, birds are recognised as being flightless when adults are unable to gain and maintain altitude by flapping their wings[1, 2], but the pathway leading to flightlessness is not the same for all birds.
Five categories of flightless birds are commonly recognised:
Post-dinosaur giants– when the large-bodied dinosaurs vanished, they left empty ecological niches that were promptly filled by giant flightless birds (up to 400kg and 3m tall). Some of these were carnivorous species.
Ratites- when we think about flightless birds, they are generally the ones we first think of: they include ostriches, emus, moas, cassowaries, kiwis, and so on. Many of these species live on continents.
Penguins– the only avian group to develop the ability to fly in a fluid thicker than air (water), modifying their wings in highly specialised flippers.
Island birds– mammal-deprived islands supported an enormous number of flightless birds worldwide, like the Dodo from Mauritius.
The others– the lesser-known exceptions who provide original evolutionary stories and who evolved flightless under a range of conditions.
This article will concentrate on flightlessness in groups other than the giant prehistoric birds and the ratites, as the evolutionary reasons for their flightlessness are different stories.
In all cases, flightlessness resulted from the reallocation of energy used to maintain flight ability into other fitness-related traits. The evolutionary loss of flight is known in at least 26 avian families (in 17 orders). This diversity encompasses cormorants, parrots, pigeons, ibises, grebes, ducks, wrens, auks, doves, rails and more. To explain this process, we generally hear that birds evolve flightless on islands, because of theabsence of predators. However, while predation is a strong determinant, it is not the ultimate* condition. In fact, flightlessness has also evolved on continents, and in the presence of predators. On the other hand, not all species on islands and/or without predators around will ever become flightless.
1) Flightlessness can evolve on islands with predators
Two island types exist, and present different environmental conditions: Oceanic islands and continental islands.
Oceanic islands (created by volcanic activity or tectonic plates movement) are barren rock at the time of their formation. Where plants, insects and birds can easily colonised, mammal species are generally absent from oceanic islands. Since mammals are generally effective bird-predators, and often active on the ground, their absence on these islands made the flightless lifestyle ideal, and probably more than 1,600 flightless species evolved on these islands [3].
“So there are no mammals. What about other predators?”
Being mammal-free doesn’t mean missing all kind of predators. Raptors are also efficient predators, and are present on many oceanic islands. So why don’t raptors prevent the evolution of flightlessness? In some cases, when only one or a few raptor species are present on islands, they don’t specialise on eating birds and rather adopt a generalist diet[4]. Their predation pressure is relaxed and becomes too low to maintain the need of a rapid escape by flying for ground-dwelling birds. Because conserving flying ability consumes lot of energy, birds that invested this energy in something else than flight performed better. To use Diamond’s analogy (1981)[5]: “A winged rail on a predator-free island is like a 60kg backpacker forced eternally to carry 15kg of bricks and to regurgitate half of each meal.” Hence, some mammal-free islands can give way to flightlessness because other non-mammal predators are not applying enough predatory pressure.
On the other hand, continental islands have different features. These islands are connected to mainlands by underwater landmass that can link the two during glacial periods. For example, islands such as Madagascar, New Guinea, Borneo and Tasmania. Since species have opportunities to disperse between the two, these islands possess a sample of the wildlife present on the continent, including mammals, whose predation prevents birds from evolving flightless. However, two continental islands carry exceptions! Both New Guinea and Tasmania are home to a species of flightless bird each: The New Guinea flightless rail (Megacrex inepta) and the Tasmanian native-hen (Tribonyx mortierii), respectively, both from the rail family (Rallidae). These birds have totally lost their flight ability on islands, while living alongside mammal predators. Amazing!
“What’s so special about the New Guinea flightless rail and the Tasmanian native-hen?”
[Note: The Tasmanian native-hen’s story is slightly more complex since it may have evolved flightless on the Australian mainland[6], but it will be the subject of a subsequent article.]
2) Flightlessness can evolve on continents with predators
Yes! Birds can also evolve flightless on continents! It actually happened 7 times independently. However, it did not happen randomly, instead, it’s only present in two specific habitat types.
In four occasions (creating 23 different species after speciation), birds have evolved flightless on continents, around coastal areas: 1) the great auk in North Atlantic (America to Europe – extinct), 2) the steamer-ducks in southern South America, 3) the goose-size sea-duck (Chendytes lawi – extinct) on the Californian coast, and 4) penguins, on all continents of the South Hemisphere.
In three other occasions (creating 4 species), species became flightless in isolated high-altitude lakes of the Andean mountains, but only species of grebes[7]: 1) the Atitlán grebe (Podilymbus giga, elevation 1,562 m – extinct), 2) the Colombian grebe (Podiceps andinus, elevation 3,000 m – extinct), 3) the Titicaca grebe (Rouandia microptera, elevation 3,840 m), and 4) the Junín grebe (Podiceps taczanowskii, elevation 4,080 m).
“Why are coastal areas and high lakes the only places on continents to host flightless birds?”
The idea of evolving flightless on continents could be puzzling because they support a diverse community of predators. To conclude on that argument once and for all that the absence of predators may not be the ultimate condition, there is a striking example occurring in New Zealand, one of the most renowned hotspots for flightless species. The Finsch’s duck (Chenonetta finschi – now extinct) was flightless. And while New Zealand has always been free from mammal predators during its evolutionary times, studies have revealed that the Finsch’s duck was flying during almost 10,000 years before finally starting to lose its flight ability![8]. Something else than the absence of predators must have triggered this change. And as in all the examples cited before, some species have evolved flightless in the presence of mammal predators, on both island and continents.
“So what is the main driver if not the absence of mammal predators?”
The main driver explaining flightlessness is the access to a stable habitat year-round.
A possible explanation of why the Finsch’s duck didn’t lose flight during 10,000 years is that the climate at the time didn’t permit an optimal use of habitat. As a glacial period was ending, new favourable and stable habitats were created, and food supply was probably less fluctuating[8]. Flight is an advantage when you’re limited by food seasonality as you can increase distances to find food.
When blown out onto new islands, birds often arrived in paradises with stable environment with year-round food supply and habitats, with no need to disperse. Moreover, the absence of mammal predators remove the need to escape predators by flying. These optimum conditions were present on over 800 islands worldwide, rapidly promoting convergent evolution of many hundreds of flightless birds (on Pacific islands in particular, where between 500 and 1,600 flightless species were present [3] ).
From another perspective, habitat features can also facilitate escape from predators, and that’s why all birds evolving flightless on continents are aquatic birds: birds on coastal areas and high-altitude lakes specialised in the aquatic environment, reallocating the energy to improve features for diving and swimming[9]. In that regard, it is interesting to note that the flightless Junín grebe is sympatric to the Junín rail (Laterallus tuerosi): they exploit the same lake and surrounding areas. Rails are the bird family who are most prone to flightlessness, but in spite of everything the Junín rail retains its flying abilities, most probably because its habitat around the lake does not permit a safe escape without flight.
So maybe only aquatic flightless birds are able to escape predators? In that case, we shouldn’t find flightless land birds living alongside mammal predators…?
But they exist in Tasmania and New Guinea!
Evolution of mammals in these two places was quite unique. Indeed, marsupial mammals dominated environments for millions of years, and if placental mammals were also present, they were only small species of bat and rodents. The type of predatory pressure applied by marsupials may explain the differences, although many big carnivorous marsupials (marsupial “lion” and “tiger”, and fanged kangaroos) existed, more detailed answers are probably found in their ecological traits.
In sum, while no predation is optimum, the low level of predation is a better explanation to the loss of flight in birds.
Ultimate reasons why birds become flightless is stable habitat year-round
Once this condition is fulfilled, flightlessness in birds is most likely to occur:
on islands where the predation rate is absent or relaxed (lack of placental mammals)
on continent (for aquatic species), when the reallocation of resources (in terms of energy) to increase diving abilities is more successful
The evolutionary story of flightlessness in birds is outlined, but gaps remain; and more scientific evidence is needed to make stronger conclusions. In particular, if year-round habitat stability is hypothesised as the main driver of flightlessness in aquatic birds, is has not always been directly proven [9, 10].
Surprisingly, studies on fossil bats may also help resolving some mysteries and especially to clarify the specific role of predation (and predator-type) on the process of flightlessness. As hypotheses tended to use insularity and the lack of predators to explain walking pattern in bats (at least two New Zealand species), a new species of walking bat discovered in Northern Australia is shifting minds.
More generally, impediments at this endeavour are simply due to our lack of knowledge about species’ biology. In particular, semi-flightless birds have the potential to reveal precious information to break down the steps of the process. Both flightless and semi-flightless species have still a lot of evolutionary stories to tell us. The real blow is that most of these unique species may disappear before giving away all their secrets.
In the meantime, next article will develop questionings about species’ potential to become flightless (why these ones and why no more?) and the proximate causes of that process.
*Proximate and ultimate causes: a proximate cause is an event that is immediately responsible for causing the observed result (e.g., a hole in a boat’s hull caused shipwreck). An ultimate cause is usually thought of as the “real” reason something occurred (e.g., the boat sank because it hit a rock).
All posts are personal reflections of the blog-post author and do not necessarily reflect the views of all other DEEP members.
References:
[1] Livezey, B.C. (2003) Evolution of flightlessness in rails (Gruiformes, Rallidae). American Ornithologists’ Union.
[2] Roots, C. (2006) Flightless birds. Greenwood Publishing Group.
[3] Steadman, D.W. (2006) Extinction and biogeography of tropical Pacific birds. University of Chicago Press.
[4] Wright, N.A., Steadman, D.W. & Witt, C.C. (2016) Predictable evolution toward flightlessness in volant island birds. Proceedings of the National Academy of Sciences, 113, 4765-4770.
[5] Diamond, J.M. (1981) Flightlessness and fear of flying in island species. Nature, 293, 507-508.
[6] Johnson, C.N. & Wroe, S. (2003) Causes of extinction of vertebrates during the Holocene of mainland Australia: arrival of the dingo, or human impact? The Holocene, 13, 941-948.
[7] Livezey, B.C. (1989) Flightlessness in grebes (Aves, Podicipedidae): its independent evolution in three genera. Evolution, 43, 29-54.
[8] Worthy, T.H. (1988) Loss of flight ability in the extinct New Zealand duck Euryanas finschi. Journal of Zoology, 215, 619-628.
[9] McCall, R.A., Nee, S. & Harvey, P.H. (1998) The role of wing length in the evolution of avian flightlessness. Evolutionary Ecology, 12, 569-580.
[10] Roff, D.A. (1994) The evolution of flightlessness: Is history important? Evolutionary Ecology, 8, 639-657.