A size comparison of the (extinct) elephant bird, an ostrich, a human and a chicken.
Birds are synonymous with flight in American culture. That's because we don't have flightless birds in our day-to-day lives. But if you lived where ostriches or penguins were in the neighborhood, your impression might be different.
Some species can fly but prefer to walk. California quail, greater sage-grouse and wild turkeys are good examples. I recall the first time I saw a flock of sage-grouse flying through a pass near Blizzard Mountain, northwest of Craters of the Moon. I had seen grouse fly many times before, short distances from leks to nearby points. But I'd never seen anything like this. They flew like rockets.
Anybody who has traipsed the foothills or montane forests in southwestern Idaho has probably been startled by the flushing of quail, grouse or even mourning doves. They often wait until the last second to fly -- surprise! These birds prefer to hide and then quietly walk away. Flying is energetically expensive.
Even robins, sparrows, juncos and other ground-foraging birds will often first try to hop away from danger rather than fly away. Walking and hopping also makes them less susceptible to predation. Once you get up in the air -- especially after being startled -- Cooper's hawks will thank you.
If you live in the Southwest, you might also have a different perspective. Greater roadrunners famously dash around on the ground, overtaking various lizards, birds and insects. But they only fly if they have to. Roadrunners have short, broad wings -- like those of quail and grouse -- providing for a sudden burst of flight.
But all of these species can fly. There are around 60 species of birds on Earth that can't fly, even if they wanted to. Of course, we don't know what they "want" -- that's another story.
Most flightless species are found in a morphological (not taxonomic) group called ratites (ostriches, rheas, emus, cassowaries and kiwi). Ratites have no keel on their sternum. Without this anchor for wing muscles, they can't fly even if they have wings.
Other flightless species occur within the Rallidae (rails, gallinules and coots), and Spheniscidae (penguins). We also have New Zealand's kakapo, the world's only flightless parrot. The kakapo is high on my wish list.
Hundreds of other flightless bird species have been driven to extinction by people, many over just the last few centuries. They were easy to hunt. The dodo is the most famous example, being posterized through no fault of its own.
Even the 1,500-pound elephant bird was relatively easy pickings for an organized group of humans. You know their drumsticks would sell at the Western Idaho State Fair.
Recent research by Klara Widrig, a postdoctoral research fellow at the Smithsonian, has put a new spin on our understanding of flightless birds. The short version is that she and her colleagues propose that distant ancestors of ostriches and other large flightless birds once flew great distances.
Ornithologists have struggled to explain why related flightless birds are found in South America, Africa, Australia and New Zealand. They obviously didn't fly around the world. Other explanations, such as that they floated around on oceanic debris, seemed unlikely.
Looking much further back in time, some scientists hypothesized that ancestors of these species were living when the Earth's land was united in the supercontinent Pangea. When it started to break up -- around 200 million years ago -- perhaps the flightless birds simply drifted apart from each other along with the continents they lived on.
But genetic research has discredited this theory. It turns out ostriches, rheas, emus, cassowaries and kiwi did not share a common flightless ancestor. This means flightlessness evolved in several independent situations. This is termed convergent evolution -- a process well demonstrated for a variety of traits. Consider the wings of birds, bats and insects for one obvious example. An attribute with great survival value evolved in unrelated groups. Consider the eyes of dragonflies, octopuses and humans.
However, the Pangea-related theory has since been discredited. Genetic research shows lineages started to separate millions of years after Pangea broke up -- far too late for the continental drift theory to be true.
Widrig's research on the skeletons of the ancestors of these flightless species shows some were built much like today's herons, and with keels on their sternums, giving them the capacity for sustained flight. They propose that they dispersed by flight to these continents and then evolved flightlessness over time.
Back to my original point, no matter how inspiring humans think flying is, it's hard to do and it's energetically expensive. If you don't have to fly to obtain food or attract mates or escape predators, why would you do it? Today, most species do fly to obtain food, attract mates and/or escape predators. The advantages of being able to go airborne vastly outweigh the costs for most species.
A few more fun facts for your crossword puzzle arsenal. The smallest flightless bird alive today is the Inaccessible Island rail (0.1 pound), a name that could hardly be more fitting. The largest is the common ostrich, with the largest males reaching 346 pounds. You may not know there is also the Somali ostrich, with males up to around 290 pounds.
The largest living penguin, the emperor penguin, only reaches around 100 pounds. The largest animals to ever live on Earth have been in the water, where buoyancy helps support weight. It's curious that no living penguin species are particularly large. However, an ancient species -- Kumimanu fordycei of New Zealand -- weighed up to 340 pounds. Would you want to swim with them?
Because birds have evolved flight and then flightlessness several times in history, you might wonder if the same thing has happened in mammals. Did flight evolve in small mammals to give us bats, and then go in reverse to give us flightless bats?
Theory says no. Bat forelimb (wings) and hindlimb (legs) evolution have been closely coupled over time. When wing shape changes, leg shape tends to follow suit. This is unlike the situation in birds.
But reality says something a little different. The common vampire bat is adapted to a terrestrial lifestyle. Vampire bats can walk, run and even jump. Fun or what?! Happy Halloween!