Q. I am pretty sure that ostriches are the biggest birds in the world. But what is the biggest bird in North America?

A. The first step is to define “big.” Andrew Lydeard, program coordinator of Alabama Audubon, points out that at least three measurements can be used—tallest when standing; body weight; wingspan. Based on the first two criteria, ostriches do indeed reign as the largest birds on earth today. But when it comes to wingspan, a flightless bird like an ostrich pales in comparison to many of the others.

In the United States the widest wingspan record is held by the California condor. According to the Cornell Lab of Ornithology, these enormous raptors can have a wingspan of 9½ feet. Compared to bald eagles with their 8-foot wingspan (max), California condors are the clear winners. We are fortunate to be able to see these small-airplane-size birds in the western states. A disquieting thought is that condors were within a few birds of extinction until some extraordinary recovery programs were established in the last century. Fortunately, outstanding conservation efforts on the condors’ behalf continue today, but making sure they can survive on their own remains a challenge.

The tallest North American bird (about 5 feet) is the whooping crane, another bird that was on the verge of extinction before the development of captive breeding programs by the Canadian Wildlife Service and the U.S. Fish and Wildlife Service. In 1941 fewer than two dozen whooping cranes existed. Today they number around 600. The award for the heaviest bird in the country depends on who you talk to and probably when and what the bird’s last meal was. Some say wild turkey whereas others say trumpeter swan. Not too far behind is the American white pelican.

Although African ostriches go uncontested as the tallest and heaviest birds on earth today, the biggest birds ever known were another group of flightless birds, moas. The tallest moas could almost have bumped their heads on the rim of a regulation basketball goal (10 feet) without jumping. Some weighed more than 400 pounds. Moas had successfully evolved on the islands of New Zealand by being able to fend off most natural predators without needing to fly. Then came the Polynesian settlers known as Maoris. The big birds had never encountered such relentless land predators as humans. The Maoris could easily capture and kill them. And they did. By the time Columbus reached America, all species of giant moas had been extinct for a century.

 Around the same time and place, Haast’s eagle had not yet gone extinct. Haast’s eagles are indisputably the largest eagles known to science. The biological evidence, based on skeletal material examined by New Zealand scientists, was published in the Journal of Vertebrate Paleontology. As with modern birds of prey, including hawks, owls and eagles, females typically got larger than males, which weighed 27 pounds. Female Haast’s eagles are estimated to have reached a body weight of more than 39 pounds. The weights are based on only a few specimens, so the largest ones probably weighed more than 40 pounds. A 40-pound flying bird is huge. To put the size in perspective, the largest bald eagles typically weigh less than 15 pounds. What was the primary prey of these giant eagles? Moas, the big flightless birds, whose disappearance as prey assured that the eagle predators would not be far behind.

Big birds are found on every continent, even Antarctica (emperor penguins), but the lesson from the moas and countless examples over environmental history make it clear that size doesn’t matter when it comes to extinction. Humans have demonstrated again and again that we can rapidly wipe out any species, whether on purpose or inadvertently. On our current trajectory, a small proportion of the world’s human population could eliminate many of our remaining large species of birds, not to mention mammals and reptiles, within our lifetimes. We should not allow this to happen.

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