Flights of Fancy in Avian Evolution
From mousebirds to terror birds, the class Aves has encompassed a remarkable diversity of species over the past 150 million years.
All of the extinct bird species illustrated so far died out due to natural factors such as sea level change, competition with other groups, or shifting forest environments. However, some astounding birds would still be extant today if not for human callousness. The list of casualties includes birds that range in size from minuscule to gigantic and in ecology from creeping insect hunters to crushers of large fruits. These birds have one factor in common: They all inhabited islands.
Throughout human history, wherever new people alighted as they migrated throughout the world, the situation turned grim for endemic birds. More than 500 extinctions of avian species on islands appear to have occurred in association with human arrivals. The timing of these extinctions ranges from roughly 50,000 years ago to just a few years in the recent past. In some cases, such as the moa of New Zealand, humans hunted a particular species for food, but in many others, indirect effects such as the introduction of nonnative predators or burning of woodlands were the drivers of extinction.
The list of vanished island species includes the largest terrestrial birds ever to have lived. In Madagascar, the elephant bird Aepyornis maximus reached heights of approximately 10 feet. In New Zealand, a radiation of a dozen or so moa species (many huge in size) browsed the land, filling the role that deer do in other ecosystems. These titans are well known to scientists and museum-goers, but many less imposing taxa disappeared as well. These oddities included the flightless, nocturnal, scent-foraging Kaua’i mole duck of Hawaii, which appears to have died out about 1,500 years ago, the iconic dodo and its cousin the solitaire (relatives of pigeons and doves wiped out in the 17th and 18th centuries), and several ground-dwelling New Zealand wrens including the tiny bush wren, which died out in 1972 after rats breached its last remaining stronghold and relocation efforts failed.
Although extinction is a natural part of evolution, the recent extinctions that have afflicted island birds are not natural. Whereas the Cretaceous mass extinction wiped out some avian lineages and opened new doors for others, today we are seeing avian biodiversity lost and replaced with homogenous sets of invasive species, reducing the resilience of these island ecosystems.
A dangerous misconception is that flightless birds like the dodo were destined for extinction because they were slow, foolish, or otherwise poorly adapted. However, these species fell victim not due to some flaw but because the environment to which they were well suited shifted too quickly. For example, species that nest on the ground on islands lacking land mammals benefit by staying hidden from aerial predators like hawks. This advantageous behavior suddenly becomes perilous when nonnative terrestrial predators such as cats are released.
Perhaps it is easier to turn away from this history of human-caused extinctions, but doing so will exacerbate the problem: The same phenomenon is occurring today in places like Guam, where introduced snakes are wiping out endemic birds. There are still opportunities to prevent the addition of more species to the island extinction toll. Whether we will act in time, though, remains a pressing question.