Earth's last passenger pigeon—Martha—died in 1914 at the Cincinnati Zoo, the final remnant of flocks that once darkened the sky. What's unknown, even nearly a century after this extinction, is how many of the plants in the eastern woodlands of the U.S. might have suffered as a result—or even gone extinct themselves. Now, new findings from halfway around the world suggest plants may indeed suffer in the absence of the animals they have relied on for pollination or dispersing seeds.

The work was done by a team of biologists, fresh from showing that New Zealand's mistletoe species are struggling due to a lack of pollinating birds, who set out to determine if other plants shared this problem. After all, the country has lost nearly half of its native land bird species since 1839.

They homed in on one native shrub—Rhabdothamnus solandri, or New Zealand gloxinia, renowned for its brilliant orange flowers—which relies on several species of birds for pollination, and is found nowhere else in the world. The tui (Prosthemadera novaeseelandiae), bellbird (Anothornis melanura) and stitchbird (Notiomystis cincta) all have beaks and long tongues perfectly suited for the shrub's 10-millimeter-long orange flowers. But bellbirds and stitchbirds are gone on the New Zealand's North Island thanks to bird-eating animals brought along by European colonizers, such as house cats (Felis catus), ship rats (Rattus rattus), stoats (Mustela erminea) and brushtail possums (Trichosurus vulpecula).

Fortunately, those bird species survive on some small islands off the coast—Little Barrier, Tiritiri Matangi and Lady Alice—too small to have had much contact with humans or the invaders they often bring along.  And in many cases mammal pests, such as the feral cats that roamed Little Barrier Island until 1980, have been eradicated.

So the biologists painstakingly catalogued the instances of the flowering shrub in plots on both the bird-friendly islands and the mainland. As a result of the missing birds, the flowering shrubs on the mainland produce smaller fruit and only 37 seeds per flower, compared with 232 seeds per flower for shrubs on the bird-friendly islands. Nearly 80 percent of flowers on the small islands showed evidence that birds had visited, whereas only 25 percent of mainland flowers did so. And although fully grown shrubs persist on the mainland in roughly similar numbers, there are less than half as many young plants sprouting up to replace them.

This pollination failure has likely been going on for a long time. "Pollination probably failed about 1870, which is when bellbirds and stitchbird densities on the North Island plummeted," says biologist Dave Kelly of the University of Canterbury in Christchurch, one of the scientists involved in the research.

Other possible pollinators, such as insects, apparently are not up to the task in the case of R. solandri, unlike the case for endemic New Zealand mistletoes or trees that have also lost bird pollinators, according to Kelly. Flowers encased in chicken wire mesh cages on the mainland and the offshore islands to keep out birds also failed to produce many seeds, "showing that birds are essential for pollination," the biologists wrote in the paper detailing their findings, published online in Science on February 3. Kelly adds: "Usually when a bird-adapted flower is not well pollinated by insects it's because the flower is too big for insects to touch the right parts [for pollination] as they visit it."

Of course, humans could take over for the missing birds, pollinating by hand and sowing the seeds. When the biologists did so, R. solandri immediately boosted its numbers. As for why the decline of the brilliantly colored flowering shrub had gone unnoticed, Kelly offers the idea of "shifting baselines"—people (and scientists) tend to ignore reports of abundance that existed before their own lifetimes—and wrote in the Science paper, "This decline could very easily have escaped notice, because it is so gradual…. It may be that similar slow plant declines as a result of failing ecological interactions have begun elsewhere."

After all, many regions of the world host plants that rely on specialized birds for pollination, from the tropics of Central and South America to South Africa and Southeast Asia. "I would bet that there are other plants declining in other parts of the world for this same reason, but it's not been measured yet," Kelly says. "In New Zealand we want to check other bird-pollinated plants to see if they too are declining. We know that a majority of them have [a] reduced seed set."

At the same time, R. solandri is still not considered in danger of extinction for two reasons: First, it still manages to produce some seed even without effective pollinators. Second, the shrub can apparently live for years. "We don't know exactly how long-lived the plant is as it does not seem to have growth rings," Kelly admits.

As for 19th-century North America, the passenger pigeon was a big consumer of the nuts of hickories, beeches and chestnuts in the vast eastern deciduous forest of North America. That forest has largely disappeared—and chestnuts were almost entirely wiped out by disease—but it is possible that the trees are also missing their one-time avian agent of dispersal. That could very easily be a problem for plants the world over, given that at least 190 species of birds have gone extinct since 1500 and more than a thousand are currently at risk of disappearing.

"The interactions among species are important for keeping ecosystems functioning properly," Kelly notes. "If we can get the birds right, the plant conservation will come as a secondary benefit."