Mosses hitch rides on the wings of birds

Before humans started traveling around the world and accidentally transporting plant seeds from one place to another, the main way that plants spread from continent to continent was by wind. But there’s a barrier that prevents that kind of dispersal across the equator — the Intertropical Convergence Zone, a band of trade winds that often manifests as a strip of clouds when seen from far above.

However, there are dozens of species of mosses, lichens and other bryophytes that can be found on either side of that barrier. Scientists had assumed that migratory birds carry seeds and spores back and forth across the equator, but they had no proof.

So Lily Lewis of the University of Connecticut in Storrs and colleagues sampled the feathers of 23 birds from eight species found in two sites in the Arctic, in Alaska and Canada. The eight species all migrate from summer in the Arctic to summer south of the equator.

Seven of the 23 birds — two semipalmated sandpipers, two red phalaropes and three American golden-plovers — had bits of bryophyte leaf or spore fragments hidden within their feathers. The birds probably picked up the plant pieces from their nests, which are depressions dug into the ground and lined with vegetation.

However, those fragments aren’t definitive evidence that birds are transporting plants from one end of the earth to the other; scientists will need to check birds at the other end of their journey.

But there are several factors that Lewis and colleagues cite as additional evidence that traveling by bird from the Arctic to South America is possible: Most of these Arctic birds don’t molt their feathers until after they reach their southern homes, so it’s likely that they can carry the vegetation that far. Bryophytes are known to be tough; they’re drought tolerant, and pieces can grow into mature plants “after severe grinding, rapid passage through a mammalian digestive tract and even after being frozen under a glacier for 400 years,” Lewis and colleagues note. Plus, traveling by bird is probably a less severe journey than one by wind, adding to the probability that those bits of vegetation can become full-grown plants even after thousands of miles of flight.

Given that about a third of the birds in the study were found carrying bits of bryophyte, that could translate to hundreds of thousands of plant spores being transported every year across the equator, the researchers report June 12 in PeerJ. It wouldn’t be all that surprising if at least a few of those plants managed to take root in the south.