IN MATTERS biological, whether large or small, it is a good bet that what Charles Darwin said back in the 19th century is correct. One idea he had, which has not hitherto been put to the test, is that waterborne invertebrates frequently disperse by hitching lifts on birds. On this, it turns out, he was right as well.

Darwin noticed that when he suspended a stuffed duck’s foot in an aquarium, invertebrate larvae attached themselves to it and held on tight for many hours, even if he removed it from the water. Sadly, he died before he could explore this phenomenon in the wild. But Joseph Simonis and Julie Ellis just have.

Dr Simonis, who works at Lincoln Park Zoo in Chicago, and Dr Ellis, of Tufts University in Massachusetts, put Darwin’s idea to the test on Appledore Island in Maine, where thousands of rain-filled rock pools sit above the high-tide mark. These pools host such animals as water fleas, rotifers and seed shrimps. Since these critters are incapable of traversing dry land by themselves, the two researchers thought they might be making use of birds to hop between pools, as Darwin suggested.

Dr Simonis and Dr Ellis therefore collected 25 young gulls that had been dabbling in the pools (juveniles are easier to catch because they cannot yet fly) and 25 others that they had followed for at least 20 minutes, and had not seen go into a pool in that time. They gave every bird a bath in a tub of water for five minutes and sieved the water afterwards to see what had floated off.

They report in Ecology that 16 of the gulls which had been dabbling carried larvae or viable eggs when they were given their enforced bath. One, indeed, had 18 such creatures attached to it. Of those gulls that had eschewed the water, only two carried hitchhiking invertebrates. Gulls, then, certainly are picking up passengers when they bathe. But are they dropping them at suitable destinations?

The answer to that seems to be “yes”, too. Pools in areas with lots of gulls shared 80% of their species. That dropped to 50% in places where the birds were few and far between. It therefore looks as though gulls are homogenising the local ecosystems by carrying invertebrates around. Chalk another one up to Charlie.