of a low herbaceous plant called Tribulus cistoides.

These larger-beaked birds met with competition upon the arrival of the bigger G. magnirostris, a few members of which flew to the island in 1982 and set up a colony. Their universally large beaks made cracking into big seeds an easy job.

A Tribulus seed is like "an orange wedge with two great big long spines sticking out the back of it," says Dolph Schluter, a zoologist at the University of British Columbia in Vancouver, Canada, who worked with the Grants when he was a graduate student. "The medium ground finches twist off the ends, but it takes a lot of force to do it. G. magnirostris has no problem with it."

The two species lived fairly happily together for many years, untiltwo factors forced the birds into harsh competition.

Tough times

The population of large finches grew, until there was enough of them to be battling with the medium finches, who were also going after Tribulus seeds. And then two years of drought, in 2003 and 2004, dramatically reduced the food supply, causing both populations to plummet as birds died of starvation.

In these mean conditions, the medium finches with smaller beaks had an advantage over those with big beaks, as they could more easily suck up smaller seeds that weren't being gobbled by the large finches.

"Small-beaked birds survived better than the large-beaked birds, to a strong extent, during the drought," says Peter Grant. And that trait was then passed down to the next generation.

In 2004 and 2005, the Grants observed a