“Wow, you look just like my father” can be a surefire way to bring a date to a screeching halt. Unless, that is, you have hollow bones and live on a remote Pacific island.
Today, in the journal PNAS, researchers report that the famous finches once studied by Darwin on the Galápagos Islands tend to choose mates that bear an uncanny resemblance to their parents. This strategy helps ensure that birds boost their chances of bunking up with partners of their own species—and may even help solve an age-old evolutionary puzzle.
On the small sliver of Earth that Darwin’s finches call home, 18 species of these birds speckle the skies. Each lineage has its own little quirks. For instance, the shapes and sizes of different finch beaks have been whittled around specific foods: Some more suited to crack open little seeds, while others are particularly adept at pecking at cacti.
But it was only in the last million years or so that these diverse lineages truly start to crystallize. Even today, their distinguishing features are few and far between—which means breeding season can be a chaotic cross between speed dating and a family reunion. For years, biologists have wondered how so many finch lineages diverged in such close proximity, in such a short amount of time. Without geographical barriers to keep species separate, how did birds avoid interbreeding?
Luckily for finches, they’re born with the most reliable role models nature can provide: their parents. By simply paying attention to the subtle features and habits of their primary caretakers, little finches can begin to acquire a sense of their species identity—and which subtle features distinguish one lineage from another. This time-sensitive process of learning is called imprinting.
Some imprinted traits are vocal. When a male finch is single and ready to mingle, he can’t be a slouch with song: His winsome, species-specific warble is a big part of what will ultimately woo a female. But young birds aren’t born belting arias. Each male must earn his vocal heritage by studying dutifully under his father’s tutelage during his youth. Because these serenades are sung or recognized on a lineage-by-lineage basis, they help maintain finch fidelity—but imprinting is far from foolproof. If impressionable young males are exposed to the vocal stylings of another species during this period, they may spend the rest of their lives chasing the wrong tail.
Clearly, deciding on a date through one trait alone can be quite the gamble. If a finch is to carry on with its own kind, there’s a good chance it’s imprinting on other cues to tell who’s who in the avian community, reasoned evolutionary biologists Peter and Rosemary Grant of Princeton University. And if the Grants have a theory about Darwin’s finches, it’s probably right.
The pair has studied the Galápagos Islands’ winged residents since 1973, observing evolution in real time as lineages of finches ebbed and flowed through the years. From 1976 to 2012, the Grants tracked thousands of finches on the tiny volcanic outcrop of Daphne Major, meticulously recording their beak and body dimensions and noting each bird’s parentage and mating habits (according to their report, the duo attempted to find every nest and identify the parents in every year of breeding from 1976 to 1998).
“There is so much power in long-term studies like these,” says James Dale, an evolutionary biologist who studies bird behavior at Massey University in New Zealand. The Grants, he adds, have “accumulated a vast database that allows them to test so many new theories.”
The breadth of the Grants’ body of work is truly staggering. By 2012, the pair had amassed nearly 40 years of anatomical and behavioral data on two species of finch: the medium ground finch (Geospiza fortis), a puff of a bird with a stubby beak suited for snacking on seeds; and the cactus finch (Geospiza scandens), which sports a svelte, spiky beak apt for chowing down on—you guessed it—prickly pear cactus.
The Grants quickly discovered each species of finch had a strong tendency to stick to its own kind—and, as they expected, the cues for their choices weren’t limited to song. For instance, in the medium ground finch, body size matched up consistently between mates and parents—especially the father. In the cactus finch, on the other hand, females also went for daddy doppelgängers, but instead prioritized the shape of their fathers’ beaks. Regardless of anatomy, these preferences meant that when most flirty finches were hot to trot, they were recalling the faces and forms they saw most often in their youth.
“It was very rewarding to discover that [features] of the mates of both sons and daughters can be predicted from the same characteristics of the parents,” the Grants wrote in an email.
Incredibly, because the differences between individual birds are so minor, many finches were selecting mates among anatomical variants of “just millimeters,” says Jason Weir, a biologist who studies the evolution of birds at the University of Toronto, but was not involved in the work. Such a subtle pattern, he says, was only detectable through the sheer numbers of birds the Grants tagged and tracked over the years. “They basically know every individual and have every pedigree,” Weir explains.
Every population, however, is graced by a few renegade romantics. Some medium ground finches ultimately strayed outside their own lineage, blurring the lines between species. But even mismatched lovebirds found common ground. Rather than sticking to their own, some medium ground finches sought the company of small ground finches (Geospiza fuliginosa), which are closely related, but, as their name indicates, a bit more diminutive in size. When the Grants examined these oddballs, they realized the medium ground finches most likely to seek mini-mates were often pint-sized themselves.
Some large finches took on a similar habit. Over the course of his 14-year life, one lothario from the large cactus finch (Geospiza conirostris) population swept no less than six female medium ground finches off their feet—several of whom were particularly big-bodied for their breed. Even in extra-species couplings, like mated with like.
Imprinting on parents to find a mate “is a learning process that helps finches to navigate the adult world,” the Grants explain.
And the findings of this study speak to much more than individual finches’ flights of fancy. By picking partners that parallel their parents, finches increase the likelihood that their mates—and future offspring—will be of their same ilk. Reusing the same imprinting template over and over means that the differences between lineages can get enhanced until they split away entirely. With this mirroring strategy, finches can breed diversity under even the most unexpected of circumstances.
Turns out, birds of a feather really do flock together—especially in the Galapagos.