The Elements of Evolution (43-14-6) Birds


Dinosaurs and their descendants diversified through several bursts of niche-filling radiations. Evolution is not necessarily the gradual process that Darwin imagined.

On his trip to the Galápagos Islands, Darwin was puzzled by the diversity of finches there: a smorgasbord of variation.

Within the past million years finches had arrived on those dry volcanic islands, blown far from where they intended. Since then, the descendants of immigrant finches evolved into 18 separate species, falling into 3 groups, ranging in size from less than 10 grams to more than 40. The 3 groupings are ground finches, tree finches, and the warbler finch.

Besides size, the most conspicuous difference in body shape is the beak, which relates to diet. Finches with short, thick beaks feed on seed, which would be too tough to crack with the slender beaks of insect feeders. The warbler finch, and other finches with long bills, probe flowers for nectar, or the holes in wood for tasty critters.

◊ ◊ ◊

In 1981, a young, male, large cactus finch immigrated 100 km from the Galápagos island of Espanola to the small island of Daphne Major. The immigration is remarkable for its extreme distance. Clearly this bird willed its way on a unique adventure.

Unable to return home to mate, the sturdy immigrant bred with a female medium ground finch on Daphne Major, who liked the male’s body, beak, and song.

One of the 1st generation of offspring bred with another medium ground finch, but all other offspring inbred. From generation 2 onwards, the lineage behaved as an independent species. Generations 4–6 came from a single brother-sister mating in generation 3. Despite close inbreeding, the lineage was highly fit, judging by their reproductive output and high survival.

Fledgling finches imprint on the features of their parents. When later choosing a mate, females discriminate on the basis of bill size and shape, as well as song and body size. Hence, this new species was immediately reproductively isolated.

The new species, dubbed big bird, is 1 of 4 different finches that live on Daphne Major. Beak morphology was a key factor in this new finch’s success. Unlike medium ground finches, big bird finches can efficiently exploit the large woody fruits available in dry seasons.

Hybrid speciation of the big bird lineage exemplifies the potential evolutionary importance of rare and chance events. Expansion of the population from 2 individuals to 3 dozen was conditioned on the founder being a male with a distinctive song, and facilitated by the chance occurrence of strong selection against large bill size in a competitor species, the medium ground finch. ~ American evolutionary biologist Peter Grant

◊ ◊ ◊

On Daphne Major, medium ground finches competed with large ground finches for seeds. Both had big beaks. Then came a drought in 2004–2005 that diminished seed supply. The medium-sized finches lost ground to their bigger cousins as many died of starvation. Medium ground finches quickly adapted by having offspring with smaller beaks, giving them greater feeding flexibility in seed size.

Besides the beak, Galápagos finches have developed specialized feeding habits. Woodpecker and mangrove finches use various tools to pry arthropods from crevices. Twigs, cactus spines, and leaf stems are popular tools of the trade.

Vampire finches peck at the base of booby feathers and suck booby blood. The boobies try to dislodge them, but the little feathered vampire finches still manage to steal their bloody meals. These finches also shove and kick seabird eggs against rocks to crack them, widen the cracks, then suck out the contents.

Life on the Galápagos is tough. Food is often difficult to come by.

As a group Darwin’s finches rip open rotting cactus pads, strip the bark off dead branches, kick over stones, probe flowers, rolled leaves, and cavities in trees, and search for arthropods on the exposed rocks of the shoreline at low tide. They consume nectar, pollen, leaves, buds, a host of arthropods, and seeds and fruits of various sizes. ~ Peter Grant