Against the loudmouth
When two people argue, it often makes third parties happy. In the case of two competing songbird species in the Galapagos, this is a pair of researchers: as the animals fight for food, they observed evolution in real time.
Galapagos finches would probably be just boring grey, brown or black sparrows with naïve, chirping vocalizations to most non-bird enthusiasts were it not for Charles Darwin's voyage to their homeland more than 170 years ago. But the animals, often named after him as Darwin's finches, became icons of research: the English scholar used their example, among other things, to explain the splitting up and development of species.
The true splendor of the finches is not presented in their plumage, but in the peculiar beak types of the 13 species on Galapagos. Each of them has its own form, although they all descend from a single ancestor. There are, for example, the ground finches (Geospiza) with their massive hawfinch beaks, which are suitable for cracking seeds, but vary in strength depending on the species and therefore skim off different food sources. The woodpecker finches (Cactospiza), on the other hand, take on the ecological role of those woodpeckers that did not make it to the Galapagos, and poke around in bark and tree holes for insects with their longer, finer beaks. And the thick-billed Darwin's finch (Camarhynchus crassirostris) has a mouthpart similar to that of the parrots that are also absent on the islands: its beak is used to crush fruit.
Despite these differences, the individual finches avoid each other spatially, as only one or two species live on many islands. And if more than two species share an island, they spend a large part of their existence in different habitats and thus largely avoid direct competition. In times of need, the birds can fall back on food resources to which they are actually not optimally adapted.
But what happens when another species invades the limited territory and sometimes resorts to the same seeds? Will one or both lose the competition? Or are they flexible enough to quickly switch to new food sources? Such a - natural - field experiment has been running for 33 years on the small Galapagos island Daphne Major and is being observed by the two biologists Peter and Rosemary Grant from Princeton University.
Originally, only middle ground finches (Geospiza fortis) lived on Daphne Major, which is not even half a square kilometer in size, and they exploited all types of seeds almost without competition. In good times they preferred small seeds, while at least some of them also resorted to the larger fruits of the zygomatic family Tribulus cistoides, which are harder to crack because of the wooden skin. Only a slightly bulkier beak granted access to their nutritious interior, so that depending on the situation, sometimes the large and sometimes the small-beaked subpopulation was favored - with corresponding survival rates and reproductive success.
When a severe drought hit the home of the finches in 1977 and prevented the emergence of small-seeded plants, this changed for the first time. Since the animals had only large grains as an alternative for a long time, the balance of power clearly shifted towards the big mouths, while slender conspecifics disappeared: Within a few generations, the average beak dimensions increased by four percent.
From 1982 this hermit life ended on Daphne, because a new breeding population of the large ground finch (Geospiza magnirostris) was established. They prefer to chew the seeds of Tribulus cistoides, but crack them much faster than their smaller cousins - but they despise small items. And: The newcomers not only use the supplies of Tribulus seeds much more extensively, but also chase away their competition as soon as it approaches the zygomatic plants.
For more than twenty years, this has had no negative impact on the numbers, behavior and physique of either finch species. On the contrary, both population groups enjoyed good growth rates. But then another severe hunger crisis hit Daphne Major. The small seeds were quickly used up, and even larger pieces of prey became scarce, so that fierce competition broke out for the remaining supplies. Many of the birds starved to death.
One year later, instead of 350, only 150 large ground finches inhabited the island, and the number of middle ground finches, half the size, had also fallen by several hundred to 235 individuals. However, the drama was far from over: as the dry period continued, the birds were left with only a few food alternatives; another 137 Geospiza magnirostris – whose population has fallen to its lowest level since immigration – and 152 Geospiza fortis perished.
Again, within the middle ground finch group, those with the largest and most powerful beaks were hit hardest; by 2005, just over a tenth of this faction survived. In contrast to their delicate fellows, they could not fall back on the last sufficient remaining food source: the tiny seeds of the borage plant Tiquilia fusca and of Sesuvium edmonstonei from the ice plant family were not within reach for them. Natural selection on the island now favored the animals with the smallest and finest mouths: Within two years, the average beak length of the middle ground finches decreased again from 11.2 to 10.6 millimeters, its diameter at the thickest point fell from 9.4 to 8.6 millimeters. At least on Daphne, the two finches are now more different than ever.
What was tantamount to a catastrophe for the animals delighted the researchers, because two drought events drove the evolution of the birds rapidly in two different directions. And for the first time they observed decisive character shifts in real time in nature: Darwin would probably have been delighted.
