![]() Two larger species of finches on Genovésa feed and nest in close proximity to the warbler and sharp-beaked finches. These same populations also feed on booby eggs by pushing and rolling them into rocks until they break, revealing remarkable behavioural adaptations that match their beak morphology. difficilis septentrionalis) use their sharp arrowhead-shaped beaks to cut wounds on large sea birds, such as the Nazca and blue-footed boobies, and drink their blood. On the neighbouring small island of Wolf, members of the same species ( G. Another species feeding nearby on a small bush is the sharp-beaked finch ( Geospiza difficilis), which has a slightly larger and more cone-shaped beak that is used to collect a more varied diet of both insects and small seeds. ![]() It has a very thin and pointed beak, which is used to probe leaves of the palo santo trees ( Bursera graveolens) to catch small insects and their larvae. ![]() One of them is called a warbler finch ( Certhidea fusca) and, as its name suggests, it looks and behaves like a warbler from the mainland. These birds look similar to each other in plumage and song, yet closer observation reveals that they all differ from one another in how their beaks look and work. High above the cliffs of the Darwin Bay on Isla Genovésa (Tower Island), one of the Galápagos archipelago islands in the Pacific Ocean, jumping around the sharp lava rocks on the ground, perched on the branches of the yellow geiger ( Cordia lutea) and croton ( Croton scouleri) bushes and flying around large yellow flowers of prickly pear cactuses ( Opuntia helleri), are small black and brown birds. Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends. The most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza, from one as large as that of a hawfinch to that of a chaffinch, and … even to that of a warbler…. Insights provided by papers collected in this Theme Issue will be of interest to a wide audience. Our objective was to bring together some of the key workers in the field of ecology and evolutionary biology who study Darwin's finches or whose studies were inspired by research on Darwin's finches. ![]() Certain advantages of studying this group allow further breakthroughs in our understanding of changes in recent island biodiversity, mechanisms of speciation and hybridization, evolution of cognitive behaviours, principles of beak/jaw biomechanics as well as the underlying developmental genetic mechanisms in generating morphological diversity. Today, as this Theme Issue illustrates, Darwin's finches continue to be a very valuable source of biological discovery. Many biology textbooks use Darwin's finches to illustrate a variety of topics of evolutionary theory, such as speciation, natural selection and niche partitioning. Since Charles Darwin and other members of the Beagle expedition collected these birds on the Galápagos Islands in 1835 and introduced them to science, they have been the subjects of intense research. One of the classic examples of adaptive radiation under natural selection is the evolution of 15 closely related species of Darwin's finches (Passeriformes), whose primary diversity lies in the size and shape of their beaks.
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