Medium ground finch
The medium ground finch is a species of bird in the family Thraupidae. It is endemic to the Galápagos Islands. Its primary natural habitat is tropical shrubland. One of Darwin's finches, the species was the first which scientists have observed evolving in real-time.
Many studies and research have been conducted on medium ground finches: there are the most famous studies conducted by Charles Darwin and more recent studies conducted in relation to the changes revolving around the medium ground finches due to natural selection. Due to an increase in urbanization on the Galápagos Islands, droughts and climate change, character displacement, changes in the finch's habitat and range, inbreeding and nesting, parasites, and viruses, medium ground finches have gone through changes. Changes that have been observed are beak size, behavior in feeding, behavior in inbreeding, behaviors in nesting, antibody development and more. The changes in the Galápagos Islands are factors that affect the medium ground finches.
Description
Like the other members of its genus, the medium ground finch is strongly sexually dimorphic; the female's plumage is brown and streaky, while the male's is solid black, with white tips to the undertail coverts. The bird measures in length—which falls between the lengths of the small and large ground finches. The bill of this species is quite variable in size, though the length of the upper mandible is always greater than the depth of the bill at its base. The wing shape, on average, seems to change with ecological shifts. Different selective pressures act on the wing shape of the finches, such as natural and sexual selection. The males have shorter, rounder wings, which help with maneuvering around a female during sexual displays.Evolution
The Galápagos Islands are famous for plant and animal life; they are also famous due to the studies conducted by Charles Darwin, which led to the development of his theory of evolution by natural selection. The Galápagos Islands have now changed and has been an area of rapid urbanization. These changes have caused an impact to the life on the island. Since the 1990s, Galápagos tourism has increased by 9.4% per year and resident populations have increased by about 6.4% per year. Rural areas on the islands, which were privately owned, had wetlands suitable for agriculture. This agriculture brought about invasive plant species, which included guava, passion fruit, etc. The urban areas had more human activity like roads, shops, and pollution. This area brought invasive species to the island like fire ants, black rats, etc. With these changes in the environment, medium ground finches with short/stubby beaks adapted due to the seeds and food preferences.Food is the main driver of beak size and shape in Darwin's finches. For medium ground finches to be able to survive in urban areas, they had to adapt to new environments. Studies have shown that medium ground finches' phenotypes have been more variable than of small ground finches on Santa Cruz Island, being consistent with previous findings that medium ground finches adapt more rapidly to local conditions than small ground finches. The speed of how medium ground finches have been able to change beaks' sizes and shape may be centered around epigenetics. A research team led by Ms. McNew on the Galápagos Islands has measured the physical traits of wild birds and the genetics and epigenetics of two Darwin's finch species living at El Garrapatero, a rural area. They then compared the findings to urban finches living near Puerto Ayora; the two sites were not far apart. Some of the earlier studies showed that only one female out of 300 medium ground finches that were marked and used in the research relocated between both sites. Later on, Ms. McNew then captured more than 1,000 small ground finch and medium ground finch specimens, taking blood samples from females, sperm from males, and physical characteristics from each finch. Researchers realized that there was a difference between urban and rural finches' feeding preferences. The finches living in urban areas preferred human foods while the rural living finches did not. They then discovered that medium ground finches from the urban living area were larger in beak size than those living in the rural area. As for the small ground finches living in urban areas, they did not have any morphological differences from the ones living in rural areas. After conducting research, Ms. McNew and her team looked for evidence through genetics and found no evidence that can prove these phenotypic changes occurred through mutations. But when looking at epigenetics, there was significant evidence. In the finches studied, epigenetic alterations between the populations were dramatic, but minimal genetic changes were observed. The evidence showed that in medium and small ground finches, most of the epigenetic mutations were related to beak size and shape.
Urbanization is not the only factor affecting medium ground finches; drought and character displacement can also act as a selective pressure that can drive evolution of medium ground finches. In 1977, a severe drought reduced the supply of seeds in the Galápagos. The finch on Daphne Major, which normally preferred small and soft seeds, was forced to turn to harder, larger seeds. This strong selective pressure favoring larger beaks, coupled with the high heritability of traits relating to beak size in finches, caused the medium ground finch population to experience evolution by natural selection, leading to an increase in average beak size in the subsequent generation. A second drought of 1984–1986 caused a similar change, made more apparent by having smaller birds with bigger beaks.
Evidence of evolution through character displacement has been found in a population of medium ground finches on the Galápagos island of Daphne Major. During a drought in 2004, overlap in the diets of the medium ground finch population and a recently settled population of large ground finches led to competition for a limited supply of seeds on which the medium ground finch population normally fed. Because the large ground finches were able to out-compete the medium ground finches for these seeds due to both a larger beak and body size, the medium ground finch population experienced a strong selective pressure against large beaks to avoid competition, ultimately leading to dramatic evolutionary change favoring smaller beaks in the subsequent generation.
Speciation
The Big Bird lineage occurred through hybrid speciation between Daphne Major G. fortis and an immigrant G. conirostris. The group underwent a severe bottleneck during the 2004 drought.The study on "Big Bird" also has a whole-genome phylogeny of some Darwin's finches. Geospiza fortis is not monophyletic in the analyses: the minimum clade that would include all G. fortis also includes G. acutirostris, G. fuliginosa, G. magnirostris, and G. conirostris. The issue has been discussed in a broader context in Zink et al.. See also.
Habitat and range
to the Galápagos, the medium ground finch is found on ten islands: Baltra, Floreana, Isabela, Fernandina, Seymour, Pinzón, San Cristóbal, Santa Cruz, Santa Fé, and Santiago. They are found on most of the main islands including the surrounding islets. They are found in semi-arid biomes. Medium ground finches make dome shaped nests, mainly in Opuntia cacti. These nests are made by the males.Medium ground finches have a better chance of survival in their habitat than small ground finches, due to their beak size. The beak size of medium ground finches can evolve in a relatively short period of time, depending on if it is a wet season or dry season. Survival and beak size of the birds are fueled by the environment. Weather conditions will favor one beak size over the other, causing birds of the unfavorable bill size to die off. The HMGA2 gene locus is responsible for the evolution of beak size and plays an important role in natural selection.
The range size of a medium ground finch varies. It relies on many factors, such as the part of year it is, age, and the sex of the bird. Females that are incubating are more likely to remain in her nesting territory. Finches are 3.7 times more likely to travel to the brooding area than any other location.
Urbanization in the Galápagos is slowly increasing which directly affects the nesting success of the finches. Nests in urban areas are built using artificial materials, such as plastic, fishing lines, paper, and human hair. These materials cause death of the birds by strangulation, ingestion, and/or entanglement. Urban areas provide more reproductive success, however medium ground finches suffer by the usage of human-related debris in their nests. 97% of the Galápagos Islands are protected national parks, however the increasing population causes more dense urban areas. The islands are easily affected by anthropogenic changes, and urban development has a large impact on the environment, ecology, and evolution of the native species. The environmental changes such as increasing light pollution and noise directly affect the finches. Urbanization causes disruptions in ecological interactions, influencing selection pressures which causes phenotypes to be selectively modified. Areas of increased urbanization seem to have a lesser density of medium ground finches, as the abundance of humans leads to an abundance of food.
Behavior
General feeding and changes in feeding behaviors
The medium ground finch feeds primarily on seeds, although it is also known to eat flowers, buds, and young leaves, and the occasional insect. It forages either on the ground or in low vegetation. In urban areas, these birds exhibit a strong preference for human food items over their natural food sources. Scientists have found that due to urbanization, medium ground finches have access to a variety of food, such as human food. Food has been a selecting agent for the various beak sizes present on the Galápagos Islands; this shows natural selection at its finest, where those who are best adapted to their environment will most likely survive and reproduce.Scientists have found that due to urbanization, medium ground finches are showing changes in the size and shape of their beaks. Scientists conducted an experiment to observe the change in behaviors for feeding preferences among these finches; they found that the food the finches ate depended on the location they were found. The scientists observed feeding behaviors in four different locations with various levels of urbanization and tourist levels. In areas of high urbanization with high tourism, finches preferred to eat human food, whereas those finches located in more rural areas ate foods present in the area. The scientists also observed that finches, when given the preference between eating mainly human food or natural foods, chose to eat human food. Urbanization has caused an influence on feeding behaviors and preferences among medium ground finches.
Changes in feeding preferences can influence survival and reproduction. Further research has been conducted to determine why medium ground finches may be preferring human food rather than natural food sources. Researchers found that birds can detect bitter, sugary, and salty foods; each provides the bird with information. The bitter foods tell the birds that certain foods may be toxic; sugary foods have high calorie gains, and salty foods have high contents of salt. In the study, beak wiping after eating human food indicated that the bird did not like the food that was just consumed; researchers found that medium ground finches in remote areas wiped their beaks when given any type of human foods, specifically oily type foods, while ground finches in more urbanized areas fed more on sweet foods and wiped their beaks less than those in rural areas. Finches in more urbanized areas have more access to human food; and therefore more calorie-rich sweet foods. Food availability can shift depending on wet and dry seasons; this may affect birds in rural areas more than those in urban areas who have abundant access to human food. The amount of food available and the type of food rural and urban medium ground finches prefer to eat can effect whether birds will be more likely to adapt to changes in their environment and survive and reproduce.
Medium ground finches eat the seeds of Tribulus cistoides, a low-lying flowering plant. Scientists tested whether urbanization would impact feeding behaviors on T. cistoides. Scientists found that there was a 1.25% increase on feeding on T. cistoides seeds in more urbanized towns rather than natural habitats. This suggests that medium ground finches located in towns or urbanized areas have a stronger preference for small mericarps than do those living in natural habitats. Scientists found that due to increased dispersal of the seeds of T.'' cistoides through birds and urbanization, there are more mericarps available in towns; this may be why more medium ground finches are foraging seeds from small mericarps. Overall, due to urbanization, there has been an increase in interactions between medium ground finches and T''. cistoides due to the increased seed dispersal.