Medium Ground-Finch

Identification

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 12.5 cm (4.9 in) 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.

Taxonomy

Scientific name

Geospiza fortis

Authority

Gould, 1837

Order

Passeriformes

Family

Thraupidae

Genus

Geospiza

Range and habitat

Endemic 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 finch make dome shaped nests, mainly in Opuntia cacti. These nests are made by the males.<p>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 finch 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.<p>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.<p>Urbanization in the Galapagos 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 Galapagos islands are protected nationals parks, however the increasing populations 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 less density of medium ground finches. as the abundance of humans leads to an abundance of food.

Migration

Resident species

Behavior

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 Galapagos islands; this shows natural selection at its finest, where those who are best adapted to their environment will most likely survive and reproduce.<p>Scientists have found that due to urbanization, medium ground finches are showing changes in 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.<p>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.<p>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 (such as seeds being dispersed by human activity), 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.<p>Inbreeding is said to affect medium ground finches; in years with low food availability inbreeding increases and this decreases reproductive success. In years with high food availability, inbreeding decreases and reproductive success increases. Finches in urban areas have more food availability than natural habitats due to human food; and due to this, reproductive success increases.  This is how natural selection can occur; the birds' environment can influence the success of offspring and can possibly bring about evolution of finches in urban areas. Even though finches in urban areas have higher reproductive success they still exhibit mortality due to human pollutants such as plastic being trapped in nests.  Overall, birds in urban areas have better outcomes for nesting than those birds in the natural habitat due to food availability. Birds in urban areas can become more adapted through having better reproductive and nesting success; this can influence a higher survival rate in urban areas.

Conservation

Population trend

stable